Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main-sequence stars . Their mass is approximately 13 to 80 times that of Jupiter ( M J ) —not big enough to sustain nuclear fusion of ordinary hydrogen ( H ) into helium in their cores, but massive enough to emit some light and heat from the fusion of deuterium ( H ). The most massive ones (> 65 M J ) can fuse lithium ( Li ).
187-535: An exoplanet or extrasolar planet is a planet outside the Solar System . The first possible evidence of an exoplanet was noted in 1917 but was not then recognized as such. The first confirmation of the detection occurred in 1992. A different planet, first detected in 1988, was confirmed in 2003. As of 7 November 2024, there are 5,787 confirmed exoplanets in 4,320 planetary systems , with 969 systems having more than one planet . The James Webb Space Telescope (JWST)
374-492: A binary star system, and several circumbinary planets have been discovered which orbit both members of a binary star. A few planets in triple star systems are known and one in the quadruple system Kepler-64 . In 2013, the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue. Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has
561-469: A geodynamo that generates a magnetic field . Similar differentiation processes are believed to have occurred on some of the large moons and dwarf planets, though the process may not always have been completed: Ceres, Callisto, and Titan appear to be incompletely differentiated. The asteroid Vesta, though not a dwarf planet because it was battered by impacts out of roundness, has a differentiated interior similar to that of Venus, Earth, and Mars. All of
748-526: A population I object with a mass less than 0.07 solar masses ( M ☉ ) or a population II object less than 0.09 M ☉ would never go through normal stellar evolution and would become a completely degenerate star . The first self-consistent calculation of the hydrogen-burning minimum mass confirmed a value between 0.07 and 0.08 solar masses for population I objects. The discovery of deuterium burning down to 0.013 M ☉ ( 13.6 M J ) and
935-486: A pulsar planet in orbit around PSR 1829-10 , using pulsar timing variations. The claim briefly received intense attention, but Lyne and his team soon retracted it. As of 24 July 2024, a total of 5,787 confirmed exoplanets are listed in the NASA Exoplanet Archive, including a few that were confirmations of controversial claims from the late 1980s. The first published discovery to receive subsequent confirmation
1122-406: A rogue planet , is believed to be orbited by a tiny protoplanetary disc , and the sub-brown dwarf OTS 44 was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth masses. The idea of planets has evolved over the history of astronomy, from the divine lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in
1309-516: A triaxial ellipsoid . The exoplanet Tau Boötis b and its parent star Tau Boötis appear to be mutually tidally locked. The defining dynamic characteristic of a planet, according to the IAU definition, is that it has cleared its neighborhood . A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the planetesimals in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with
1496-420: A white dwarf that has cooled to the point that it no longer emits significant amounts of light. However, the time required for even the lowest-mass white dwarf to cool to this temperature is calculated to be longer than the current age of the universe; hence such objects are expected to not yet exist. Early theories concerning the nature of the lowest-mass stars and the hydrogen-burning limit suggested that
1683-432: A binary brown dwarf system. Lithium is generally present in brown dwarfs and not in low-mass stars. Stars, which reach the high temperature necessary for fusing hydrogen, rapidly deplete their lithium. Fusion of lithium-7 and a proton occurs, producing two helium-4 nuclei. The temperature necessary for this reaction is just below that necessary for hydrogen fusion. Convection in low-mass stars ensures that lithium in
1870-494: A boundary, even though deuterium burning does not last very long and most brown dwarfs have long since finished burning their deuterium. This is not universally agreed upon: the exoplanets Encyclopaedia includes objects up to 60 M J , and the Exoplanet Data Explorer up to 24 M J . The smallest known exoplanet with an accurately known mass is PSR B1257+12A , one of the first exoplanets discovered, which
2057-435: A composition more similar to their host star than accretion-formed planets, which would contain increased abundances of heavier elements. Most directly imaged planets as of April 2014 are massive and have wide orbits so probably represent the low-mass end of a brown dwarf formation. One study suggests that objects above 10 M Jup formed through gravitational instability and should not be thought of as planets. Also,
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#17328556182842244-679: A disk remnant left over from the supernova that produced the pulsar. The first confirmed discovery of an exoplanet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of 51 Pegasi b , an exoplanet around 51 Pegasi . From then until the Kepler space telescope mission, most of the known exoplanets were gas giants comparable in mass to Jupiter or larger as they were more easily detected. The catalog of Kepler candidate planets consists mostly of planets
2431-403: A faint companion to the white dwarf star GD 165 was found in an infrared search of white dwarfs. The spectrum of the companion GD 165B was very red and enigmatic, showing none of the features expected of a low-mass red dwarf . It became clear that GD 165B would need to be classified as a much cooler object than the latest M dwarfs then known. GD 165B remained unique for almost a decade until
2618-401: A gaseous protoplanetary disk , they accrete hydrogen / helium envelopes. These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space. This means that even terrestrial planets may start off with large radii if they form early enough. An example is Kepler-51b which has only about twice the mass of Earth but
2805-430: A heterogeneous iron-containing atmosphere was imaged around the B component in the nearby Luhman 16 system. For late T-type brown dwarfs only a few variable searches were carried out. Thin cloud layers are predicted to form in late T-dwarfs from chromium and potassium chloride , as well as several sulfides . These sulfides are manganese sulfide , sodium sulfide and zinc sulfide . The variable T7 dwarf 2M0050–3322
2992-471: A high mass (possibly a low-mass star) or a young brown dwarf with a very low mass. For Y dwarfs this is less of a problem, as they remain low-mass objects near the sub-brown dwarf limit, even for relatively high age estimates. For L and T dwarfs it is still useful to have an accurate age estimate. The luminosity is here the less concerning property, as this can be estimated from the spectral energy distribution . The age estimate can be done in two ways. Either
3179-442: A list of omens and their relationships with various celestial phenomena including the motions of the planets. The inferior planets Venus and Mercury and the superior planets Mars , Jupiter , and Saturn were all identified by Babylonian astronomers . These would remain the only known planets until the invention of the telescope in early modern times. The ancient Greeks initially did not attach as much significance to
3366-408: A lower thick iron cloud layer and an upper silicate cloud layer. This upper silicate cloud layer can consist out of quartz , enstatite , corundum and/or fosterite . It is however not clear if silicate clouds are always necessary for young objects. Silicate absorption can be directly observed in the mid-infrared at 8 to 12 μm. Observations with Spitzer IRS have shown that silicate absorption
3553-400: A magenta color, and Kappa Andromedae b , which if seen up close would appear reddish in color. Helium planets are expected to be white or grey in appearance. The apparent brightness ( apparent magnitude ) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from
3740-411: A mass 5.5–10.4 times the mass of Earth, attracted attention upon its discovery for potentially being in the habitable zone, though later studies concluded that it is actually too close to its star to be habitable. Planets more massive than Jupiter are also known, extending seamlessly into the realm of brown dwarfs. Exoplanets have been found that are much closer to their parent star than any planet in
3927-479: A metallic or rocky core today, or a reaccumulation of the resulting debris. Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a differentiated interior consisting of a dense planetary core surrounded by a mantle that either is or was a fluid . The terrestrial planets' mantles are sealed within hard crusts , but in
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#17328556182844114-474: A minimum bolometric luminosity that they can sustain through steady fusion. This luminosity varies from star to star, but is generally at least 0.01% that of the Sun. Brown dwarfs cool and darken steadily over their lifetimes; sufficiently old brown dwarfs will be too faint to be detectable. Clouds are used to explain the weakening of the iron hydride (FeH) spectral line in late L-dwarfs. Iron clouds deplete FeH in
