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133-525: Located in its parent star's habitable zone , PH2b (or Kepler-86b) is a Jupiter-size gas giant which could in theory host a natural satellite suitable for hosting life. The report of the confirmed detection of PH2b was submitted on January 3, 2013. It was discovered by amateur Pole Rafał Herszkowicz using his laptop and access to the Internet project with data from the Kepler space observatory . PH2 b

266-454: 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

399-616: A brown dwarf . It was detected by gravitational microlensing. In June 2008, European researchers announced the discovery of three super-Earths around the star HD 40307 , a star that is only slightly less massive than the Sun . Planets have at least the following minimum masses: 4.2, 6.7, and 9.4 times Earth's. The planets were detected by the radial velocity method by the HARPS (High Accuracy Radial Velocity Planet Searcher) in Chile . In addition,

532-510: A runaway greenhouse effect like Venus. Two further possible super-Earths were discovered in 2006: OGLE-2005-BLG-390Lb with a mass of 5.5 Earth masses, which was found by gravitational microlensing , and HD 69830 b with a mass of 10 Earth masses. The smallest super-Earth found as of 2008 was MOA-2007-BLG-192Lb . The planet was announced by astrophysicist David P. Bennett for the international MOA collaboration on June 2, 2008. This planet has approximately 3.3 Earth masses and orbits

665-406: A Harvard professor of astronomy and lead author of an article on the discovery. However, interior models of this planet suggest that under most conditions it does not have liquid water. By November 2009, a total of 30 super-Earths had been discovered, 24 of which were first observed by HARPS. Discovered on 5 January 2010, a planet HD 156668 b with a minimum mass of 4.15 Earth masses ,

798-488: 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

931-441: 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

1064-456: A distance from the star at which the temperature is just right for water to exist in liquid form rather than being stripped away by stellar radiation or locked permanently in ice. In May 2014, previously discovered Kepler-10c was determined to have the mass comparable to Neptune (17 Earth masses). With the radius of 2.35 R 🜨 , it is currently the largest known planet likely to have a predominantly rocky composition. At 17 Earth masses, it

1197-485: A distance where life in theory could exist, according to a record-breaking tally announced on Tuesday by the European Southern Observatory. They are part of a cluster of as many as seven planets that circle Gliese 667C , one of three stars located a relatively close 22 light years from Earth in the constellation of Scorpio, it said. The planets orbit Gliese 667C in the so-called Goldilocks Zone —

1330-498: A diversity of compositions that is not well-explained by a single mass-radius relation as that found in rocky planets). After measuring 65 super-Earths smaller than 4 Earth-radii, the empirical data points out that Gas Dwarves would be the most usual composition: there is a trend where planets with radii up to 1.5 Earth-radii increase in density with increasing radius, but above 1.5 radii the average planet density rapidly decreases with increasing radius, indicating that these planets have

1463-615: 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

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1596-435: A greenhouse atmosphere of some sort that could have liquid water on its surface" in the words of the researchers, thus further improving its prospects for habitability. In 2019, the mass of the planet was measured by radial velocity , showing it to be close in mass to Saturn . Habitable zone In astronomy and astrobiology , the habitable zone ( HZ ), or more precisely the circumstellar habitable zone ( CHZ ),

1729-443: 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

1862-515: 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

1995-473: 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

2128-401: A large fraction of volatiles by volume overlying a rocky core. Another discovery about exoplanets' composition is that about the gap or rarity observed for planets between 1.5 and 2.0 Earth-radii , which is explained by a bimodal formation of planets (rocky Super-Earths below 1.75 and sub-Neptunes with thick gas envelopes being above such radii). Additional studies, conducted with lasers at

2261-503: 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

2394-490: 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

2527-519: A major constituent ( ocean planets ), or have a denser core enshrouded with an extended gaseous envelope ( gas dwarf or sub-Neptune). A super-Earth of high density is believed to be rocky and/or metallic, like Earth and the other terrestrial planets of the Solar System. A super-Earth's interior could be undifferentiated, partially differentiated, or completely differentiated into layers of different composition. Researchers at Harvard Astronomy Department have developed user-friendly online tools to characterize

2660-399: A period of 0.28 days orbiting the host star K2-141 (EPIC 246393474) was reported. Another Super-Earth, K2-155d , is discovered. In July 2018, the discovery of 40 Eridani b was announced. At 16 light-years it is the closest super-Earth known, and its star is the second-brightest hosting a super-Earth. In July 2019, the discovery of GJ 357 d was announced. Thirty-one light-years from

