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108-583: Breakthrough Starshot is a research and engineering project by the Breakthrough Initiatives to develop a proof-of-concept fleet of light sail interstellar probes named Starchip , to be capable of making the journey to the Alpha Centauri star system 4.34 light-years away. It was founded in 2016 by Yuri Milner , Stephen Hawking , and Mark Zuckerberg . A flyby mission has been proposed to Proxima Centauri b , an Earth-sized exoplanet in

216-449: A Sun-like star . With a radius 2.4 times that of Earth, Kepler-22b has been predicted by some to be an ocean planet. Gliese 667 Cc , discovered in 2011 but announced in 2012, is a super-Earth orbiting in the circumstellar habitable zone of Gliese 667 C . It is one of the most Earth-like planets known. Gliese 163 c , discovered in September 2012 in orbit around the red dwarf Gliese 163

324-485: A magnetic sail . Breakthrough Initiatives Breakthrough Initiatives is a science-based program founded in 2015 and funded by Julia and Yuri Milner , also of Breakthrough Prize , to search for extraterrestrial intelligence over a span of at least 10 years. The program is divided into multiple projects. Breakthrough Listen will comprise an effort to search over 1,000,000 stars for artificial radio or laser signals. A parallel project called Breakthrough Message

432-557: A very small centimeter-sized vehicle weighing a few grams. They would be propelled by a square-kilometre array of 10 kW ground-based lasers with a combined output of up to 100 GW. A swarm of about 1000 units would compensate for the losses caused by interstellar dust collisions en route to the target. In a detailed study in 2016, Thiem Hoang and coauthors found that mitigating the collisions with dust , hydrogen , and galactic cosmic rays may not be as severe an engineering problem as first thought, although it will likely limit

540-468: A 1W diode laser level, are planned. A 150 mg atomic battery , powered by plutonium-238 or americium-241 , is planned. A coating, possibly made of beryllium copper , is planned to protect the nanocraft from dust collisions and atomic particle erosion. The light sail is envisioned to be no larger than 4 by 4 meters (13 by 13 feet), possibly of composite graphene -based material. The material would have to be very thin and be able to reflect

648-485: A circumstellar habitable zone centered at 1.34 AU from the Sun, a star with 0.25 times the luminosity of the Sun would have a habitable zone centered at 0.25 {\displaystyle {\sqrt {0.25}}} , or 0.5, the distance from the star, corresponding to a distance of 0.67 AU. Various complicating factors, though, including the individual characteristics of stars themselves, mean that extrasolar extrapolation of

756-441: A dehydrated state temperature between 0.150 K (−273 °C) and 424 K (151 °C). Life on a planetary object orbiting outside HZ might hibernate on the cold side as the planet approaches the apastron where the planet is coolest and become active on approach to the periastron when the planet is sufficiently warm. A 2015 review concluded that the exoplanets Kepler-62f , Kepler-186f and Kepler-442b were likely

864-615: A given period. Like the general circumstellar habitable zone, the continuously habitable zone of a star is divided into a conservative and extended region. In red dwarf systems, gigantic stellar flares which could double a star's brightness in minutes and huge starspots which can cover 20% of the star's surface area, have the potential to strip an otherwise habitable planet of its atmosphere and water. As with more massive stars, though, stellar evolution changes their nature and energy flux, so by about 1.2 billion years of age, red dwarfs generally become sufficiently constant to allow for

972-449: A global debate at high levels of science and politics on the risks and rewards of contacting advanced civilizations". Breakthrough Starshot, announced 12 April 2016, is a US$ 100 million program to develop a proof-of-concept light sail spacecraft fleet capable of making the journey to Alpha Centauri at 20% the speed of light (60,000 km/s or 215 million km/h) taking about 20 years to get there, and about 4 years to notify Earth of

1080-443: A habitable zone in the outer parts of stellar systems may exist during the pre-main-sequence phase of stellar evolution, especially around M-dwarfs, potentially lasting for billion-year timescales. Circumstellar habitable zones change over time with stellar evolution. For example, hot O-type stars, which may remain on the main sequence for fewer than 10 million years, would have rapidly changing habitable zones not conducive to

1188-515: A helium-burning star, important life processes like photosynthesis could only happen around planets where the atmosphere has carbon dioxide, as by the time a solar-mass star becomes a red giant, planetary-mass bodies would have already absorbed much of their free carbon dioxide. Moreover, as Ramirez and Kaltenegger (2016) showed, intense stellar winds would completely remove the atmospheres of such smaller planetary bodies, rendering them uninhabitable anyway. Thus, Titan would not be habitable even after

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1296-467: A high component of greenhouse gas and terrestrial planets much more massive than Earth ( super-Earth class planets), that have retained atmospheres with surface pressures of up to 100 kbar. There are no examples of such objects in the Solar System to study; not enough is known about the nature of atmospheres of these kinds of extrasolar objects, and their position in the habitable zone cannot determine

1404-503: A larger "extended habitable zone" in which a planet like Venus, with stronger greenhouse effects , can have the right temperature for liquid water to exist at the surface. Estimates for the habitable zone within the Solar System range from 0.38 to 10.0 astronomical units , though arriving at these estimates has been challenging for a variety of reasons. Numerous planetary mass objects orbit within, or close to, this range and as such receive sufficient sunlight to raise temperatures above