4301-512: A mixture of very-low-mass stars and sub-stellar objects (brown dwarfs), whereas the T dwarf class is composed entirely of brown dwarfs. Because of the absorption of sodium and potassium in the green part of the spectrum of T dwarfs, the actual appearance of T dwarfs to human visual perception is estimated to be not brown, but magenta . Early observations limited how distant T-dwarfs could be observed. T-class brown dwarfs, such as WISE 0316+4307 , have been detected more than 100 light-years from
4488-572: A multitude of similar-sized objects. As described above, this characteristic was mandated as part of the IAU 's official definition of a planet in August 2006. Although to date this criterion only applies to the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs . Gravity causes planets to be pulled into
4675-402: A negligible axial tilt as a result of their proximity to their stars. Similarly, the axial tilts of the planetary-mass moons are near zero, with Earth's Moon at 6.687° as the biggest exception; additionally, Callisto's axial tilt varies between 0 and about 2 degrees on timescales of thousands of years. The planets rotate around invisible axes through their centres. A planet's rotation period
4862-424: A planet may be able to be formed in their orbit. In the early 1990s, a group of astronomers led by Donald Backer , who were studying what they thought was a binary pulsar ( PSR B1620−26 b ), determined that a third object was needed to explain the observed Doppler shifts . Within a few years, the gravitational effects of the planet on the orbit of the pulsar and white dwarf had been measured, giving an estimate of
5049-413: A planet reaches a mass somewhat larger than Mars's mass, it begins to accumulate an extended atmosphere , greatly increasing the capture rate of the planetesimals by means of atmospheric drag . Depending on the accretion history of solids and gas, a giant planet , an ice giant , or a terrestrial planet may result. It is thought that the regular satellites of Jupiter, Saturn, and Uranus formed in
5236-426: A plausible base for future human exploration . Titan has the only nitrogen -rich planetary atmosphere in the Solar System other than Earth's. Just as Earth's conditions are close to the triple point of water, allowing it to exist in all three states on the planet's surface, so Titan's are to the triple point of methane . Planetary atmospheres are affected by the varying insolation or internal energy, leading to
5423-457: A roughly spherical shape, so a planet's size can be expressed roughly by an average radius (for example, Earth radius or Jupiter radius ). However, planets are not perfectly spherical; for example, the Earth's rotation causes it to be slightly flattened at the poles with a bulge around the equator . Therefore, a better approximation of Earth's shape is an oblate spheroid , whose equatorial diameter
5610-407: A significant effect. There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase -dependent, this is not always the case in the near infrared. Temperatures of gas giants reduce over time and with distance from their stars. Lowering the temperature increases optical albedo even without clouds. At
5797-454: A significant impact on mythology , religious cosmology , and ancient astronomy . In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the " fixed stars ", which maintained a constant relative position in the sky. Ancient Greeks called these lights πλάνητες ἀστέρες ( planētes asteres ) ' wandering stars ' or simply πλανῆται ( planētai ) ' wanderers ' from which today's word "planet"
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5984-403: A significantly lower mass than the gas giants (only 14 and 17 Earth masses). Dwarf planets are gravitationally rounded, but have not cleared their orbits of other bodies . In increasing order of average distance from the Sun, the ones generally agreed among astronomers are Ceres , Orcus , Pluto , Haumea , Quaoar , Makemake , Gonggong , Eris , and Sedna . Ceres is the largest object in
6171-407: A similar way; however, Triton was likely captured by Neptune, and Earth's Moon and Pluto's Charon might have formed in collisions. When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation , the solar wind , Poynting–Robertson drag and other effects. Thereafter there still may be many protoplanets orbiting
6358-504: A statistical technique called "verification by multiplicity". Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they were more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth. On 23 July 2015, NASA announced Kepler-452b , a near-Earth-size planet orbiting the habitable zone of a G2-type star. On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in
6545-452: A sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures, ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths. Planet A planet is a large, rounded astronomical body that is generally required to be in orbit around a star , stellar remnant , or brown dwarf , and is not one itself. The Solar System has eight planets by
6732-608: A system is designated "b" (the parent star is considered "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets . A limited number of exoplanets have IAU-sanctioned proper names . Other naming systems exist. For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there
6919-467: A terrestrial planet could sustain liquid water on its surface, given enough atmospheric pressure. One in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone, which suggests that the nearest would be expected to be within 12 light-years distance from Earth. The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation , which estimates
7106-496: A very-low-mass star, because observationally it is very difficult to distinguish between the two. Soon after the discovery of GD 165B, other brown-dwarf candidates were reported. Most failed to live up to their candidacy, however, because the absence of lithium showed them to be stellar objects. True stars burn their lithium within a little over 100 Myr , whereas brown dwarfs (which can, confusingly, have temperatures and luminosities similar to true stars) will not. Hence,
7293-465: A wide range of other factors in determining the suitability of a planet for hosting life. Rogue planets are those that do not orbit any star. Such objects are considered a separate category of planets, especially if they are gas giants , often counted as sub-brown dwarfs . The rogue planets in the Milky Way possibly number in the billions or more. The official definition of the term planet used by
7480-438: Is 43 kilometers (27 mi) larger than the pole -to-pole diameter. Generally, a planet's shape may be described by giving polar and equatorial radii of a spheroid or specifying a reference ellipsoid . From such a specification, the planet's flattening, surface area, and volume can be calculated; its normal gravity can be computed knowing its size, shape, rotation rate, and mass. A planet's defining physical characteristic
7667-444: Is a rule of thumb rather than a quantity with precise physical significance. Larger objects will burn most of their deuterium and smaller ones will burn only a little, and the 13‑Jupiter-mass value is somewhere in between. The amount of deuterium burnt also depends to some extent on the composition of the object, specifically on the amount of helium and deuterium present and on the fraction of heavier elements, which determines
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7854-569: Is almost the size of Saturn, which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old. There is at least one planet on average per star. About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone . Most known exoplanets orbit stars roughly similar to the Sun , i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars ( red dwarfs , of spectral category M) are less likely to have planets massive enough to be detected by
8041-446: Is also debated whether brown dwarfs would be better defined by their formation process rather than by theoretical mass limits based on nuclear fusion reactions. Under this interpretation brown dwarfs are those objects that represent the lowest-mass products of the star formation process, while planets are objects formed in an accretion disk surrounding a star. The coolest free-floating objects discovered, such as WISE 0855 , as well as
8228-429: Is also seen in very young stars, which have not yet had enough time to burn it all. Heavier stars, like the Sun, can also retain lithium in their outer layers, which never get hot enough to fuse lithium, and whose convective layer does not mix with the core where the lithium would be rapidly depleted. Those larger stars are easily distinguishable from brown dwarfs by their size and luminosity. Conversely, brown dwarfs at
8415-484: Is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter. Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in