2793-623: A period of 36.6 days, placing it in the middle of the habitable zone where liquid water could exist and midway between the planets c and d. It was discovered using the radial velocity method by scientists at the University of California at Santa Cruz and the Carnegie Institution of Washington. However, the existence of Gliese 581 g has been questioned by another team of astronomers, and it is currently listed as unconfirmed at The Extrasolar Planets Encyclopaedia . On 2 February,

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2926-478: 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

3059-505: A potentially habitable super-Earth HD 85512 b was found using the HARPS as well as a three super-Earth system 82 G. Eridani . On HD 85512 b, it would be habitable if it exhibits more than 50% cloud cover. Then less than a month later, a flood of 41 new exoplanets, including 10 super-Earths, were announced. On 5 December 2011, the Kepler space telescope discovered its first planet within

3192-503: 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

3325-411: A red dwarf star about 137 light-years away. The Solar System contains no known super-Earths, because Earth is the largest terrestrial planet in the Solar System, and all larger planets have both at least 14 times the mass of Earth and thick gaseous envelopes without well-defined rocky or watery surfaces; that is, they are either gas giants or ice giants , not terrestrial planets. In January 2016,

3458-463: A relatively high probability of spurious detection and may well come from non-planetary sources. Although the planet's initial detection was made using Kepler data, PH2's stellar spectra, required to rule out background stars or faint companions with planets as sources for the observed transits, were collected using the HIRES instrument at the W. M. Keck Observatory . Results of observations confirmed

3591-571: 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

3724-469: A super-Earth is detectable by both the radial-velocity and the transit methods, then both its mass and its radius can be determined; thus its average bulk density can be calculated. The actual empirical observations are giving similar results as theoretical models, as it's found that planets larger than approximately 1.6 Earth-radius (more massive than approximately 6 Earth-masses) contain significant fractions of volatiles or H/He gas (such planets appear to have

3857-463: A true temperature of 737 K (464 °C or 867 °F ). Though the atmosphere of Venus traps more heat than Earth's, NASA lists the black-body temperature of Venus based on the fact that Venus has an extremely high albedo ( Bond albedo 0.90, Visual geometric albedo 0.67), giving it a lower black body temperature than the more absorbent (lower albedo ) Earth. Earth's magnetic field results from its flowing liquid metallic core, but in super-Earths

3990-533: A very short orbital period of about 2 days. Due to the proximity of Gliese 876 d to its host star (a red dwarf ), it may have a surface temperature of 430–650 kelvin and be too hot to support liquid water. In April 2007, a team headed by Stéphane Udry based in Switzerland announced the discovery of two new super-Earths within the Gliese 581 planetary system , both on the edge of the habitable zone around

4123-412: A watery and icy planet. Within this range of radii the super-Earth Gliese 876 d would have a surface gravity between 1.9 g and 3.3g (19 and 32 m/s ). However, this planet is not known to transit its host star. The limit between rocky planets and planets with a thick gaseous envelope is calculated with theoretical models. Calculating the effect of the active XUV saturation phase of G-type stars over

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4256-409: 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

4389-410: Is a type of exoplanet with a mass higher than Earth 's, but substantially below those of the Solar System's ice giants , Uranus and Neptune , which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to the mass of the planet, and so does not imply anything about the surface conditions or habitability . The alternative term "gas dwarfs" may be more accurate for those at

4522-403: 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

4655-507: Is found to be a subject of powerful flares, so it is now considered uninhabitable. 16 January, K2-3d a planet of 1.5 Earth radii was found orbiting within the habitable zone of K2-3 , receiving 1.4 times the intensity of visible light as Earth. Kepler-452b , announced on 23 July 2015 is 50% bigger than Earth, likely rocky and takes approximately 385 Earth days to orbit the habitable zone of its G-class (solar analog) star Kepler-452 . Super-Earth A Super-Earth or super-terran

4788-524: Is in fact much larger and hotter than first reported. Based on the latest Kepler findings, astronomer Seth Shostak estimates "within a thousand light-years of Earth" there are "at least 30,000 of these habitable worlds." Also based on the findings, the Kepler Team has estimated "at least 50 billion planets in the Milky Way" of which "at least 500 million" are in the habitable zone. On 17 August,

