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43-464: Under a geophysical definition of planet , the small icy worlds of the Solar System qualify as icy planets. These include most of the planetary-mass moons , such as Ganymede , Titan , Europa , Enceladus , and Triton ; and also most of the known dwarf planets , such as Pluto , Orcus , Haumea , Makemake , Quaoar , Sedna , Gonggong , and Eris . In June 2020, NASA scientists reported that it

86-748: A main-sequence star. OGLE-2005-BLG-390Lb's signature was first detected on January 25, 2006, by observations at the Danish 1.54-m telescope at ESO La Silla Observatory in Chile . The telescope was part of a network of telescopes used by the PLANET / RoboNet gravitational microlensing campaign. Much of the follow-up observational data was gathered by a 0.6-m telescope at the Perth Observatory in Western Australia. Gravitational lensing occurs when light from

129-686: A very slight chance that it is a neutron star or black hole (<1% probability). Regardless of the star's classification, its radiant energy output would be significantly less than that of the Sun . It has a mass of 0.22 M ☉ , but an unknown radius. If it is a red dwarf, it would likely have a radius of 0.17 R ☉ . The age is estimated to be around 9.587 billion years old. OGLE-2005-BLG-390Lb orbits its star every 3,500 days (about 10 years) at an average distance of 2.0 to 4.1  AU (300 to 610 million  km ; 190 to 380 million  mi ), or an orbit that would fall between

172-507: A body with a rocky core, uncertainties in the masses of exoplanets, and debate over whether deuterium-fusion or the mechanism of formation is the most appropriate criterion to distinguish a planet from a star. These uncertainties apply equally to the IAU conception of a planet. Both the IAU definition and the geophysical definitions that differ from it consider the shape of the object, with consideration given to hydrostatic equilibrium . Determining

215-435: A distant star is bent and magnified by the gravitational field of a foreground star. A gravitational microlensing event occurs when a planet accompanying this foreground star can cause an additional small increase in the intensity of magnified light as it passes between the background star and the observer as well. The PLANET / RoboNet campaign regularly investigates promising microlensing event alerts that are issued by

258-406: A joint announcement of the discovery. The planet does not appear to meet conditions presumed necessary to support life . The planet is estimated to be about five times Earth's mass (5.5 −2.7 M E ). Some astronomers have speculated that it may have a rocky core like Earth, with a thin atmosphere . Its distance from the star, and the star's relatively low temperature, means that

301-483: A period of about two weeks. During this series of observations, a 15% "spike" in intensity occurred, lasting approximately 12 hours. From the intensity of the increase, and its length, the PLANET astronomers were able to derive the planet's mass, and its approximate displacement from the star. The paper submitted to Nature bears the names of all members of PLANET , RoboNet , OGLE , and MOA . The planet has been given

344-446: A planet and were not interested in debating or discussing how the term "planet" should be defined in geoscience. An early petition rejecting the IAU definition attracted more than 300 signatures, though not all of these critics supported an alternative definition. Other critics took issue with the definition itself and wished to create alternative definitions that could be used in different disciplines. The geophysical definition of

387-478: A planet is have been used by planetary scientists for decades, and continued after the IAU definition was established, and that asteroids have routinely been regarded as "minor" planets, though usage varies considerably. Geophysical definitions have been used to define exoplanets. The 2006 IAU definition purposefully does not address the complication of exoplanets, though in 2003 the IAU declared that "the minimum mass required for an extrasolar object to be considered

430-476: A planet is: a substellar-mass body that has never undergone nuclear fusion and has enough gravitation to be round due to hydrostatic equilibrium, regardless of its orbital parameters. Some variation can be found in how planetary scientists classify borderline objects, such as the asteroids Pallas and Vesta . These two are probably surviving protoplanets , and are larger than some clearly ellipsoidal objects, but currently are not very round (although Vesta likely

473-402: A planet put forth by Stern and Levinson is an alternative to the IAU's definition of what is and is not a planet and is meant to stand as the geophysical definition, while the IAU definition, they argue, is intended more for astronomers. Nonetheless, some geologists favor the IAU's definition. Proponents of Stern and Levinson's geophysical definition have shown that such conceptions of what

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516-459: A planet should be the same as that used in the Solar System". While some geophysical definitions that differ from the IAU definition apply, in theory, to exoplanets and rogue planets , they have not been used in practice, due to ignorance of the geophysical properties of most exoplanets. Geophysical definitions typically exclude objects that have ever undergone nuclear fusion, and so may exclude