8602-401: Is common, but not ubiquitous, for L2-L8 dwarfs. Additionally, MIRI has observed silicate absorption in the planetary-mass companion VHS 1256b . Iron rain as part of atmospheric convection processes is possible only in brown dwarfs, and not in small stars. The spectroscopy research into iron rain is still ongoing, but not all brown dwarfs will always have this atmospheric anomaly. In 2013,
8789-543: Is delineated by a set of elements: Planets have varying degrees of axial tilt; they spin at an angle to the plane of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the Northern Hemisphere points away from its star, the Southern Hemisphere points towards it, and vice versa. Each planet therefore has seasons , resulting in changes to
8976-407: Is expected to discover more exoplanets, and to give more insight into their traits, such as their composition , environmental conditions , and potential for life . There are many methods of detecting exoplanets . Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of
9163-749: Is explained to have a top layer of potassium chloride clouds, a mid layer of sodium sulfide clouds and a lower layer of manganese sulfide clouds. Patchy clouds of the top two cloud layers could explain why the methane and water vapor bands are variable. At the lowest temperatures of the Y-dwarf WISE 0855-0714 patchy cloud layers of sulfide and water ice clouds could cover 50% of the surface. Like stars, brown dwarfs form independently, but, unlike stars, they lack sufficient mass to "ignite" hydrogen fusion. Like all stars, they can occur singly or in close proximity to other stars. Some orbit stars and can, like planets, have eccentric orbits. Brown dwarfs are all roughly
9350-410: Is influenced by exceptionally broad absorption features from the alkali metals Na and K . These differences led J. Davy Kirkpatrick to propose the T spectral class for objects exhibiting H- and K-band CH 4 absorption. As of 2013 , 355 T dwarfs were known. NIR classification schemes for T dwarfs have recently been developed by Adam Burgasser and Tom Geballe. Theory suggests that L dwarfs are
9537-458: Is known as a stellar day . Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the Sun's north pole . The exceptions are Venus and Uranus, which rotate clockwise, though Uranus's extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise. Regardless of which convention
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#17328556182849724-704: Is not known why TrES-2b is so dark—it could be due to an unknown chemical compound. For gas giants , geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have
9911-497: Is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets. Kepler-16 contains the first discovered planet that orbits a binary main-sequence star system. On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope . These exoplanets were checked using
10098-421: Is one of the first two instances of clear evidence for a brown dwarf, along with Teide 1 . Confirmed in 1995, both were identified by the presence of the 670.8 nm lithium line. The latter was found to have a temperature and luminosity well below the stellar range. Its near-infrared spectrum clearly exhibited a methane absorption band at 2 micrometres, a feature that had previously only been observed in
10285-437: Is prevented, by electron degeneracy pressure, from reaching the densities and pressures needed. Further gravitational contraction is prevented and the result is a brown dwarf that simply cools off by radiating away its internal thermal energy. Note that, in principle, it is possible for a brown dwarf to slowly accrete mass above the hydrogen burning limit without initiating hydrogen fusion. This could happen via mass transfer in
10472-470: Is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of hydrostatic equilibrium . This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until
10659-415: Is the first M-type brown dwarf discovered, and LP 944-20 , the closest M-type brown dwarf. The defining characteristic of spectral class M, the coolest type in the long-standing classical stellar sequence, is an optical spectrum dominated by absorption bands of titanium(II) oxide (TiO) and vanadium(II) oxide (VO) molecules. However, GD 165 B, the cool companion to the white dwarf GD 165 , had none of
10846-437: Is the largest known detached object , a population that never comes close enough to the Sun to interact with any of the classical planets; the origins of their orbits are still being debated. All nine are similar to terrestrial planets in having a solid surface, but they are made of ice and rock rather than rock and metal. Moreover, all of them are smaller than Mercury, with Pluto being the largest known dwarf planet and Eris being
11033-500: Is the smallest object generally agreed to be a geophysical planet , at about six millionths of Earth's mass, though there are many larger bodies that may not be geophysical planets (e.g. Salacia ). An exoplanet is a planet outside the Solar System. As of 24 July 2024, there are 7,026 confirmed exoplanets in 4,949 planetary systems , with 1007 systems having more than one planet . Known exoplanets range in size from gas giants about twice as large as Jupiter down to just over
11220-421: Is the smallest, at 0.055 Earth masses. The planets of the Solar System can be divided into categories based on their composition. Terrestrials are similar to Earth, with bodies largely composed of rock and metal: Mercury, Venus, Earth, and Mars. Earth is the largest terrestrial planet. Giant planets are significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, and Neptune. They differ from
11407-463: Is through the gravitational collapse of a cold interstellar cloud of gas and dust. As the cloud contracts, it heats due to the Kelvin–Helmholtz mechanism . Early in the process the contracting gas quickly radiates away much of the energy, allowing the collapse to continue. Eventually, the central region becomes sufficiently dense to trap radiation. Consequently, the central temperature and density of
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#173285561828411594-583: Is too massive to be a planet and might be a brown dwarf . Known orbital times for exoplanets vary from less than an hour (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all planets detected so far are within the Milky Way. However, there is evidence that extragalactic planets , exoplanets located in other galaxies, may exist. The nearest exoplanets are located 4.2 light-years (1.3 parsecs ) from Earth and orbit Proxima Centauri ,
11781-454: Is unusual to find exoplanets with sizes between 1.5 and 2 times the radius of the Earth. In January 2020, scientists announced the discovery of TOI 700 d , the first Earth-sized planet in the habitable zone detected by TESS. As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed, several of them being nearly Earth-sized and located in
11968-457: Is used, Uranus has a retrograde rotation relative to its orbit. The rotation of a planet can be induced by several factors during formation. A net angular momentum can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the giant planets contributes to the angular momentum. Finally, during the last stages of planet building, a stochastic process of protoplanetary accretion can randomly alter
12155-405: The International Astronomical Union (IAU) only covers the Solar System and thus does not apply to exoplanets. The IAU Working Group on Extrasolar Planets issued a position statement containing a working definition of "planet" in 2001 and which was modified in 2003. An exoplanet was defined by the following criteria: This working definition was amended by the IAU's Commission F2: Exoplanets and
12342-404: The International Astronomical Union considers an object above 13 M J (the limiting mass for thermonuclear fusion of deuterium) to be a brown dwarf, whereas an object under that mass (and orbiting a star or stellar remnant) is considered a planet. The minimum mass required to trigger sustained hydrogen burning (about 80 M J ) forms the upper limit of the definition. It
12529-534: The Keck ;1 telescope in November 1995 showed that Teide 1 still had the initial lithium abundance of the original molecular cloud from which Pleiades stars formed, proving the lack of thermonuclear fusion in its core. These observations fully confirmed that Teide 1 is a brown dwarf, as well as the efficiency of the spectroscopic lithium test . For some time, Teide 1 was the smallest known object outside
12716-564: The Luhman 16 system, a binary of L- and T-type brown dwarfs about 6.5 light-years (2.0 parsecs ) from the Sun. Luhman 16 is the third closest system to the Sun after Alpha Centauri and Barnard's Star . The objects now called "brown dwarfs" were theorized by Shiv S. Kumar in the 1960s to exist and were originally called black dwarfs , a classification for dark substellar objects floating freely in space that were not massive enough to sustain hydrogen fusion. However, (a) the term black dwarf
12903-450: The Milky Way galaxy . Planets are extremely faint compared to their parent stars. For example, a Sun-like star is about a billion times brighter than the reflected light from any exoplanet orbiting it. It is difficult to detect such a faint light source, and furthermore, the parent star causes a glare that tends to wash it out. It is necessary to block the light from the parent star to reduce