4921-476: 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

5054-419: 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

5187-472: Is the least massive planet detected by the radial velocity method . The only confirmed radial velocity planet smaller than this planet is Gliese 581e at 1.9 Earth masses (see above). On 24 August, astronomers using ESO's HARPS instrument announced the discovery of a planetary system with up to seven planets orbiting a Sun-like star, HD 10180 , one of which, although not yet confirmed, has an estimated minimum mass of 1.35 ± 0.23 times that of Earth, which would be

5320-492: Is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure . The bounds of the HZ are based on Earth 's position in the Solar System and the amount of radiant energy it receives from the Sun . Due to the importance of liquid water to Earth's biosphere , the nature of the HZ and the objects within it may be instrumental in determining

5453-528: 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,

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5586-452: Is well above the 10 Earth mass upper limit that is commonly used for the term 'super-Earth' so the term mega-Earth has been proposed. However, in July 2017, more careful analysis of HARPS-N and HIRES data showed that Kepler-10c was much less massive than originally thought, instead around 7.37 (6.18 to 8.69) M E with a mean density of 3.14 g/cm . Instead of a primarily rocky composition,

5719-606: 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

5852-737: The Kepler Space Observatory mission team released a list of 1235 extrasolar planet candidates , including 68 candidates of approximately "Earth-size" (Rp < 1.25 Re) and 288 candidates of "super-Earth-size" (1.25 Re < Rp < 2 Re). In addition, 54 planet candidates were detected in the " habitable zone ." Six candidates in this zone were less than twice the size of the Earth [namely: KOI 326.01 (Rp=0.85), KOI 701.03 (Rp=1.73), KOI 268.01 (Rp=1.75), KOI 1026.01 (Rp=1.77), KOI 854.01 (Rp=1.91), KOI 70.03 (Rp=1.96) – Table 6] A more recent study found that one of these candidates (KOI 326.01)

5985-497: The Kepler space observatory announced the discovery of Kepler-69c (formerly KOI-172.02 ), an Earth -like exoplanet candidate (1.5 times the radius of Earth) orbiting a star similar to the Sun in the habitable zone and possibly a "prime candidate to host alien life ". In April 2013, using observations by NASA's Kepler mission team led by William Borucki , of the agency's Ames Research Center, found five planets orbiting in

6118-502: The Kepler-11 planetary system show that the two innermost planets Kepler-11b and c, whose calculated mass is ≈2 M 🜨 and between ≈5 and 6 M 🜨 respectively (which are within measurement errors), are extremely vulnerable to envelope loss. In particular, the complete removal of the primordial H/He envelope by energetic stellar photons appears almost inevitable in the case of Kepler-11b, regardless of its formation hypothesis. If

6251-808: The Lawrence Livermore National Laboratory and the OMEGA laboratory at the University of Rochester , show that the magnesium-silicate internal regions of the planet would undergo phase changes under the immense pressures and temperatures of a super-Earth planet, and that the different phases of this liquid magnesium silicate would separate into layers. Further theoretical work by Valencia and others suggests that super-Earths would be more geologically active than Earth, with more vigorous plate tectonics due to thinner plates under more stress. In fact, their models suggested that Earth

6384-462: 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

6517-576: 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

6650-514: 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

6783-479: The pulsar PSR B1257+12 in 1992. The two outer planets ( Poltergeist and Phobetor ) of the system have masses approximately four times Earth—too small to be gas giants. The first super-Earth around a main-sequence star was discovered by a team under Eugenio Rivera in 2005. It orbits Gliese 876 and received the designation Gliese 876 d (two Jupiter-sized gas giants had previously been discovered in that system). It has an estimated mass of 7.5 Earth masses and

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6916-409: 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

7049-534: The February 2011 figures, the number of Earth-size and super-Earth-size planets increased by 200% and 140% respectively. Moreover, 48 planet candidates were found in the habitable zones of surveyed stars, marking a decrease from the February figure; this was due to the more stringent criteria in use in the December data. In 2011, a density of 55 Cancri e was calculated which turned out to be similar to Earth's. At