559-578: A similar threshold for chemical evolution in the trans-Neptunian region. Such a high threshold suggests that at most nine known trans-Neptunian objects could possibly be geophysical planets: Pluto, Eris , Haumea , Makemake , Gonggong , Charon , Quaoar , Orcus , and Sedna pass the 900-kilometre (560 mi) threshold. The bodies generally agreed to be geophysical planets include the eight major planets: nine dwarf planets that geophysicists generally agree are planets: and nineteen planetary-mass moons : Some other objects are sometimes included at

602-561: Is a super-Earth exoplanet orbiting OGLE-2005-BLG-390L , a star 21,500 ± 3,300 light-years (6,600 ± 1,000 parsecs ) from Earth near the center of the Milky Way , making it one of the most distant planets known. On January 25, 2006, Probing Lensing Anomalies NETwork / Robotic Telescope Network (PLANET/Robonet), Optical Gravitational Lensing Experiment (OGLE), and Microlensing Observations in Astrophysics (MOA) made

645-463: Is already the case with the IAU definition; Mercury is now known to not be in hydrostatic equilibrium, but it is universally considered to be a planet regardless.) In 2019, Grundy et al. argued that trans-Neptunian objects up to 900 to 1,000 kilometres (560 to 620 mi) in diameter (e.g. (55637) 2002 UX 25 and Gǃkúnǁʼhòmdímà ) have never compressed out their internal porosity, and are thus not planetary bodies. In 2023, Emery et al. argued for

688-592: Is also often considered an ice planet due to its surface ice, though its high density indicates that its interior is mostly rocky. The same is true for the scattered-disc objects Sedna , Gonggong and Eris . Ten known exoplanets are likely ice planets, given their compositions, orbits, densities, and host stars. Possible examples of ice planets include OGLE-2005-BLG-390Lb , OGLE-2013-BLG-0341LBb , Gliese 667 C d , Kepler-441b , OGLE-2016-BLG-1195Lb and MOA-2007-BLG-192Lb . Geophysical definition of planet The International Union of Geological Sciences (IUGS)

731-403: Is approximately the threshold of geological activity. However, there are exceptions such as Callisto and Mimas , which have equilibrium shapes (historical in the case of Mimas) but show no signs of past or present endogenous geological activity, and Enceladus , which is geologically active due to tidal heating but is apparently not currently in equilibrium. Some geophysical definitions are

774-748: Is likely that exoplanets with oceans, including some with oceans that may lie beneath a layer of surface ice, may be common in the Milky Way galaxy , based on mathematical modeling studies . Gliese 667 C d , orbiting the trinary star Gliese 667 , is a possible example of ice planet. An ice planet's surface can be composed of water , methane , ammonia , carbon dioxide (known as " dry ice "), carbon monoxide , nitrogen , and other volatiles, depending on its surface temperature. Ice planets would have surface temperatures below 260 K (−13 °C) if composed primarily of water , below 180 K (−93 °C) if primarily composed of CO 2 and ammonia , and below 80 K (−193 °C) if composed primarily of methane. On

817-450: Is no sunlight to use for photosynthesis . Microorganisms can produce nutrients using specific chemicals ( chemosynthesis ) that may provide food and energy for other organisms. Some planets, if conditions are right, may have significant atmospheres and surface liquids like Saturn's moon Titan , which could be habitable for exotic forms of life . Although there are many icy objects in the Solar System , none of them qualify as planets under

860-406: Is notable for its large distance from its star for such a relatively small exoplanet - these planets are challenging to find with other detection methods. Prior to this, "small" exoplanets such as Gliese 876 d , which has an orbital period of less than 2 Earth-days, were detected very close to their stars. OGLE-2005-BLG-390Lb shows a combination of size and orbit that would not make it out of place in

903-407: Is round at 396-kilometre (246 mi) diameter, but rocky Vesta is not at 525-kilometre (326 mi) diameter. Thus they stated that some uncertainty could be tolerated in classifying an object as a world, while its dynamical classification could be simply determined from mass and orbital period. The number of geophysical planets in the Solar System cannot be objectively listed, as it depends on