13090-535: The Mount Wilson Observatory , produced a spectrum of the star using Mount Wilson's 60-inch telescope . He interpreted the spectrum to be of an F-type main-sequence star , but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized by the gravity of the star, the resulting dust then falling onto the star. The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards,
13277-569: The Observatoire de Haute-Provence , ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics . Technological advances, most notably in high-resolution spectroscopy , led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing
13464-520: The asteroid belt , located between the orbits of Mars and Jupiter. The other eight all orbit beyond Neptune. Orcus, Pluto, Haumea, Quaoar, and Makemake orbit in the Kuiper belt , which is a second belt of small Solar System bodies beyond the orbit of Neptune. Gonggong and Eris orbit in the scattered disc , which is somewhat further out and, unlike the Kuiper belt, is unstable towards interactions with Neptune. Sedna
13651-454: The climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its solstice . Each planet has two in the course of its orbit; when one hemisphere has its summer solstice with its day being the longest, the other has its winter solstice when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of
13838-540: The radial-velocity method . Despite this, several tens of planets around red dwarfs have been discovered by the Kepler space telescope , which uses the transit method to detect smaller planets. Using data from Kepler , a correlation has been found between the metallicity of a star and the probability that the star hosts a giant planet, similar to the size of Jupiter . Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity. Some planets orbit one member of
14025-450: The sin i ambiguity ." The NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses. Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core pressure is dominated by Coulomb pressure or electron degeneracy pressure with the dividing line at around 5 Jupiter masses. The convention for naming exoplanets
14212-485: The 13-Jupiter-mass cutoff does not have a precise physical significance. Deuterium fusion can occur in some objects with a mass below that cutoff. The amount of deuterium fused depends to some extent on the composition of the object. As of 2011, the Extrasolar Planets Encyclopaedia included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around 13 M Jup in
14399-489: The 80 cm telescope (IAC 80) at Teide Observatory , and its spectrum was first recorded in December 1994 using the 4.2 m William Herschel Telescope at Roque de los Muchachos Observatory (La Palma). The distance, chemical composition, and age of Teide 1 could be established because of its membership in the young Pleiades star cluster. Using the most advanced stellar and substellar evolution models at that moment,
14586-415: The Kuiper belt, particularly Eris , spurred debate about how exactly to define a planet. In 2006, the International Astronomical Union (IAU) adopted a definition of a planet in the Solar System, placing the four terrestrial planets and the four giant planets in the planet category; Ceres, Pluto, and Eris are in the category of dwarf planet . Many planetary scientists have nonetheless continued to apply
14773-610: The Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. After the fall of the Western Roman Empire , astronomy developed further in India and the medieval Islamic world. In 499 CE, the Indian astronomer Aryabhata propounded a planetary model that explicitly incorporated Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of
14960-588: The Moon. The smallest object in the Solar System generally agreed to be a geophysical planet is Saturn's moon Mimas, with a radius about 3.1% of Earth's and a mass about 0.00063% of Earth's. Saturn's smaller moon Phoebe , currently an irregular body of 1.7% Earth's radius and 0.00014% Earth's mass, is thought to have attained hydrostatic equilibrium and differentiation early in its history before being battered out of shape by impacts. Some asteroids may be fragments of protoplanets that began to accrete and differentiate, but suffered catastrophic collisions, leaving only
15147-509: The NIR spectrum of Gliese 229B is dominated by absorption bands from methane (CH 4 ), a feature which in the Solar System is found only in the giant planets and Titan . CH 4 , H 2 O, and molecular hydrogen (H 2 ) collision-induced absorption (CIA) give Gliese 229B blue near-infrared colors. Its steeply sloped red optical spectrum also lacks the FeH and CrH bands that characterize L dwarfs and instead
15334-522: The Solar System in August 2018. The official working definition of an exoplanet is now as follows: The IAU's working definition is not always used. One alternate suggestion is that planets should be distinguished from brown dwarfs on the basis of their formation. It is widely thought that giant planets form through core accretion , which may sometimes produce planets with masses above the deuterium fusion threshold; massive planets of that sort may have already been observed. Brown dwarfs form like stars from
15521-452: The Solar System is to the Sun. Mercury, the closest planet to the Sun at 0.4 AU , takes 88 days for an orbit, but ultra-short period planets can orbit in less than a day. The Kepler-11 system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury. There are hot Jupiters , such as 51 Pegasi b, that orbit very close to their star and may evaporate to become chthonian planets , which are
15708-404: The Solar System planets except Mercury have substantial atmospheres because their gravity is strong enough to keep gases close to the surface. Saturn's largest moon Titan also has a substantial atmosphere thicker than that of Earth; Neptune's largest moon Triton and the dwarf planet Pluto have more tenuous atmospheres. The larger giant planets are massive enough to keep large amounts of
15895-420: The Solar System that had been identified by direct observation. Since then, over 1,800 brown dwarfs have been identified, even some very close to Earth, like Epsilon Indi Ba and Bb, a pair of brown dwarfs gravitationally bound to a Sun-like star 12 light-years from the Sun, and Luhman 16, a binary system of brown dwarfs at 6.5 light-years from the Sun. The standard mechanism for star birth
16082-437: The Solar System, but in multitudes of other extrasolar systems. The consensus as to what counts as a planet, as opposed to other objects, has changed several times. It previously encompassed asteroids , moons , and dwarf planets like Pluto , and there continues to be some disagreement today. The five classical planets of the Solar System , being visible to the naked eye, have been known since ancient times and have had
16269-489: The Solar System, whereas others are commonly observed in exoplanets. In the Solar System, all the planets orbit the Sun in the same direction as the Sun rotates : counter-clockwise as seen from above the Sun's north pole. At least one exoplanet, WASP-17b , has been found to orbit in the opposite direction to its star's rotation. The period of one revolution of a planet's orbit is known as its sidereal period or year . A planet's year depends on its distance from its star;
16456-445: The Sun and are likewise accompanied by planets. In the eighteenth century, the same possibility was mentioned by Isaac Newton in the " General Scholium " that concludes his Principia . Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One ." In 1938, D.Belorizky demonstrated that it
16643-416: The Sun, Jupiter and Saturn are both made primarily of hydrogen and helium. Saturn is nearly as large as Jupiter, despite having only 30% the mass. Three of the giant planets in the Solar System (Jupiter, Saturn, and Neptune ) emit much more (up to about twice) heat than they receive from the Sun. All four giant planets have their own "planetary" systems, in the form of extensive moon systems. Currently,
16830-802: The Two Micron All Sky Survey ( 2MASS ), the Deep Near Infrared Survey of the Southern Sky (DENIS), and the Sloan Digital Sky Survey (SDSS). This spectral class also contains the coolest main-sequence stars (> 80 M J ), which have spectral classes L2 to L6. As GD 165B is the prototype of the L dwarfs, Gliese 229 B is the prototype of a second new spectral class, the T dwarfs . T dwarfs are pinkish-magenta. Whereas near-infrared (NIR) spectra of L dwarfs show strong absorption bands of H 2 O and carbon monoxide (CO),
17017-468: The advent of the Two Micron All-Sky Survey ( 2MASS ) in 1997, which discovered many objects with similar colors and spectral features. Today, GD 165B is recognized as the prototype of a class of objects now called "L dwarfs". Although the discovery of the coolest dwarf was highly significant at the time, it was debated whether GD 165B would be classified as a brown dwarf or simply
17204-475: The advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity ." The NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses. Objects below 13 M J , called sub-brown dwarfs or planetary-mass brown dwarfs , form in