7182-408: 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

7315-470: 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

7448-409: 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

7581-403: 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

7714-410: 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

7847-476: 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

7980-413: The Solar System's four inner planets, a new and significant discovery. COROT-7b, discovered right after HD 7924 b , is the first super-Earth discovered that orbits a main sequence star that is G class or larger. The discovery of Gliese 581e with a minimum mass of 1.9 Earth masses was announced on 21 April 2009. It was at the time the smallest extrasolar planet discovered around a normal star and

8113-402: The Solar System, the planet is at least 6.1 M E . In 2021, the exoplanet G 9-40 b was discovered. In 2022, the discovery of a super-Earth around the red dwarf star Ross 508 was reported. Part of the planet's elliptical orbit takes it within the habitable zone . On 31 January 2024 NASA reported the discovery of a super-Earth called TOI-715 b located in the habitable zone of

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8246-454: 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

8379-472: 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

8512-517: The Super-Earth GJ 1214 b ), or even to rocky planets known as chthonian planets (after migrating towards the proximity of their parent star). The amount of the outermost layers that is lost depends on the size and the material of the planet and the distance from the star. In a typical system, a gas giant orbiting 0.02 AU around its parent star loses 5–7% of its mass during its lifetime, but orbiting closer than 0.015 AU can mean evaporation of

8645-433: The atmospheres, albedo and greenhouse effects of super-Earths are unknown, the surface temperatures are unknown and generally only an equilibrium temperature is given. For example, the black-body temperature of the Earth is 255.3 K (−18 °C or 0 °F ). It is the greenhouse gases that keep the Earth warmer. Venus has a black-body temperature of only 184.2 K (−89 °C or −128 °F ) even though Venus has

8778-404: 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

8911-446: The bulk composition of the super-Earths. A study on Gliese 876 d by a team around Diana Valencia revealed that it would be possible to infer from a radius measured by the transit method of detecting planets and the mass of the relevant planet what the structural composition is. For Gliese 876 d, calculations range from 9,200 km (1.4 Earth radii) for a rocky planet and very large iron core to 12,500 km (2.0 Earth radii) for

9044-467: The candidates in the study, including PH2b, were identified by citizen scientists Abe J. Hoekstra, Thomas Lee Jacobs, Daryll LaCourse, Hans Martin Schwengler, Rafał Herszkowicz and Mike Chopin among others, with the help of Yale University astronomers. In addition to PH2b itself, twenty other planet candidates were found which are located in the habitable zones of their host stars; however, these have

9177-411: 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

9310-581: 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

9443-450: The closest in mass to Earth. Being at an orbital distance of just 0.03 AU and orbiting its star in just 3.15 days, it is not in the habitable zone, and may have 100 times more tidal heating than Jupiter's volcanic satellite Io . A planet found in December 2009, GJ 1214 b , is 2.7 times as large as Earth and orbits a star much smaller and less luminous than the Sun. "This planet probably does have liquid water," said David Charbonneau,

9576-406: 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

9709-522: The detection of Proxima b , an Earth-sized exoplanet that is in the habitable zone of the red dwarf star Proxima Centauri , the closest star to the Sun . Due to its closeness to Earth , Proxima b may be a flyby destination for a fleet of interstellar StarChip spacecraft currently being developed by the Breakthrough Starshot project. In February 2018, K2-141b, a rocky ultra-short period planet (USP) Super-Earth, with

9842-423: 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

9975-469: 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

10108-434: 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

10241-400: The existence of PH2b with "99.9 percent confidence." PH2 is host to one confirmed planet, PH2b, orbiting with a period of about 282 days, placing it and any possible moons in the habitable zone. The temperature in the upper atmosphere of the planet could range from 185 K (−88 °C; −127 °F) to 303 K (30 °C; 86 °F). A moon of PH2b would likely have "a rocky core, plus

10374-461: The existence of a hypothetical super-Earth ninth planet in the Solar System, referred to as Planet Nine , was proposed as an explanation for the orbital behavior of six trans-Neptunian objects , but it is speculated to also be an ice giant like Uranus or Neptune. A refined model in 2019 constrains it to around five Earth masses; planets of this mass are probably mini-Neptunes. The fact that there are barely any asteroids or planetesimals inside

10507-580: The forces of magma to break the crust into plates. New research suggests that the rocky centres of super-Earths are unlikely to evolve into terrestrial rocky planets like the inner planets of the Solar System because they appear to hold on to their large atmospheres. Rather than evolving into a planet composed mainly of rock with a thin atmosphere, the small rocky core remains engulfed by its large hydrogen-rich envelope. Theoretical models show that Hot Jupiters and Hot Neptunes can evolve by hydrodynamic loss of their atmospheres to Mini-Neptunes (as it could be