1512-486: A liquid. Although traditional definitions of the habitable zone assume that carbon dioxide and water vapor are the most important greenhouse gases (as they are on the Earth), a study led by Ramses Ramirez and co-author Lisa Kaltenegger has shown that the size of the habitable zone is greatly increased if prodigious volcanic outgassing of hydrogen is also included along with the carbon dioxide and water vapor. The outer edge in

1620-462: A mass limit. All components must be engineered to endure extreme acceleration , cold, vacuum, and protons. The spacecraft will have to survive collisions with space dust ; Starshot expects each square centimeter of frontal cross-section to collide at high speed with about a thousand particles of size at least 0.1 μm. Focusing a set of lasers totaling one hundred gigawatts onto the solar sail will be difficult due to atmospheric turbulence , so there

1728-524: A mission to Saturn 's moon Enceladus , in search for life in its warm ocean, and in 2018 signed a partnership agreement with NASA for the project. The Breakthrough Initiatives were announced to the public on 20 July 2015, at London's Royal Society by physicist Stephen Hawking . Russian tycoon Yuri Milner created the Initiatives to search for intelligent extraterrestrial life in the Universe and consider

1836-413: A multimillion-dollar astronomical program to develop Earth- and space-based technologies that can find Earth-like planets in our cosmic neighborhood – and try to establish whether they host life. The project aims to identify and characterize Earth-sized, rocky planets around Alpha Centauri and other stars within 20 light years of Earth, in search of oxygen and other "biosignatures." Breakthrough Enceladus

1944-437: A plan for possibly transmitting messages out into space . The announcement included an open letter co-signed by multiple scientists, including Hawking, expressing support for an intensified search for alien radio communications. During the public launch, Hawking said: "In an infinite Universe, there must be other life. There is no bigger question. It is time to commit to finding the answer." The US$ 100 million cash infusion

2052-478: A planet where the orbits of natural satellites would not be disrupted, and at the same time tidal heating from the planet would not cause liquid water to boil away. It has been noted that the current term of 'circumstellar habitable zone' poses confusion as the name suggests that planets within this region will possess a habitable environment. However, surface conditions are dependent on a host of different individual properties of that planet. This misunderstanding

2160-495: A radius estimated at 1.1 Earth, Kepler-186f , discovery announced in April 2014, is the closest yet size to Earth of an exoplanet confirmed by the transit method though its mass remains unknown and its parent star is not a Solar analog. Kapteyn b , discovered in June 2014 is a possible rocky world of about 4.8 Earth masses and about 1.5 Earth radii were found orbiting the habitable zone of

2268-510: A speed 4.6% of the speed of light, which would take at least 90 years to get to Alpha Centauri A. The sail could be configured so that the stellar pressure from Alpha Centauri A brakes and deflects the probe toward Alpha Centauri B, where it would arrive after a few days. The sail would then be slowed again to 0.4% of the speed of light and catapulted towards Proxima Centauri. At that speed it will arrive there after another 46 years – about 140 years after its launch. It could hence be decelerated using

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2376-568: A star with high orbital eccentricity may spend only some of its year in the HZ and experience a large variation in temperature and atmospheric pressure. This would result in dramatic seasonal phase shifts where liquid water may exist only intermittently. It is possible that subsurface habitats could be insulated from such changes and that extremophiles on or near the surface might survive through adaptions such as hibernation ( cryptobiosis ) and/or hyperthermostability . Tardigrades , for example, can survive in

2484-571: A successful arrival. The interstellar journey may include a flyby of Proxima Centauri b , an Earth-sized exoplanet that is in the habitable zone of its host star in the Alpha Centauri system. From a distance of 1 Astronomical Unit (150 million kilometers or 93 million miles), the four cameras on each of the spacecraft could potentially capture an image of high enough quality to resolve surface features. The spacecraft fleet would have 1000 craft, and each craft, named StarChip , would be

2592-506: A very small centimeter-sized craft weighing several grams. They would be propelled by several ground-based lasers of up to 100 gigawatts. Each tiny spacecraft would transmit data back to Earth using a compact on-board laser communications system. Pete Worden is the head of this project. The conceptual principles to enable this interstellar travel project were described in "A Roadmap to Interstellar Flight", by Philip Lubin of UC Santa Barbara . METI president Douglas Vakoch summarized

2700-409: Is a star that resembles the Sun. No solar twin with an exact match as that of the Sun has been found. However, some stars are nearly identical to the Sun and are considered solar twins. An exact solar twin would be a G2V star with a 5,778 K temperature, be 4.6 billion years old, with the correct metallicity and a 0.1% solar luminosity variation. Stars with an age of 4.6 billion years are at

2808-437: Is an astrobiology space probe mission concept to explore the possibility of life on Saturn 's moon, Enceladus . In September 2018, NASA signed a collaboration agreement with Breakthrough to jointly create the mission concept. This mission would be the first privately funded deep space mission. It would study the content of the plumes ejecting from Enceladus's warm ocean through its southern ice crust. Enceladus's ice crust