17391-428: The atmospheres of giant planets and that of Saturn 's moon Titan . Methane absorption is not expected at any temperature of a main-sequence star. This discovery helped to establish yet another spectral class even cooler than L dwarfs, known as " T dwarfs", for which Gliese 229B is the prototype. The first confirmed class "M" brown dwarf was discovered by Spanish astrophysicists Rafael Rebolo (head of
17578-480: The atmospheric dynamics that affect the day-night temperature difference are complex. One important characteristic of the planets is their intrinsic magnetic moments , which in turn give rise to magnetospheres. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of electrically conducting material in their interiors, which generate their magnetic fields. These fields significantly change
17765-504: The atmospheric opacity and thus the radiative cooling rate. As of 2011 the Extrasolar Planets Encyclopaedia included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around 13 M Jup in the observed mass spectrum reinforces the choice to forget this mass limit". As of 2016, this limit was increased to 60 Jupiter masses, based on a study of mass–density relationships. The Exoplanet Data Explorer includes objects up to 24 Jupiter masses with
17952-405: The brown dwarf is young and still has spectral features that are associated with youth, or the brown dwarf co-moves with a star or stellar group ( star cluster or association ), where age estimates are easier to obtain. A very young brown dwarf that was further studied with this method is 2M1207 and the companion 2M1207b . Based on the location, proper motion and spectral signature, this object
18139-408: The closest star to the Sun. The discovery of exoplanets has intensified interest in the search for extraterrestrial life . There is special interest in planets that orbit in a star's habitable zone (sometimes called "goldilocks zone"), where it is possible for liquid water, a prerequisite for life as we know it, to exist on the surface. However, the study of planetary habitability also considers
18326-559: The collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk . Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets . After
18513-410: The collapsed cloud increase dramatically with time, slowing the contraction, until the conditions are hot and dense enough for thermonuclear reactions to occur in the core of the protostar . For a typical star, gas and radiation pressure generated by the thermonuclear fusion reactions within its core will support it against any further gravitational contraction. Hydrostatic equilibrium is reached, and
18700-454: The color of light they emit but from their falling between white dwarf stars and "dark" planets in size. To the naked eye, brown dwarfs would appear in different colors depending on their temperature. The warmest ones are possibly orange or red, while cooler brown dwarfs would likely appear magenta or black to the human eye. Brown dwarfs may be fully convective , with no layers or chemical differentiation by depth. Though their existence
18887-464: The constellation Virgo. This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called small planet radius gap . The gap, sometimes called the Fulton gap, is the observation that it
19074-406: The definition, regarding where exactly to draw the line between a planet and a star. Multiple exoplanets have been found to orbit in the habitable zones of their stars (where liquid water can potentially exist on a planetary surface ), but Earth remains the only planet known to support life . It is not known with certainty how planets are formed. The prevailing theory is that they coalesce during
19261-421: The density reaches the point where electrons become closely packed enough to create quantum electron degeneracy pressure . According to the brown dwarf interior models, typical conditions in the core for density, temperature and pressure are expected to be the following: This means that the protostar is not massive or dense enough ever to reach the conditions needed to sustain hydrogen fusion. The infalling matter
19448-434: The detection of lithium in the atmosphere of an object older than 100 Myr ensures that it is a brown dwarf. The first class "T" brown dwarf was discovered in 1994 by Caltech astronomers Shrinivas Kulkarni , Tadashi Nakajima, Keith Matthews and Rebecca Oppenheimer , and Johns Hopkins scientists Samuel T. Durrance and David Golimowski. It was confirmed in 1995 as a substellar companion to Gliese 229 . Gliese 229b
19635-423: The development of the telescope , the meaning of planet broadened to include objects only visible with assistance: the moons of the planets beyond Earth; the ice giants Uranus and Neptune; Ceres and other bodies later recognized to be part of the asteroid belt ; and Pluto , later found to be the largest member of the collection of icy bodies known as the Kuiper belt . The discovery of other large objects in
19822-426: The direct gravitational collapse of clouds of gas, and this formation mechanism also produces objects that are below the 13 M Jup limit and can be as low as 1 M Jup . Objects in this mass range that orbit their stars with wide separations of hundreds or thousands of Astronomical Units (AU) and have large star/object mass ratios likely formed as brown dwarfs; their atmospheres would likely have
20009-473: The eight planets in the Solar System, only Venus and Mars lack such a magnetic field. Of the magnetized planets, the magnetic field of Mercury is the weakest and is barely able to deflect the solar wind . Jupiter's moon Ganymede has a magnetic field several times stronger, and Jupiter's is the strongest in the Solar System (so intense in fact that it poses a serious health risk to future crewed missions to all its moons inward of Callisto ). The magnetic fields of
20196-579: The existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars. However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable. During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star . Astronomers now generally regard all early reports of detection as erroneous. In 1991, Andrew Lyne , M. Bailes and S. L. Shemar claimed to have discovered
20383-408: The exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion." On 21 March 2022, the 5000th exoplanet beyond the Solar System was confirmed. On 11 January 2023, NASA scientists reported the detection of LHS 475 b , an Earth-like exoplanet – and
20570-441: The exoplanets detected are inside the tidal locking zone. In several cases, multiple planets have been observed around a star. About 1 in 5 Sun-like stars are estimated to have an " Earth -sized" planet in the habitable zone . Assuming there are 200 billion stars in the Milky Way , it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting
20757-418: The farther a planet is from its star, the longer the distance it must travel and the slower its speed, since it is less affected by its star's gravity . No planet's orbit is perfectly circular, and hence the distance of each from the host star varies over the course of its year. The closest approach to its star is called its periastron , or perihelion in the Solar System, whereas its farthest separation from
20944-412: The first confirmation of detection came in 1992 when Aleksander Wolszczan announced the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12 . The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi . Some exoplanets have been imaged directly by telescopes, but
21131-570: The first exoplanet discovered by the James Webb Space Telescope . This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own. In the sixteenth century, the Italian philosopher Giordano Bruno , an early supporter of the Copernican theory that Earth and other planets orbit the Sun ( heliocentrism ), put forward the view that fixed stars are similar to
21318-522: The formation of dynamic weather systems such as hurricanes (on Earth), planet-wide dust storms (on Mars), a greater-than-Earth-sized anticyclone on Jupiter (called the Great Red Spot ), and holes in the atmosphere (on Neptune). Weather patterns detected on exoplanets include a hot region on HD 189733 b twice the size of the Great Red Spot, as well as clouds on the hot Jupiter Kepler-7b ,
21505-626: The giant planets have numerous moons in complex planetary-type systems. Except for Ceres and Sedna, all the consensus dwarf planets are known to have at least one moon as well. Many moons of the giant planets have features similar to those on the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa and Enceladus). The four giant planets are orbited by planetary rings of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny ' moonlets ' whose gravity shapes and maintains their structure. Although
21692-467: The giant planets the mantle simply blends into the upper cloud layers. The terrestrial planets have cores of elements such as iron and nickel and mantles of silicates . Jupiter and Saturn are believed to have cores of rock and metal surrounded by mantles of metallic hydrogen . Uranus and Neptune, which are smaller, have rocky cores surrounded by mantles of water, ammonia , methane , and other ices . The fluid action within these planets' cores creates