10640-401: 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

10773-403: 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

10906-415: 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

11039-406: The habitable zone of a Sun-like star, Kepler-62 , 1,200 light years from Earth. These new super-Earths have radii of 1.3, 1.4, 1.6, and 1.9 times that of Earth. Theoretical modelling of two of these super-Earths, Kepler-62e and Kepler-62f , suggests both could be solid, either rocky or rocky with frozen water. On 25 June 2013, three "super Earth" planets have been found orbiting a nearby star at

11172-632: The habitable zone or "Goldilocks region" of its Sun-like star. Kepler-22b is 2.4 times the radius of the Earth and occupies an orbit 15% closer to its star than the Earth to the Sun. This is compensated for, however, as the star, with a spectral type G5V , is slightly dimmer than the Sun (G2V). Thus, surface temperatures would still allow liquid water on its surface. On 5 December 2011, the Kepler team announced that they had discovered 2,326 planetary candidates, of which 207 are similar in size to Earth, 680 are super-Earth-size, 1,181 are Neptune-size, 203 are Jupiter-size and 55 are larger than Jupiter. Compared to

11305-535: 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

11438-410: The higher end of the mass scale, although " mini-Neptunes " is a more common term. In general, super-Earths are defined by their masses . The term does not imply temperatures, compositions, orbital properties, habitability, or environments. While sources generally agree on an upper bound of 10 Earth masses (~69% of the mass of Uranus , which is the Solar System's giant planet with the least mass),

11571-411: 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

11704-404: 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,

11837-488: 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

11970-728: 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

12103-426: 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

12236-438: The loss of the primitive nebula-captured hydrogen envelopes in extrasolar planets, it's obtained that planets with a core mass of more than 1.5 Earth-mass (1.15 Earth-radius max.), most likely cannot get rid of their nebula captured hydrogen envelopes during their whole lifetime. Other calculations point out that the limit between envelope-free rocky super-Earths and sub-Neptunes is around 1.75 Earth-radii, as 2 Earth-radii would be

12369-414: The lower bound varies from 1 or 1.9 to 5, with various other definitions appearing in the popular media. The term "super-Earth" is also used by astronomers to refer to planets bigger than Earth-like planets (from 0.8 to 1.2 Earth-radius), but smaller than mini-Neptunes (from 2 to 4 Earth-radii). This definition was made by the Kepler space telescope personnel. Some authors further suggest that

12502-476: The lowest mass of any exoplanet found to date orbiting a main-sequence star. Although unconfirmed, there is a 98.6% probability that this planet does exist. The National Science Foundation announced on 29 September the discovery of a fourth super-Earth ( Gliese 581g ) orbiting within the Gliese 581 planetary system. The planet has a minimum mass 3.1 times that of Earth and a nearly circular orbit at 0.146 AU with

12635-411: 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

12768-427: The more accurately determined mass of Kepler-10c suggests a world made almost entirely of volatiles, mainly water. 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;

12901-418: 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

13034-488: 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

13167-435: 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

13300-415: 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

13433-520: 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

13566-557: The orbit of Mercury led some astronomers believing that a super-Earth might have formed in proximity to the Sun, cleared its neighborhood and rapidly get disrupted by the Sun. Due to the larger mass of super-Earths, their physical characteristics may differ from Earth's; theoretical models for super-Earths provide four possible main compositions according to their density: low-density super-Earths are inferred to be composed mainly of hydrogen and helium ( mini-Neptunes ); super-Earths of intermediate density are inferred to either have water as

13699-422: 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

13832-442: 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

13965-404: 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

14098-513: 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

14231-432: 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

14364-449: The same European research team announced a planet 7.5 times the mass of Earth orbiting the star HD 181433 . This star also has a Jupiter-like planet that orbits it every three years. Planet COROT-7b , with a mass estimated at 4.8 Earth masses and an orbital period of only 0.853 days, was announced on 3 February 2009. The density estimate obtained for COROT-7b points to a composition including rocky silicate minerals similar to that of

14497-432: 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

14630-416: 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

14763-554: The scope and distribution of planets capable of supporting Earth-like extraterrestrial life and intelligence . 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 the ones that are too extreme (large or small, hot or cold, etc.), and settling on