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946-469: Is the internationally recognized body charged with fostering agreement on nomenclature and classification across geoscientific disciplines. However, they have yet to create a formal definition of the term " planet ". As a result, there are various geophysical definitions in use among professional geophysicists, planetary scientists, and other professionals in the geosciences. Many professionals opt to use one of several of these geophysical definitions instead of

989-512: The IAU definition of planet . However, most planetary-mass moons are ice-rock (e.g. Ganymede , Callisto , Enceladus , Titan , and Triton ) or even primarily ice (e.g. Mimas , Tethys , Dione , Rhea , and Iapetus ) and so qualify as ice planets under geophysical definitions of the term. The largest Kuiper belt objects, such as Pluto , Haumea , Makemake , Charon , Quaoar , and Orcus also qualify as such under geophysical definitions. Europa

1032-618: The Polish OGLE or the Japanese–New Zealand MOA survey. The observation of just such an event led to the discovery of OGLE-2005-BLG-390Lb. OGLE detected the microlensing effect produced by the star OGLE-2005-BLG-390L , and it was the PLANET team's follow-up observations and analysis which uncovered evidence of the planet itself. The PLANET team conducted close observation of the OGLE-2005-BLG-390 microlensing event over

1075-468: The Solar System . "The team has discovered the most Earthlike planet yet", said Michael Turner, assistant director for the mathematical and physical sciences directorate at the National Science Foundation , which supported the work. At the time of discovery, with 5.5 Earth masses, the planet was less massive than the previous candidate for lowest-mass exoplanet around a main-sequence star,

1118-408: The higher-mass objects included in exoplanet catalogs as well as the lower-mass objects. The Extrasolar Planets Encyclopaedia , Exoplanet Data Explorer and NASA Exoplanet Archive all include objects significantly more massive than the theoretical 13- Jupiter mass threshold at which deuterium fusion is believed to be supported, for reasons including: uncertainties in how this limit would apply to

1161-456: The 7.5 Earth mass Gliese 876 d. Since 2013, many Earth-sized or smaller planets around main-sequence stars have been detected by the Kepler spacecraft and others. OGLE-2005-BLG-390L (located in the constellation Scorpius , RA 17:54:19.2, Dec −30°22 ′ 38″, J2000 , 6.6 ± 1.0 kpc distance) is thought to likely be a cool red dwarf (95% probability), or a white dwarf (4% probability), with

1204-481: The IAU: Ceres , Pluto (the dwarf planet with the largest known radius), Eris (the dwarf planet with the largest known mass), Haumea , and Makemake , though the last three have not actually been demonstrated to be dwarf planets. Astronomers normally include these five, as well as four more: Quaoar , Sedna , Orcus , and Gonggong . Many critics of the IAU decision were focused specifically on retaining Pluto as

1247-415: The Solar System suggests that this may not be enough for a rocky body to be a planet. Another, WD 1145+017 b , is only 0.0007 Earth masses, while SDSS J1228+1040 b may be only 0.01 Earth radii in size, well below the upper equilibrium limit for icy bodies in the Solar System. (See List of smallest exoplanets .) OGLE-2005-BLG-390Lb OGLE-2005-BLG-390Lb (known sometimes as Hoth by NASA )

1290-499: The body must: They explain their reasoning by noting that this definition delineates the evolutionary stages and primary features of planets more clearly. Specifically, they claim that the hallmark of planethood is, "the collective behavior of the body's mass to overpower mechanical strength and flow into an equilibrium ellipsoid whose shape is dominated by its own gravity" and that the definition allows for "an early period during which gravity may not yet have fully manifested itself to be

1333-561: The borderlines, such as the asteroids Pallas, Vesta, and Hygiea (larger than Mimas, but Pallas and Vesta are noticeably not round); Neptune's second-largest moon Proteus (larger than Mimas, but still not round); or some other trans-Neptunian objects that might or might not be dwarf planets. An examination of spacecraft imagery suggests that the threshold at which an object is large enough to be rounded by self-gravity (whether due to purely gravitational forces, as with Pluto and Titan , or augmented by tidal heating, as with Io and Europa )

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1376-575: The definition voted on by the International Astronomical Union , the dominant organization for setting planetary nomenclature. Some geoscientists adhere to the formal definition of a planet that was proposed by the International Astronomical Union (IAU) in August 2006. According to IAU definition of planet , a planet is an astronomical body orbiting the Sun that is massive enough to be rounded by its own gravity , and has cleared