21879-430: The glare while leaving the light from the planet detectable; doing so is a major technical challenge which requires extreme optothermal stability . All exoplanets that have been directly imaged are both large (more massive than Jupiter ) and widely separated from their parent stars. Specially designed direct-imaging instruments such as Gemini Planet Imager , VLT-SPHERE , and SCExAO will image dozens of gas giants, but
22066-458: The gradual accumulation of material driven by gravity , a process called accretion . The word planet comes from the Greek πλανήται ([[[wikt:πλανήτης|planḗtai]]] Error: {{Transliteration}}: transliteration text not Latin script ( help ) ) ' wanderers ' . In antiquity , this word referred to the Sun , Moon , and five points of light visible to the naked eye that moved across the background of
22253-417: The grounds that the internal physics of objects does not change between approximately one Saturn mass (beginning of significant self-compression) and the onset of hydrogen burning and becoming a red dwarf star. Beyond roughly 13 M J (at least for objects with solar-type isotopic abundance ), an object achieves conditions suitable for nuclear fusion of deuterium : this has sometimes been advocated as
22440-554: The habitable zone, some around Sun-like stars. In September 2020, astronomers reported evidence, for the first time, of an extragalactic planet , M51-ULS-1b , detected by eclipsing a bright X-ray source (XRS), in the Whirlpool Galaxy (M51a). Also in September 2020, astronomers using microlensing techniques reported the detection , for the first time, of an Earth-mass rogue planet unbounded by any star, and free floating in
22627-486: The hallmark TiO features of M dwarfs. The subsequent identification of many objects like GD 165B ultimately led to the definition of a new spectral class , the L dwarfs , defined in the red optical region of the spectrum not by metal-oxide absorption bands (TiO, VO), but by metal hydride emission bands ( FeH , CrH , MgH , CaH ) and prominent atomic lines of alkali metals (Na, K, Rb, Cs). As of 2013 , over 900 L dwarfs had been identified, most by wide-field surveys:
22814-560: The high end of their mass range can be hot enough to deplete their lithium when they are young. Dwarfs of mass greater than 65 M J can burn their lithium by the time they are half a billion years old; thus the lithium test is not perfect. Unlike stars, older brown dwarfs are sometimes cool enough that, over very long periods of time, their atmospheres can gather observable quantities of methane , which cannot form in hotter objects. Dwarfs confirmed in this fashion include Gliese 229 B. Main-sequence stars cool, but eventually reach
23001-784: The impact of dust formation in the cool outer atmospheres of brown dwarfs in the late 1980s brought these theories into question. However, such objects were hard to find because they emit almost no visible light. Their strongest emissions are in the infrared (IR) spectrum, and ground-based IR detectors were too imprecise at that time to readily identify any brown dwarfs. Since then, numerous searches by various methods have sought these objects. These methods included multi-color imaging surveys around field stars, imaging surveys for faint companions of main-sequence dwarfs and white dwarfs , surveys of young star clusters , and radial velocity monitoring for close companions. For many years, efforts to discover brown dwarfs were fruitless. In 1988, however,
23188-416: The interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called the magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which cannot effectively protect the planet. Of
23375-520: The leftover cores. There are also exoplanets that are much farther from their star. Neptune is 30 AU from the Sun and takes 165 years to orbit, but there are exoplanets that are thousands of AU from their star and take more than a million years to orbit (e.g. COCONUTS-2b ). Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in
23562-447: The light gases hydrogen and helium, whereas the smaller planets lose these gases into space . Analysis of exoplanets suggests that the threshold for being able to hold on to these light gases occurs at about 2.0 +0.7 −0.6 M E , so that Earth and Venus are near the maximum size for rocky planets. The composition of Earth's atmosphere is different from the other planets because the various life processes that have transpired on
23749-501: The likelihood that a star will have planets. Hence, a metal-rich population I star is more likely to have a substantial planetary system than a metal-poor, population II star . According to the IAU definition , there are eight planets in the Solar System, which are (in increasing distance from the Sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Jupiter is the largest, at 318 Earth masses , whereas Mercury
23936-438: The lowest-mass young objects known, like PSO J318.5−22 , are thought to have masses below 13 M J , and as a result are sometimes referred to as planetary-mass objects due to the ambiguity of whether they should be regarded as rogue planets or brown dwarfs. There are planetary-mass objects known to orbit brown dwarfs, such as 2M1207b , MOA-2007-BLG-192Lb , 2MASS J044144b and Oph 98 B. The 13-Jupiter-mass cutoff
24123-480: The magnetosphere of an orbiting hot Jupiter. Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in resonance with each other or with smaller bodies. This is common in satellite systems (e.g. the resonance between Io, Europa , and Ganymede around Jupiter, or between Enceladus and Dione around Saturn). All except Mercury and Venus have natural satellites , often called "moons". Earth has one, Mars has two, and
24310-421: The mass of the third object that was too small for it to be a star. The conclusion that the third object was a planet was announced by Stephen Thorsett and his collaborators in 1993. On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi . This discovery, made at
24497-432: The mass range (over 60 M J ) cool quickly enough that after 10 million years they no longer undergo fusion . X-ray and infrared spectra are telltale signs of brown dwarfs. Some emit X-rays ; and all "warm" dwarfs continue to glow tellingly in the red and infrared spectra until they cool to planet-like temperatures (under 1,000 K). Gas giants have some of the characteristics of brown dwarfs. Like
24684-470: The mass to come to a conclusion. The theory behind the mass estimate is that brown dwarfs with a similar mass form in a similar way and are hot when they form. Some have spectral types that are similar to low-mass stars, such as 2M1101AB . As they cool down the brown dwarfs should retain a range of luminosities depending on the mass. Without the age and luminosity, a mass estimate is difficult; for example, an L-type brown dwarf could be an old brown dwarf with
24871-582: The most massive. There are at least nineteen planetary-mass moons or satellite planets—moons large enough to take on ellipsoidal shapes: The Moon, Io, and Europa have compositions similar to the terrestrial planets; the others are made of ice and rock like the dwarf planets, with Tethys being made of almost pure ice. Europa is often considered an icy planet, though, because its surface ice layer makes it difficult to study its interior. Ganymede and Titan are larger than Mercury by radius, and Callisto almost equals it, but all three are much less massive. Mimas
25058-421: The most restrictive definition of the term: the terrestrial planets Mercury , Venus , Earth , and Mars , and the giant planets Jupiter , Saturn , Uranus , and Neptune . The best available theory of planet formation is the nebular hypothesis , which posits that an interstellar cloud collapses out of a nebula to create a young protostar orbited by a protoplanetary disk . Planets grow in this disk by
25245-557: The nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi . In 1855, William Stephen Jacob at the East India Company 's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system. In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved
25432-485: The number of intelligent, communicating civilizations that exist in the Milky Way. There are types of planets that do not exist in the Solar System: super-Earths and mini-Neptunes , which have masses between that of Earth and Neptune. Objects less than about twice the mass of Earth are expected to be rocky like Earth; beyond that, they become a mixture of volatiles and gas like Neptune. The planet Gliese 581c , with
25619-516: The numerous red dwarfs are included. The least massive exoplanet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon . The most massive exoplanet listed on the NASA Exoplanet Archive is HR 2562 b , about 30 times the mass of Jupiter . However, according to some definitions of a planet (based on the nuclear fusion of deuterium ), it
25806-414: The object collapses into a sphere. Mass is the prime attribute by which planets are distinguished from stars. No objects between the masses of the Sun and Jupiter exist in the Solar System, but there are exoplanets of this size. The lower stellar mass limit is estimated to be around 75 to 80 times that of Jupiter ( M J ). Some authors advocate that this be used as the upper limit for planethood, on