14896-423: 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

15029-449: The shortest orbit is HD 219134 b , and is Earth's closest known rocky, and transiting, exoplanet. In February 2016, it was announced that NASA 's Hubble Space Telescope had detected hydrogen and helium (and suggestions of hydrogen cyanide ), but no water vapor , in the atmosphere of 55 Cancri e , the first time the atmosphere of a super-Earth exoplanet was analyzed successfully. In August 2016, astronomers announced

15162-447: The size of about 2 Earth radii, it was the largest planet until 2014, which was determined to lack a significant hydrogen atmosphere. On 20 December 2011, the Kepler team announced the discovery of the first Earth-size exoplanets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20 . Planet Gliese 667 Cb (GJ 667 Cb) was announced by HARPS on 19 October 2009, together with 29 other planets, while Gliese 667 Cc (GJ 667 Cc)

15295-454: 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

15428-546: The star where liquid water may be possible on the surface. With Gliese 581c having a mass of at least 5 Earth masses and a distance from Gliese 581 of 0.073 astronomical units (6.8 million mi, 11 million km), it is on the "warm" edge of the habitable zone around Gliese 581 with an estimated mean temperature (without considering effects from an atmosphere) of −3 degrees Celsius with an albedo comparable to Venus and 40 degrees Celsius with an albedo comparable to Earth. Subsequent research suggested Gliese 581c had likely suffered

15561-603: 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")

15694-506: 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

15827-505: 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

15960-470: 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

16093-439: 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

16226-553: The term Super-Earth might be limited to rocky planets without a significant atmosphere, or planets that have not just atmospheres but also solid surfaces or oceans with a sharp boundary between liquid and atmosphere, which the four giant planets in the Solar System do not have. Planets above 10 Earth masses are termed massive solid planets , mega-Earths , or gas giant planets , depending on whether they are mostly made of rock and ice or mostly gas. The first super-Earths were discovered by Aleksander Wolszczan and Dale Frail around

16359-461: The third, Kepler-440b , is a super-Earth. On 30 July 2015, Astronomy & Astrophysics said they found a planetary system with three super-Earths orbiting a bright, dwarf star. The four-planet system, dubbed HD 219134 , had been found 21 light years from Earth in the M-shaped northern hemisphere of constellation Cassiopeia , but it is not in the habitable zone of its star. The planet with

16492-479: 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

16625-433: 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

16758-452: The upper limit to be rocky (a planet with 2 Earth-radii and 5 Earth-masses with a mean Earth-like core composition would imply that 1/200 of its mass would be in a H/He envelope, with an atmospheric pressure near to 2.0 GPa or 20,000 bar). Whether or not the primitive nebula-captured H/He envelope of a super-Earth is entirely lost after formation also depends on the orbital distance. For example, formation and evolution calculations of

16891-408: The whole planet except for its core. The low densities inferred from observations imply that a fraction of the super-Earth population has substantial H/He envelopes, which may have been even more massive soon after formation. Therefore, contrary to the terrestrial planets of the solar system, these super-Earths must have formed during the gas-phase of their progenitor protoplanetary disk . Since

17024-491: 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

17157-476: Was detected, along with 42 other planet candidates, in archival data from Kepler by the Planet Hunters project, in which human volunteers analyze the light curves of Kepler target stars, searching for planetary transit signals which may be missed by computer programs. Previous work by Planet Hunters helped to confirm the existence of PH1b , a Neptune -mass planet within a four-star system. All of

17290-416: 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

17423-401: Was included in a paper published on 21 November 2011. More detailed data on Gliese 667 Cc were published in early February 2012. In September 2012, the discovery of two planets orbiting Gliese 163 was announced. One of the planets, Gliese 163 c , about 6.9 times the mass of Earth and somewhat hotter, was considered to be within the habitable zone . On 7 January 2013, astronomers from

17556-478: Was itself a "borderline" case, just barely large enough to sustain plate tectonics. These findings were corroborated by van Heck et al., who determined that plate tectonics may be more likely on super-Earths than on Earth itself, assuming similar composition. However, other studies determined that strong convection currents in the mantle acting on strong gravity would make the crust stronger and thus inhibit plate tectonics. The planet's surface would be too strong for

17689-618: 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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