2916-593: Is an advisor to the Starshot project. In July 2017, scientists announced that precursors to StarChip , called Sprites, were successfully launched and flown through Polar Satellite Launch Vehicle by ISRO from Satish Dhawan Space Centre . 105 Sprites were also flown to the ISS on the KickSat -2 mission that launched on 17 November 2018, from where they were deployed on 18 March 2019. They successfully transmitted data before reentering

3024-430: Is an effort to create a message "representative of humanity and planet Earth". The project Breakthrough Starshot , co-founded with Mark Zuckerberg , aims to send a swarm of probes to the nearest star at about 20% the speed of light . The project Breakthrough Watch aims to identify and characterize Earth-sized, rocky planets around Alpha Centauri and other stars within 20 light years of Earth. Breakthrough plans to send

3132-403: Is coincidental with the red-dwarf habitable zone, it has been suggested that the volcanism caused by tidal heating could cause a "tidal Venus" planet with high temperatures and no hospitable environment for life. Others maintain that circumstellar habitable zones are more common and that it is indeed possible for water to exist on planets orbiting cooler stars. Climate modeling from 2013 supports

3240-482: Is in the habitable zone of its host star. The ultra-light StarChip robotic nanocraft, fitted with light sails , are planned to travel at speeds of 20% and 15% of the speed of light , taking between 20 and 30 years to reach the star system, respectively, and about 4 years to notify Earth of a successful arrival. The conceptual principles to enable practical interstellar travel were described in "A Roadmap to Interstellar Flight", by Philip Lubin of UC Santa Barbara , who

3348-471: Is located 49 light years from Earth. The planet has 6.9 Earth masses and 1.8–2.4 Earth radii, and with its close orbit receives 40 percent more stellar radiation than Earth, leading to surface temperatures of about 60° C . HD 40307 g , a candidate planet tentatively discovered in November 2012, is in the circumstellar habitable zone of HD 40307 . In December 2012, Tau Ceti e and Tau Ceti f were found in

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3456-411: Is projected to mark up the pace of SETI research over the early 2000s rate, and will nearly double the rate NASA was spending on SETI research annually in approximately 1973–1993. Breakthrough Listen is a program to search for intelligent extraterrestrial communications in the Universe. With $ 100 million in funding and thousands of hours of dedicated telescope time on state-of-the-art facilities, it

3564-419: Is reflected in excited reports of 'habitable planets'. Since it is completely unknown whether conditions on these distant HZ worlds could host life, different terminology is needed. Whether a body is in the circumstellar habitable zone of its host star is dependent on the radius of the planet's orbit (for natural satellites, the host planet's orbit), the mass of the body itself, and the radiative flux of

3672-674: Is the most comprehensive search for alien communications to date. The project began in January 2016, and is expected to continue for 10 years. The project uses radio wave observations from the Green Bank Observatory and the Parkes Observatory , and visible light observations from the Automated Planet Finder . Targets for the project include one million nearby stars and the centers of 100 galaxies. All data generated from

3780-490: Is the name used by Breakthrough Initiatives for a very small, centimeter-sized, gram-scale, interstellar spacecraft envisioned for the Breakthrough Starshot program, a proposed mission to propel a fleet of a thousand StarChips on a journey to Alpha Centauri , the nearest star system , about 4.37 light-years from Earth . The journey may include a flyby of Proxima Centauri b , an Earth-sized exoplanet that

3888-415: Is the suggestion to use space-based laser infrastructure. In addition, due to the size of the light sail and distance the light sail will be from the laser at the end of the acceleration, very large coherent combining optics would be required to focus the laser. The diffraction limit of the laser light used sets the minimum diameter of the coherently focused laser beam at the source. For example, to accelerate

3996-515: Is thought to be around two to five kilometers thick, and a probe could use an ice-penetrating radar to constrain its structure. Circumstellar habitable zone The habitable zone is also called the Goldilocks zone , a metaphor , allusion and antonomasia of the children's fairy tale of " Goldilocks and the Three Bears ", in which a little girl chooses from sets of three items, rejecting

4104-528: Is unclear whether such satellites could form in the first place. HD 69830 d , a gas giant with 17 times the mass of Earth, was found in 2006 orbiting within the circumstellar habitable zone of HD 69830 , 41 light years away from Earth. The following year, 55 Cancri f was discovered within the HZ of its host star 55 Cancri A . Hypothetical satellites with sufficient mass and composition are thought to be able to support liquid water at their surfaces. Though, in theory, such giant planets could possess moons,

4212-406: Is within the Alpha Centauri system's habitable zone . Ideally, the Breakthrough Starshot would aim its spacecraft within one astronomical unit (150 million kilometers or 93 million miles) of that world. From this distance, a craft's cameras could capture an image of high enough resolution to resolve surface features. The fleet would have about 1000 spacecraft. Each one, called a StarChip, would be

4320-602: The Hill radius of the planet so that they are not pulled out of the orbit of their host planet. Red dwarfs that have masses less than 20% of that of the Sun cannot have habitable moons around giant planets, as the small size of the circumstellar habitable zone would put a habitable moon so close to the star that it would be stripped from its host planet. In such a system, a moon close enough to its host planet to maintain its orbit would have tidal heating so intense as to eliminate any prospects of habitability. A planetary object that orbits