25993-425: The observed mass spectrum reinforces the choice to forget this mass limit". As of 2016, this limit was increased to 60 Jupiter masses based on a study of mass–density relationships. The Exoplanet Data Explorer includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to
26180-401: The origins of planetary rings are not precisely known, they are believed to be the result of natural satellites that fell below their parent planets' Roche limits and were torn apart by tidal forces . The dwarf planets Haumea and Quaoar also have rings. No secondary characteristics have been observed around exoplanets. The sub-brown dwarf Cha 110913−773444 , which has been described as
26367-486: The other giant planets, measured at their surfaces, are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative to the planets' rotational axes and displaced from the planets' centres. In 2003, a team of astronomers in Hawaii observing the star HD 179949 detected a bright spot on its surface, apparently created by
26554-761: The other in perpetual night. Mercury and Venus, the closest planets to the Sun, similarly exhibit very slow rotation: Mercury is tidally locked into a 3:2 spin–orbit resonance (rotating three times for every two revolutions around the Sun), and Venus's rotation may be in equilibrium between tidal forces slowing it down and atmospheric tides created by solar heating speeding it up. All the large moons are tidally locked to their parent planets; Pluto and Charon are tidally locked to each other, as are Eris and Dysnomia, and probably Orcus and its moon Vanth . The other dwarf planets with known rotation periods rotate faster than Earth; Haumea rotates so fast that it has been distorted into
26741-420: The planet have introduced free molecular oxygen . The atmospheres of Mars and Venus are both dominated by carbon dioxide , but differ drastically in density: the average surface pressure of Mars's atmosphere is less than 1% that of Earth's (too low to allow liquid water to exist), while the average surface pressure of Venus's atmosphere is about 92 times that of Earth's. It is likely that Venus's atmosphere
26928-404: The planet's existence to be confirmed. On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12 . This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived
27115-597: The planet. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either continually in sunlight or continually in darkness around the time of its solstices . In the Solar System, Mercury, Venus, Ceres, and Jupiter have very small tilts; Pallas, Uranus, and Pluto have extreme ones; and Earth, Mars, Vesta, Saturn, and Neptune have moderate ones. Among exoplanets, axial tilts are not known for certain, though most hot Jupiters are believed to have
27302-572: The planets as the Babylonians. In the 6th and 5th centuries BC, the Pythagoreans appear to have developed their own independent planetary theory , which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe. Pythagoras or Parmenides is said to have been the first to identify the evening star ( Hesperos ) and morning star ( Phosphoros ) as one and
27489-475: The positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness and account for most of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the Almagest written by Ptolemy in the 2nd century CE. So complete
27676-413: The range of possible masses. Moreover, the mass–radius relationship shows no change from about one Saturn mass to the onset of hydrogen burning ( 0.080 ± 0.008 M ☉ ), suggesting that from this perspective brown dwarfs are simply high-mass Jovian planets. This can make distinguishing them from planets difficult. In addition, many brown dwarfs undergo no fusion; even those at the high end of
27863-736: The same ( Aphrodite , Greek corresponding to Latin Venus ), though this had long been known in Mesopotamia. In the 3rd century BC, Aristarchus of Samos proposed a heliocentric system, according to which Earth and the planets revolved around the Sun. The geocentric system remained dominant until the Scientific Revolution . By the 1st century BC, during the Hellenistic period , the Greeks had begun to develop their own mathematical schemes for predicting
28050-412: The same manner as stars and brown dwarfs (i.e. through the collapse of a gas cloud ) but have a mass below the limiting mass for thermonuclear fusion of deuterium . Some researchers call them free-floating planets, whereas others call them planetary-mass brown dwarfs. While spectroscopic features can help to distinguish between low-mass stars and brown dwarfs, it is often necessary to estimate
28237-407: The same radius as Jupiter. At the high end of their mass range ( 60–90 M J ), the volume of a brown dwarf is governed primarily by electron-degeneracy pressure, as it is in white dwarfs; at the low end of the range ( 10 M J ), their volume is governed primarily by Coulomb pressure , as it is in planets. The net result is that the radii of brown dwarfs vary by only 10–15% over
28424-463: The second millennium BC. The MUL.APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun, Moon, and planets over the course of the year. Late Babylonian astronomy is the origin of Western astronomy and indeed all Western efforts in the exact sciences . The Enuma anu enlil , written during the Neo-Assyrian period in the 7th century BC, comprises
28611-421: The size of Neptune and smaller, down to smaller than Mercury. In 2011, the Kepler space telescope team reported the discovery of the first Earth-sized exoplanets orbiting a Sun-like star , Kepler-20e and Kepler-20f . Since that time, more than 100 planets have been identified that are approximately the same size as Earth , 20 of which orbit in the habitable zone of their star—the range of orbits where
28798-450: The size of the Moon . Analysis of gravitational microlensing data suggests a minimum average of 1.6 bound planets for every star in the Milky Way . In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12 . This discovery was confirmed and is generally considered to be the first definitive detection of exoplanets. Researchers suspect they formed from
28985-413: The smaller planetesimals (as well as radioactive decay ) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by density, with higher density materials sinking toward the core . Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from
29172-470: The spin axis of the planet. There is great variation in the length of day between the planets, with Venus taking 243 days to rotate, and the giant planets only a few hours. The rotational periods of exoplanets are not known, but for hot Jupiters , their proximity to their stars means that they are tidally locked (that is, their orbits are in sync with their rotations). This means, they always show one face to their stars, with one side in perpetual day,
29359-476: The star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with a high albedo that is far from the star. The darkest known planet in terms of geometric albedo is TrES-2b , a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres, but it
29546-400: The star is called its apastron ( aphelion ). As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy , just as a falling object on Earth accelerates as it falls. As the planet nears apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory . Each planet's orbit
29733-523: The star or each other, but over time many will collide, either to form a larger, combined protoplanet or release material for other protoplanets to absorb. Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies . The energetic impacts of
29920-428: The star will spend most of its lifetime fusing hydrogen into helium as a main-sequence star. If, however, the initial mass of the protostar is less than about 0.08 M ☉ , normal hydrogen thermonuclear fusion reactions will not ignite in the core. Gravitational contraction does not heat the small protostar very effectively, and before the temperature in the core can increase enough to trigger fusion,
30107-694: The stars. He also theorized that the orbits of planets were elliptical . Aryabhata's followers were particularly strong in South India , where his principles of the diurnal rotation of Earth, among others, were followed and a number of secondary works were based on them. Brown dwarf Astronomers classify self-luminous objects by spectral type , a distinction intimately tied to the surface temperature, and brown dwarfs occupy types M, L, T, and Y. As brown dwarfs do not undergo stable hydrogen fusion, they cool down over time, progressively passing through later spectral types as they age. Their name comes not from
30294-426: The stars—namely, Mercury, Venus, Mars, Jupiter, and Saturn. Planets have historically had religious associations: multiple cultures identified celestial bodies with gods, and these connections with mythology and folklore persist in the schemes for naming newly discovered Solar System bodies. Earth itself was recognized as a planet when heliocentrism supplanted geocentrism during the 16th and 17th centuries. With