1419-495: The dominant force". They subclassified planetary bodies as, Furthermore, there are important dynamical categories: A 2018 encapsulation of the above definition defined all planetary bodies as planets. It was worded for a more general audience, and was intended as an alternative to the IAU definition of a planet . It noted that planetary scientists find a different definition of "planet" to be more useful for their field, just as different fields define "metal" differently. For them,

1462-533: The neighbourhood around its orbit . Another widely accepted geophysical definition of a planet includes that which was put forth by planetary scientists Alan Stern and Harold Levison in 2002. The pair proposed the following rules to determine whether an object in space satisfies the definition for a planetary body. A planetary body is defined as any body in space that satisfies the following testable upper and lower bound criteria on its mass: If isolated from external perturbations (e.g., dynamical and thermal),

1505-401: The orbits of Mars and Jupiter in the Solar System (This range of distances is the range of error in measurement and calculation; it does not represent the planet's orbital eccentricity , as its orbital elements are not known, other than its orbital period). Until this discovery, no small exoplanet had been found farther than 0.15 AU (22 million km; 14 million mi) from

1548-502: The planet's likely surface temperature is around 50  K (−220  °C ; −370  °F ), making it one of the coldest known. If it is a rocky world, this temperature would make it likely that the surface would be made of frozen volatiles , substances which would be liquids or gases on Earth: water, ammonia , methane and nitrogen would all be frozen solid. If it is not a rocky planet, it would more closely resemble an icy gas planet like Uranus , although much smaller. The planet

1591-503: The precise definition as well as detailed knowledge of a number of poorly-observed bodies, and there are some borderline cases. At the time of the IAU definition in 2006, it was thought that the limit at which icy astronomical bodies were likely to be in hydrostatic equilibrium was around 400 kilometres (250 mi) in diameter, suggesting that there were a large number of dwarf planets in the Kuiper belt and scattered disk . However, by 2010 it

1634-481: The roundness of a body requires measurements across multiple chords (and even that is not enough to determine whether it is actually in equilibrium), but exoplanet detection techniques provide only the planet's mass, the ratio of its cross-sectional area to that of the host star, or its relative brightness. One small exoplanet, Kepler-1520b , has a mass of less than 0.02 times that of the Earth, and analogy to objects within

1677-529: The same as the IAU definition, while other geophysical definitions tend to be more or less equivalent to the second clause of the IAU definition of planet. Stern's 2018 definition, but not his 2002 definition, excludes the first clause of the IAU definition (that a planet be in orbit around a star) and the third clause (that a planet has cleared the neighborhood around its orbit). It thus counts dwarf planets and planetary-mass moons as planets. Five bodies are currently recognized as or named as dwarf planets by

1720-416: The surface, ice planets are hostile to life forms like those living on Earth because they are extremely cold. Many ice worlds likely have subsurface oceans , warmed by internal heat or tidal forces from another nearby body. Liquid subsurface water would provide habitable conditions for life , including fish , plankton , and microorganisms . Subsurface plants as we know them could not exist because there

1763-566: Was known that icy moons up to 1,500 kilometres (930 mi) in diameter (e.g. Iapetus ) are not in equilibrium. Iapetus is round, but is too oblate for its current spin: it has an equilibrium shape for a rotation period of 16 hours, not its actual spin of 79 days. This might be because the shape of Iapetus was frozen by formation of a thick crust shortly after its formation, while its rotation continued to slow afterwards due to tidal dissipation , until it became tidally locked . Most geophysical definitions list such bodies anyway. (In fact, this

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1806-545: Was not necessarily in conflict with the dynamical conception of a planet: for them, "planet" is defined dynamically, and is a subset of "world" (which also includes dwarf planets, round moons, and free floaters). However, they pointed out that a taxonomy based on roundness is highly problematic because roundness is very rarely directly observable, is a continuum, and proxying it based on size or mass leads to inconsistencies because planetary material strength depends on temperature, composition, and mixing ratios. For example, icy Mimas

1849-409: Was round in the past). Some definitions include them, while others do not. In 2009, Jean-Luc Margot (who proposed a mathematical criterion for clearing the neighborhood) and Levison suggested that "roundness" should refer to bodies whose gravitational forces exceed their material strength, and that round bodies could be called "worlds". They noted that such a geophysical classification was sound and

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