4428-458: The Moon , Mars , and numerous asteroids also lie within various estimates of the habitable zone. Only at Mars' lowest elevations (less than 30% of the planet's surface) is atmospheric pressure and temperature sufficient for water to, if present, exist in liquid form for short periods. At Hellas Basin , for example, atmospheric pressures can reach 1,115 Pa and temperatures above zero Celsius (about

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4536-454: The habitable zone of its host star, Proxima Centauri , in the Alpha Centauri system. At a speed between 15% and 20% of the speed of light , it would take between 20 and 30 years to complete the journey, and approximately 4 years for a return message from the starship to Earth. The conceptual principles to enable this interstellar travel project were described in "A Roadmap to Interstellar Flight", by Philip Lubin of UC Santa Barbara . Sending

4644-746: The habitable zone of its star. It could be a target for one of the Breakthrough Initiatives' projects. In January 2017, Breakthrough Initiatives and the European Southern Observatory began collaborating to search for habitable planets in the nearby star system Alpha Centauri. The agreement involves Breakthrough Initiatives providing funding for an upgrade to the VISIR (VLT Imager and Spectrometer for mid-Infrared) instrument on ESO's Very Large Telescope (VLT) in Chile . This upgrade will increase

4752-576: The lithosphere , and photolysis . For an extrasolar system, an icy body from beyond the frost line could migrate into the habitable zone of its star, creating an ocean planet with seas hundreds of kilometers deep such as GJ 1214 b or Kepler-22b may be. Maintenance of liquid surface water also requires a sufficiently thick atmosphere. Possible origins of terrestrial atmospheres are currently theorised to outgassing, impact degassing and ingassing. Atmospheres are thought to be maintained through similar processes along with biogeochemical cycles and

4860-514: The orbital period , causing one side to permanently face the host star and the other side to face away. In the past, such tidal locking was thought to cause extreme heat on the star-facing side and bitter cold on the opposite side, making many red dwarf planets uninhabitable; however, three-dimensional climate models in 2013 showed that the side of a red dwarf planet facing the host star could have extensive cloud cover, increasing its bond albedo and reducing significantly temperature differences between

4968-409: The solar wind make it impossible for these bodies to sustain liquid water on their surface. Despite this, studies are strongly suggestive of past liquid water on the surface of Venus, Mars, Vesta and Ceres, suggesting a more common phenomenon than previously thought. Since sustainable liquid water is thought to be essential to support complex life, most estimates, therefore, are inferred from

5076-408: The HZ concept is more complex. Some scientists argue that the concept of a circumstellar habitable zone is actually limited to stars in certain types of systems or of certain spectral types . Binary systems, for example, have circumstellar habitable zones that differ from those of single-star planetary systems, in addition to the orbital stability concerns inherent with a three-body configuration. If

5184-470: The HZ might outnumber planets. In subsequent decades, the HZ concept began to be challenged as a primary criterion for life, so the concept is still evolving. Since the discovery of evidence for extraterrestrial liquid water , substantial quantities of it are now thought to occur outside the circumstellar habitable zone. The concept of deep biospheres , like Earth's, that exist independently of stellar energy, are now generally accepted in astrobiology given

5292-409: The HZ, such an orbit would causes extreme seasonal effects. In spite of this, simulations have suggested that a sufficiently large companion could support surface water year-round. Gliese 876 b , discovered in 1998, and Gliese 876 c , discovered in 2001, are both gas giants discovered in the habitable zone around Gliese 876 that may also have large moons. Another gas giant, Upsilon Andromedae d

5400-503: The Milky Way. However, this is merely a statistical prediction; only a small fraction of these possible planets have yet been discovered. Previous studies have been more conservative. In 2011, Seth Borenstein concluded that there are roughly 500 million habitable planets in the Milky Way. NASA's Jet Propulsion Laboratory 2011 study, based on observations from the Kepler mission, raised

5508-410: The Solar System were such a binary system, the outer limits of the resulting circumstellar habitable zone could extend as far as 2.4 AU. With regard to spectral types, Zoltán Balog proposes that O-type stars cannot form planets due to the photoevaporation caused by their strong ultraviolet emissions. Studying ultraviolet emissions, Andrea Buccino found that only 40% of stars studied (including

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5616-473: The Solar System would extend out as far as 2.4 AU in that case. Similar increases in the size of the habitable zone were computed for other stellar systems. An earlier study by Ray Pierrehumbert and Eric Gaidos had eliminated the CO 2 -H 2 O concept entirely, arguing that young planets could accrete many tens to hundreds of bars of hydrogen from the protoplanetary disc, providing enough of a greenhouse effect to extend

5724-454: The Sun becomes a red giant. Nevertheless, life need not originate during this stage of stellar evolution for it to be detected. Once the star becomes a red giant, and the habitable zone extends outward, the icy surface would melt, forming a temporary atmosphere that can be searched for signs of life that may have been thriving before the start of the red giant stage. A planet's atmospheric conditions influence its ability to retain heat so that

5832-469: The Sun) had overlapping liquid water and ultraviolet habitable zones. Stars smaller than the Sun, on the other hand, have distinct impediments to habitability. For example, Michael Hart proposed that only main-sequence stars of spectral class K0 or brighter could offer habitable zones, an idea which has evolved in modern times into the concept of a tidal locking radius for red dwarfs . Within this radius, which