30481-460: The subsequent impact of comets (smaller planets will lose any atmosphere they gain through various escape mechanisms ). With the discovery and observation of planetary systems around stars other than the Sun, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity —an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 ( helium )—appears to determine
30668-412: The super-Earth Gliese 1214 b , and others. Hot Jupiters, due to their extreme proximities to their host stars, have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets. These planets may have vast differences in temperature between their day and night sides that produce supersonic winds, although multiple factors are involved and the details of
30855-523: The team estimated for Teide 1 a mass of 55 ± 15 M J , which is below the stellar-mass limit. The object became a reference in subsequent young brown dwarf related works. In theory, a brown dwarf below 65 M J is unable to burn lithium by thermonuclear fusion at any time during its evolution. This fact is one of the lithium test principles used to judge the substellar nature of low-luminosity and low-surface-temperature astronomical bodies. High-quality spectral data acquired by
31042-579: The team), María Rosa Zapatero-Osorio, and Eduardo L. Martín in 1994. This object, found in the Pleiades open cluster, received the name Teide 1 . The discovery article was submitted to Nature in May 1995, and published on 14 September 1995. Nature highlighted "Brown dwarfs discovered, official" on the front page of that issue. Teide 1 was discovered in images collected by the IAC team on 6 January 1994 using
31229-550: The term planet more broadly, including dwarf planets as well as rounded satellites like the Moon. Further advances in astronomy led to the discovery of over five thousand planets outside the Solar System, termed exoplanets . These often show unusual features that the Solar System planets do not show, such as hot Jupiters —giant planets that orbit close to their parent stars, like 51 Pegasi b —and extremely eccentric orbits , such as HD 20782 b . The discovery of brown dwarfs and planets larger than Jupiter also spurred debate on
31416-433: The terrestrial planets in composition. The gas giants , Jupiter and Saturn, are primarily composed of hydrogen and helium and are the most massive planets in the Solar System. Saturn is one third as massive as Jupiter, at 95 Earth masses. The ice giants , Uranus and Neptune, are primarily composed of low-boiling-point materials such as water, methane , and ammonia , with thick atmospheres of hydrogen and helium. They have
31603-458: The time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs , objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei, but subsequent work in 1992 again raised serious doubts. Finally, in 2003, improved techniques allowed
31790-404: The topic in the popular press. These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits. As pulsars are aggressive stars, it was considered unlikely at the time that
31977-585: The upper atmosphere, and the cloud layer blocks the view to lower layers still containing FeH. The later strengthening of this chemical compound at cooler temperatures of mid- to late T-dwarfs is explained by disturbed clouds that allows a telescope to look into the deeper layers of the atmosphere that still contains FeH. Young L/T-dwarfs (L2-T4) show high variability , which could be explained with clouds, hot spots, magnetically driven aurorae or thermochemical instabilities. The clouds of these brown dwarfs are explained as either iron clouds with varying thickness or
32164-439: The variation in a star's apparent luminosity as an orbiting planet transited in front of it. Initially, the most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these " hot Jupiters ", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it
32351-510: The vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method . In February 2018, researchers using the Chandra X-ray Observatory , combined with a planet detection technique called microlensing , found evidence of planets in a distant galaxy, stating, "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of
32538-551: The vast majority of known extrasolar planets have only been detected through indirect methods. Planets may form within a few to tens (or more) of millions of years of their star forming. The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming to planetary systems of over 10 Gyr old. When planets form in
32725-414: The whole volume of the star is eventually depleted. Therefore, the presence of the lithium spectral line in a candidate brown dwarf is a strong indicator that it is indeed a substellar object. The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the lithium test , and was pioneered by Rafael Rebolo , Eduardo Martín and Antonio Magazzu . However, lithium
32912-419: Was already in use to refer to a cold white dwarf ; (b) red dwarfs fuse hydrogen; and (c) these objects may be luminous at visible wavelengths early in their lives. Because of this, alternative names for these objects were proposed, including planetar and substar . In 1975, Jill Tarter suggested the term "brown dwarf", using "brown" as an approximate color. The term "black dwarf" still refers to
33099-413: Was derived. In ancient Greece , China , Babylon , and indeed all pre-modern civilizations, it was almost universally believed that Earth was the center of the Universe and that all the "planets" circled Earth. The reasons for this perception were that stars and planets appeared to revolve around Earth each day and the apparently common-sense perceptions that Earth was solid and stable and that it
33286-403: Was determined to belong to the ~8-million-year-old TW Hydrae association , and the mass of the secondary was determined to be 8 ± 2 M J , below the deuterium burning limit. An example of a very old age obtained by the co-movement method is the brown dwarf + white dwarf binary COCONUTS-1, with the white dwarf estimated to be 7.3 +2.8 −1.6 billion years old. In this case the mass
33473-416: Was found in 1992 in orbit around a pulsar . Its mass is roughly half that of the planet Mercury. Even smaller is WD 1145+017 b , orbiting a white dwarf; its mass is roughly that of the dwarf planet Haumea, and it is typically termed a minor planet. The smallest known planet orbiting a main-sequence star other than the Sun is Kepler-37b , with a mass (and radius) that is probably slightly higher than that of
33660-443: Was initially theorized in the 1960s, it was not until the mid-1990s that the first unambiguous brown dwarfs were discovered. As brown dwarfs have relatively low surface temperatures, they are not very bright at visible wavelengths, emitting most of their light in the infrared . However, with the advent of more capable infrared detecting devices, thousands of brown dwarfs have been identified. The nearest known brown dwarfs are located in
33847-467: Was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia . Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei . Partly because the observations were at the very limits of instrumental capabilities at
34034-417: Was no way of knowing whether they were real in fact, how common they were, or how similar they might be to the planets of the Solar System . Various detection claims made in the nineteenth century were rejected by astronomers. The first evidence of a possible exoplanet, orbiting Van Maanen 2 , was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams , who later became director of
34221-508: Was not estimated with the derived age, but the co-movement provided an accurate distance estimate, using Gaia parallax . Using this measurement the authors estimated the radius, which was then used to estimate the mass for the brown dwarf as 15.4 +0.9 −0.8 M J . These are brown dwarfs with a spectral class of M5.5 or later; they are also called late-M dwarfs. Some scientists regard them as red dwarfs . All brown dwarfs with spectral type M are young objects, such as Teide 1 , which
34408-569: Was not moving but at rest. The first civilization known to have a functional theory of the planets were the Babylonians , who lived in Mesopotamia in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa , a 7th-century BC copy of a list of observations of the motions of the planet Venus, that probably dates as early as
34595-467: Was realistic to search for exo-Jupiters by using transit photometry . In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason that planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits. Claims of exoplanet detections have been made since
34782-508: Was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries. To the Greeks and Romans, there were seven known planets, each presumed to be circling Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order and using modern names):
34969-458: Was the result of a runaway greenhouse effect in its history, which today makes it the hottest planet by surface temperature, hotter even than Mercury. Despite hostile surface conditions, temperature, and pressure at about 50–55 km altitude in Venus's atmosphere are close to Earthlike conditions (the only place in the Solar System beyond Earth where this is so), and this region has been suggested as
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