5940-558: The atmosphere and burning up on 21 March. Each StarChip nanocraft is expected to carry miniaturized cameras, navigation gear, communication equipment, photon thrusters and a power supply. In addition, each nanocraft would be fitted with a meter-scale light sail , made of lightweight materials, with a gram-scale mass. Five sub-gram scale digital cameras , each with a minimum 2- megapixels resolution , are envisioned. Four sub-gram scale processors are planned. Four sub-gram scale photon thrusters, each minimally capable of performing at

6048-404: The best candidates for being potentially habitable. These are at a distance of 990, 490 and 1,120 light-years away, respectively. Of these, Kepler-186f is closest in size to Earth with 1.2 times Earth's radius, and it is located towards the outer edge of the habitable zone around its red dwarf star. Among nearest terrestrial exoplanet candidates , Tau Ceti e is 11.9 light-years away. It is in

6156-459: The case of the Sun would range from 7 to 22 AU. At such stage, Saturn's moon Titan would likely be habitable in Earth's temperature sense. Given that this new equilibrium lasts for about 1 Gyr , and because life on Earth emerged by 0.7 Gyr from the formation of the Solar System at latest, life could conceivably develop on planetary mass objects in the habitable zone of red giants. However, around such

6264-581: The circumstellar habitable zone of Tau Ceti , a Sun-like star 12 light years away. Although more massive than Earth, they are among the least massive planets found to date orbiting in the habitable zone; however, Tau Ceti f, like HD 85512 b, did not fit the new circumstellar habitable zone criteria established by the 2013 Kopparapu study. It is now considered as uninhabitable. Recent discoveries have uncovered planets that are thought to be similar in size or mass to Earth. "Earth-sized" ranges are typically defined by mass. The lower range used in many definitions of

6372-406: The concept be extended to other solvents, including dihydrogen, sulfuric acid, dinitrogen, formamide, and methane, among others, which would support hypothetical life forms that use an alternative biochemistry . In 2013, further developments in habitable zone concepts were made with the proposal of a circum planetary habitable zone, also known as the "habitable edge", to encompass the region around

6480-423: The development of life. Once a star has evolved sufficiently to become a red giant, its circumstellar habitable zone will change dramatically from its main-sequence size. For example, the Sun is expected to engulf the previously habitable Earth as a red giant. However, once a red giant star reaches the horizontal branch , it achieves a new equilibrium and can sustain a new circumstellar habitable zone, which in

6588-464: The development of life. Red dwarf stars, on the other hand, which can live for hundreds of billions of years on the main sequence, would have planets with ample time for life to develop and evolve. Even while stars are on the main sequence, though, their energy output steadily increases, pushing their habitable zones farther out; our Sun, for example, was 75% as bright in the Archaean as it is now, and in

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6696-431: The distance to Alpha Centauri, assuming a 30 m diameter receiving telescope on Earth. The Starshot project is for fly-by missions, which pass the target at high velocity. Heller et al. proposed that a photo-gravitational assist could be used to slow such a probe and allow it to enter orbit (using photon pressure in maneuvers similar to aerobraking ). This requires a sail that is both much lighter and much larger than

6804-434: The effect that a repositioned orbit would have on the habitability of Earth or Venus as their surface gravity allows sufficient atmosphere to be retained for several billion years. According to the extended habitable zone concept, planetary-mass objects with atmospheres capable of inducing sufficient radiative forcing could possess liquid water farther out from the Sun. Such objects could include those whose atmospheres contain

6912-418: The final mission cost at $ 5–10 billion, and estimates the first craft could launch by around 2036. Pete Worden is the project's executive director and Harvard Professor Avi Loeb chairs the advisory board for the project. The Breakthrough Starshot program aims to demonstrate a proof-of-concept for ultra-fast, light-driven nano-spacecraft, and lay the foundations for a first launch to Alpha Centauri within

7020-401: The freezing point of water. However, their atmospheric conditions vary substantially. The aphelion of Venus, for example, touches the inner edge of the zone in most estimates and while atmospheric pressure at the surface is sufficient for liquid water, a strong greenhouse effect raises surface temperatures to 462 °C (864 °F) at which water can only exist as vapor. The entire orbits of

7128-403: The future, continued increases in energy output will put Earth outside the Sun's habitable zone, even before it reaches the red giant phase. In order to deal with this increase in luminosity, the concept of a continuously habitable zone has been introduced. As the name suggests, the continuously habitable zone is a region around a star in which planetary-mass bodies can sustain liquid water for

7236-415: The gravitational instabilities of those systems. The concept of habitable zones was further developed in 1964 by Stephen H. Dole in his book Habitable Planets for Man , in which he discussed the concept of the circumstellar habitable zone as well as various other determinants of planetary habitability, eventually estimating the number of habitable planets in the Milky Way to be about 600 million. At

7344-530: The habitable zones of Sun-like stars and red dwarfs in the Milky Way . About 11 billion of these may be orbiting Sun-like stars. Proxima Centauri b , located about 4.2 light-years (1.3 parsecs ) from Earth in the constellation of Centaurus , is the nearest known exoplanet, and is orbiting in the habitable zone of its star. The HZ is also of particular interest to the emerging field of habitability of natural satellites , because planetary-mass moons in

7452-411: The host star. Given the large spread in the masses of planets within a circumstellar habitable zone, coupled with the discovery of super-Earth planets which can sustain thicker atmospheres and stronger magnetic fields than Earth, circumstellar habitable zones are now split into two separate regions—a "conservative habitable zone" in which lower-mass planets like Earth can remain habitable, complemented by

7560-404: The idea that red dwarf stars can support planets with relatively constant temperatures over their surfaces in spite of tidal locking. Astronomy professor Eric Agol argues that even white dwarfs may support a relatively brief habitable zone through planetary migration. At the same time, others have written in similar support of semi-stable, temporary habitable zones around brown dwarfs . Also,

7668-488: The inner edge of its planetary system's habitable zone, giving it an estimated average surface temperature of 68 °C (154 °F). Studies that have attempted to estimate the number of terrestrial planets within the circumstellar habitable zone tend to reflect the availability of scientific data. A 2013 study by Ravi Kumar Kopparapu put η e , the fraction of stars with planets in the HZ, at 0.48, meaning that there may be roughly 95–180 billion habitable planets in

7776-728: The large amount of liquid water known to exist in lithospheres and asthenospheres of the Solar System. Sustained by other energy sources, such as tidal heating or radioactive decay or pressurized by non-atmospheric means, liquid water may be found even on rogue planets , or their moons. Liquid water can also exist at a wider range of temperatures and pressures as a solution , for example with sodium chlorides in seawater on Earth, chlorides and sulphates on equatorial Mars , or ammoniates, due to its different colligative properties . In addition, other circumstellar zones, where non-water solvents favorable to hypothetical life based on alternative biochemistries could exist in liquid form at

7884-411: The laser beam while absorbing only a small fraction of the incident energy, or it will vaporize the sail. The light sail may also double as power source during cruise, because collisions with atoms of interstellar medium would deliver 60 watt/m of power. A laser communicator, utilizing the light sail as the primary reflector, would be capable of data rates of 2.6-15 baud per watt of transmitted power at

7992-602: The lightweight spacecraft involves a multi-kilometer phased array of beam-steerable lasers with a combined coherent power output of up to 100 GW . The project was announced on 12 April 2016 in an event held in New York City by physicist and venture capitalist Yuri Milner , together with cosmologist Stephen Hawking , who was serving as board member of the initiatives. Other board members include Facebook, Inc. (now known as Meta Platforms ) CEO Mark Zuckerberg . The project has an initial funding of US$ 100 million. Milner places

8100-432: The likelihood of planet detection in the system. The Starshot concept envisions launching a " mothership " carrying about a thousand tiny spacecraft (on the scale of centimeters) to a high-altitude Earth orbit for deployment. A phased array of ground-based lasers would then focus a light beam on the sails of these spacecraft to accelerate them one by one to the target speed within 10 minutes, with an average acceleration on

8208-426: The location of the habitable zone is also specific to each type of planet: desert planets (also known as dry planets), with very little water, will have less water vapor in the atmosphere than Earth and so have a reduced greenhouse effect , meaning that a desert planet could maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is less ice to reflect heat into space, so

8316-411: The mitigation of atmospheric escape . In a 2013 study led by Italian astronomer Giovanni Vladilo , it was shown that the size of the circumstellar habitable zone increased with greater atmospheric pressure. Below an atmospheric pressure of about 15 millibars, it was found that habitability could not be maintained because even a small shift in pressure or temperature could render water unable to form as

8424-418: The most conservative estimates, only Earth lies within the zone; in the most permissive estimates, even Saturn at perihelion, or Mercury at aphelion, might be included. Astronomers use stellar flux and the inverse-square law to extrapolate circumstellar habitable zone models created for the Solar System to other stars. For example, according to Kopparapu's habitable zone estimate, although the Solar System has

8532-537: The most stable state. Proper metallicity and size are also critical to low luminosity variation. Using data collected by NASA's Kepler space telescope and the W. M. Keck Observatory , scientists have estimated that 22% of solar-type stars in the Milky Way galaxy have Earth-sized planets in their habitable zone. On 7 January 2013, astronomers from the Kepler team announced the discovery of Kepler-69c (formerly KOI-172.02 ), an Earth-size exoplanet candidate (1.7 times

8640-435: The net temperature effect of such atmospheres including induced albedo , anti-greenhouse or other possible heat sources. For reference, the average distance from the Sun of some major bodies within the various estimates of the habitable zone is: Mercury, 0.39 AU; Venus, 0.72 AU; Earth, 1.00 AU; Mars, 1.52 AU; Vesta, 2.36 AU; Ceres and Pallas, 2.77 AU; Jupiter, 5.20 AU; Saturn, 9.58 AU. In

8748-500: The next generation. The spacecraft would make a flyby, and possibly photograph, of any Earth-like worlds that might exist in the system. Secondary goals are Solar System exploration and detection of Earth-crossing asteroids . The European Southern Observatory (ESO) announced the detection of a planet orbiting the third star in the Alpha Centauri system, Proxima Centauri in August 2016. The planet, called Proxima Centauri b , orbits within

8856-415: The number somewhat, estimating that about "1.4 to 2.7 percent" of all stars of spectral class F , G , and K are expected to have planets in their HZs. The first discoveries of extrasolar planets in the HZ occurred just a few years after the first extrasolar planets were discovered. However, these early detections were all gas giant-sized, and many were in eccentric orbits. Despite this, studies indicate

8964-603: The ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right". Since the concept was first presented in 1953, many stars have been confirmed to possess an HZ planet, including some systems that consist of multiple HZ planets. Most such planets, being either super-Earths or gas giants , are more massive than Earth, because massive planets are easier to detect . On November 4, 2013, astronomers reported, based on Kepler space telescope data, that there could be as many as 40 billion Earth-sized planets orbiting in

9072-438: The order of 100 km/s (10,000  ɡ ), and an illumination energy on the order of 1 TJ delivered to each sail. A preliminary sail model is suggested to have a surface area of 4 m × 4 m. An October 2017 presentation of the Starshot system model examined circular sails and finds that the beam director capital cost is minimized by having a sail diameter of 5 meters. The Earth-sized planet Proxima Centauri b

9180-422: The outer edge of desert-planet habitable zones is further out. A planet cannot have a hydrosphere —a key ingredient for the formation of carbon-based life—unless there is a source for water within its stellar system. The origin of water on Earth is still not completely understood; possible sources include the result of impacts with icy bodies, outgassing , mineralization , leakage from hydrous minerals from

9288-442: The possibility of large, Earth-like moons around these planets supporting liquid water. One of the first discoveries was 70 Virginis b , a gas giant initially nicknamed "Goldilocks" due to it being neither "too hot" nor "too cold". Later study revealed temperatures analogous to Venus, ruling out any potential for liquid water. 16 Cygni Bb , also discovered in 1996, has an extremely eccentric orbit that spends only part of its time in

9396-429: The previously mentioned 4m sail at 10,000Gs to 0.2 c requires combining optics which are approximately 3 kilometers in diameter to focus the laser light on the sail. This could be implemented using a phased array system which is being researched at The University of California Santa Barbara. According to The Economist , at least a dozen off-the-shelf technologies will need to improve by orders of magnitude . StarChip

9504-595: The project are available to the public, and SETI@Home is used for some of the data analysis. The first results were published in April 2017, with further updates expected every 6 months. The Breakthrough Message program is to study the ethics of sending messages into deep space. It also launched an open competition with a US$ 1 million prize pool, to design a digital message that could be transmitted from Earth to an extraterrestrial civilization. The message should be "representative of humanity and planet Earth". The program pledges "not to transmit any message until there has been

9612-542: The proposed Starshot sail. The table below lists possible target stars for photogravitational assist rendezvous. The travel times are the calculated times for an optimized spacecraft to travel to the star and then enter orbit around the star. The German physicist Claudius Gros has proposed that the technology of the Breakthrough Starshot initiative may be used in a second step to establish a biosphere of unicellular microbes on otherwise only transiently habitable exoplanets . A Genesis probe would travel at lower speeds, at

9720-426: The quality of the sensors on board. Light propulsion requires enormous power: a laser with a gigawatt of power (approximately the output of a large nuclear plant) would provide only a few newtons of thrust . The spaceship will compensate for the low thrust by having a mass of only a few grams. The camera, computer, communications laser, a nuclear power source , and the solar sail must be miniaturized to fit within

9828-404: The radius of Earth) orbiting Kepler-69 , a star similar to the Sun, in the HZ and expected to offer habitable conditions. The discovery of two planets orbiting in the habitable zone of Kepler-62 , by the Kepler team was announced on April 19, 2013. The planets, named Kepler-62e and Kepler-62f , are likely solid planets with sizes 1.6 and 1.4 times the radius of Earth, respectively. With

9936-503: The red subdwarf Kapteyn's Star , 12.8 light-years away. On 6 January 2015, NASA announced the 1000th confirmed exoplanet discovered by the Kepler Space Telescope. Three of the newly confirmed exoplanets were found to orbit within habitable zones of their related stars : two of the three, Kepler-438b and Kepler-442b , are near-Earth-size and likely rocky ; the third, Kepler-440b , is a super-Earth . However, Kepler-438b

10044-432: The region where life is most likely to emerge in a galaxy, encompasses those regions close enough to a galactic center that stars there are enriched with heavier elements , but not so close that star systems, planetary orbits, and the emergence of life would be frequently disrupted by the intense radiation and enormous gravitational forces commonly found at galactic centers. Subsequently, some astrobiologists propose that

10152-432: The same system and thought to be a better candidate for habitability, was also announced in 2007. Its existence was later disconfirmed in 2014, but only for a short time. As of 2015, the planet has no newer disconfirmations. Gliese 581 g , yet another planet thought to have been discovered in the circumstellar habitable zone of the system, was considered to be more habitable than both Gliese 581 c and d. However, its existence

10260-416: The same time, science-fiction author Isaac Asimov introduced the concept of a circumstellar habitable zone to the general public through his various explorations of space colonization . The term " Goldilocks zone " emerged in the 1970s, referencing specifically a region around a star whose temperature is "just right" for water to be present in the liquid phase. In 1993, astronomer James Kasting introduced

10368-423: The series of planets, in the rank which was necessary, and at exactly the right distance from the sun, to accomplish its secondary object of vegetation; and from this we might infer that the other planets are either too near or too remote from the sun, to vegetate." The concept of a circumstellar habitable zone was first introduced in 1913, by Edward Maunder in his book "Are The Planets Inhabited?". The concept

10476-585: The significance of the project, saying that "by sending hundreds or thousands of space probes the size of postage stamps, Breakthrough Starshot gets around the hazards of spaceflight that could easily end a mission relying on a single spacecraft. Only one nanocraft needs to make its way to Alpha Centauri and send back a signal for the mission to be successful. When that happens, Starshot will make history." In July 2017, scientists announced that precursors to StarChip , named Sprites , were successfully launched and flown. Breakthrough Watch is

10584-451: The solar system outer edge to 10 AU. In this case, though, the hydrogen is not continuously replenished by volcanism and is lost within millions to tens of millions of years. In the case of planets orbiting in the HZs of red dwarf stars, the extremely close distances to the stars cause tidal locking , an important factor in habitability. For a tidally locked planet, the sidereal day is as long as

10692-603: The super-Earth class is 1.9 Earth masses; likewise, sub-Earths range up to the size of Venus (~0.815 Earth masses). An upper limit of 1.5 Earth radii is also considered, given that above 1.5  R 🜨 the average planet density rapidly decreases with increasing radius, indicating these planets have a significant fraction of volatiles by volume overlying a rocky core. A genuinely Earth-like planet – an Earth analog or "Earth twin" – would need to meet many conditions beyond size and mass; such properties are not observable using current technology. A solar analog (or "solar twin")

10800-498: The surface, have been proposed. An estimate of the range of distances from the Sun allowing the existence of liquid water appears in Newton's Principia (Book III, Section 1, corol. 4). The philosopher Louis Claude de Saint-Martin speculated in his 1802 work Man: His True Nature and Ministry , "... we may presume, that, being susceptible of vegetation, it [the Earth] has been placed, in

10908-505: The technology did not exist to detect moons around them, and no extrasolar moons had been discovered. Planets within the zone with the potential for solid surfaces were therefore of much higher interest. The 2007 discovery of Gliese 581c , the first super-Earth in the circumstellar habitable zone, created significant interest in the system by the scientific community, although the planet was later found to have extreme surface conditions that may resemble Venus. Gliese 581 d, another planet in

11016-470: The term "circumstellar habitable zone" to refer more precisely to the region then (and still) known as the habitable zone. Kasting was the first to present a detailed model for the habitable zone for exoplanets. An update to habitable zone concept came in 2000 when astronomers Peter Ward and Donald Brownlee introduced the idea of the " galactic habitable zone ", which they later developed with Guillermo Gonzalez . The galactic habitable zone, defined as

11124-439: The term "habitable zone" in 1959 to refer to the area around a star where liquid water could exist on a sufficiently large body, and was the first to introduce it in the context of planetary habitability and extraterrestrial life. A major early contributor to the habitable zone concept, Huang argued in 1960 that circumstellar habitable zones, and by extension extraterrestrial life, would be uncommon in multiple star systems , given

11232-474: The triple point for water) for 70 days in the Martian year. Despite indirect evidence in the form of seasonal flows on warm Martian slopes , no confirmation has been made of the presence of liquid water there. While other objects orbit partly within this zone, including comets, Ceres is the only one of planetary mass. A combination of low mass and an inability to mitigate evaporation and atmosphere loss against

11340-433: The two sides. Planetary mass natural satellites have the potential to be habitable as well. However, these bodies need to fulfill additional parameters, in particular being located within the circumplanetary habitable zones of their host planets. More specifically, moons need to be far enough from their host giant planets that they are not transformed by tidal heating into volcanic worlds like Io , but must remain within

11448-489: Was also disconfirmed in 2014, and astronomers are divided about its existence. Discovered in August 2011, HD 85512 b was initially speculated to be habitable, but the new circumstellar habitable zone criteria devised by Kopparapu et al. in 2013 place the planet outside the circumstellar habitable zone. Kepler-22 b , discovered in December 2011 by the Kepler space probe, is the first transiting exoplanet discovered around

11556-416: Was discovered in 1999 orbiting Upsilon Andromidae's habitable zone. Announced on April 4, 2001, HD 28185 b is a gas giant found to orbit entirely within its star's circumstellar habitable zone and has a low orbital eccentricity, comparable to that of Mars in the Solar System. Tidal interactions suggest it could harbor habitable Earth-mass satellites in orbit around it for many billions of years, though it

11664-606: Was later discussed in 1953 by Hubertus Strughold , who in his treatise The Green and the Red Planet: A Physiological Study of the Possibility of Life on Mars , coined the term "ecosphere" and referred to various "zones" in which life could emerge. In the same year, Harlow Shapley wrote "Liquid Water Belt", which described the same concept in further scientific detail. Both works stressed the importance of liquid water to life. Su-Shu Huang , an American astrophysicist, first introduced

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