# data[].attributes.body
-Πιθανότητες νερού
-Έχει παρόμοια θερμοκρασία και μέγεθος με αυτά της Γης
-Δεν έχει εποχές
-Είναι 2,3 φορές η μάζα της Γης και η ακτίνα του είναι 1,34 φορές αυτή της Γης
-Έχει θερμοκρασία 233Κ

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-Λίγο πιο θερμός από τη Γη λόγω της ηλεκτρομαγνητικής ακτινοβολίας
-Έχει μια ζεστή και μια κρύα πλευρά με πιθανότητες ζωής στην μέση
-Μεγαλύτερο κομμάτι του πλανήτη θα μπορούσε να κατοικηθεί αν είχε πυκνή ατμόσφαιρα για να θερμαίνεται η κρύα πλευρά

# data[].attributes.body
-Πιθανώς να έχει νερό
-Είναι ο πρώτος στη λίστα για κατοικήσιμους πλανήτες όσων αφορά την ακτίνα και την αστρική ροή
-Πάρα πολλές καταιγίδες
-Υπερβολική ακτινοβολία

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-Άγνωστα ατμοσφαιρικά χαρακτηριστικά
-Θερμοκρασία πάνω από 273Κ
-Δεν έχει εποχές λόγω της μικρής αξονικής στροφής
-Είναι 1,4 φορές η μάζα της Γης και η ακτίνα του είναι 1,17 φορές αυτή της Γης

# data[].attributes.body
-Πιθανώς να είναι βραχώδης λόγω της μικρής ακτίνας του
-Η μάζα του είναι 5 φορές αυτή της Γης κάτι το οποίο δεν επιτρέπει την ύπαρξη του φαινομένου του θερμοκηπίου
-Υπάρχει ηφαιστιακή δραστηριότητα. Αυτό θα βοηθούσε στο να αναπτύσεται ζωή
-Η ακτίνα του είναι 1,5 φορές αυτή της Γης
-Έχει θερμοκρασία 265

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-Ακτινοβολία 0,05 φορές αυτή της Γης
-Ακτίνα 0,07 φορές αυτή της Γης
-Είναι βραχώδης
-Πιθανότητες νερού σε ωκεανούς
-Έχει την ίδια πυκνότητα με την Γη
-Η μάζα του είναι 2,8 φορές αυτή της Γης
-Πιθανώς να έχει κάποιον δορυφόρο αφού δείχνει στοιχεία παλλίροιας
-Λόγω της μικρής ποσότητας CO2 υπάρχει πιθανότητα να είναι όλο πάγος
-Έχει περίπου την ίδια περιστροφή με αυτή της Γης

# data[].attributes.body
-Βραχώδης πλανήτης
-Η μάζα του είναι περίπου 4,3 φορές αυτή της Γης
-Η ακτίνα είναι 0,23 φορές η ακτίνα της Γης
-Η βαρύτητα είναι 1,6 φορές αυτή της Γης
-Ανακαλύφθηκε 17 Δεκεμβρίου 2015
-Η θερμοκρασία είναι 223Κ (-60C, -58C), λίγο πιο υψηλή από αυτή του Άρη
-Η μια πλευρά είναι κρύα ενώ η άλλη ζεστή. Υπάρχει πιθανότητα νερού στην μέση

# data[].attributes.body
-Σωστές ατμοσφαιρικές συνθήκες
-Περίπτωση νερού στην επιφάνεια
-Η ακτίνα είναι 0,92 φορές η ακτίνα της Γης
-Πιθανώς να είναι βραχώδης

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Extraterrestrial liquid water in the Solar System is likely uncommon, although it has been hypothesized to exist in some of its moons, and to have formerly existed on Mars and Venus. Extrasolar liquid water has not yet been found to exist. The following list contains candidates from the list of confirmed objects that meet the following criteria:

Confirmed object orbiting within a circumstellar habitable zone of Earth mass or greater (because smaller objects may not have the gravitational means to retain water) but not a star

Has been studied for more than a year

Confirmed surface with strong evidence for it being either solid or liquid

Water vapour detected in its atmosphere

Gravitational, radio or differentation models that predict a wet stratum

Most known extrasolar planetary systems appear to have very different compositions from the Solar System, though there is sampling bias arising from the detection methods.

The goal of current searches is to find Earth-sized planets in the habitable zone of their planetary systems (also called the Goldilocks zone). Planets with oceans could include Earth-sized moons of giant planets, though it remains speculative whether such 'moons' really exist. The Kepler telescope might be sensitive enough to detect them. But there is evidence that rocky planets hosting water may be commonplace throughout the Milky Way.

Image

Planet

Star

Mass
(x M⊕)

Temp.
(surf. mean, K)

Distance
(ly)

Age
(gy)

Orbit
(days)

Discovery
(year)

Notes

Earth

Sun

1

288

0

4.54

365

For comparison.

Description (alphabetical order)

Gliese 581 c, d and g

Later work suggests that Gliese 581 c would probably be too hot for liquid water. It was then suggested that Gliese 581 d might be warm enough for oceans if a greenhouse effect was operating. Gliese 581 d is eight times the mass of the Earth and might have a thick atmosphere.

Gliese 581 d looks an even better candidate. The orbital period was originally estimated at 83 days and has now been revised to 66 days. This was announced along with another new world, Gliese 581 e, which is next to twice the mass of Earth but too close to its sun for liquid water. In May 2011, a new study suggested that the planet might have a thick atmosphere, oceans and even life.

The unconfirmed planet Gliese 581 g is another good candidate. This planet is estimated to be between three and four times as massive as the earth, and as such it is too small to be a gas giant. The orbital period is estimated at 37 days, which places its orbit right in the middle of the habitable zone of the star Gliese 581.

Gliese 667 Cc was originally described as one of two 'super-Earth' planets around Gliese 667 C, a dim red star that is part of a triple star system. The stars of this system have a concentration of heavy elements only 25% that of our Sun's. Such elements are the building blocks of terrestrial planets so it was thought to be unusual for such star systems to have an abundance of low mass planets. It seems that habitable planets can form in a greater variety of environments than previously believed.

Gliese 667 Cc, in a tight 28-day orbit of a dim red star, must receive 90% of the light that Earth receives, but most of its incoming light is in the infrared, so a higher percentage of this incoming energy should be absorbed by the planet. The planet is expected to absorb about the same amount of energy from its star that Earth absorbs from the Sun, which would allow surface temperatures similar to Earth and perhaps liquid water.

Further work published in June 2013 suggests that the system has six planets, and that three of them are in the habitable zone.

HD 28185 b was the first exoplanet to be detected in the habitable zone. The planet has only been detected indirectly, but is believed to be a gas giant, with no solid surface. Some scientists have argued that it could have moons large and stable enough to have oceans.

HD 85512 b was discovered in August 2011. It is larger than Earth, but small enough to be probably a rocky world. It is on the borders of its star's habitable zone and might have liquid water, and is a potential candidate for a life-supporting world.

MOA-2007-BLG-192Lb is a small planet orbiting a small star. It is about 3 Earth masses, currently the second smallest detected extrasolar planet orbiting a normal star, after Gliese 581 e.

The planet orbits its host star or brown dwarf with an orbital radius similar to that of Venus. But the host is likely to be between 3,000 and 1 million times fainter than the Sun, so the top of the planet's atmosphere is likely to be colder than Pluto. However, the planet is likely to maintain a massive atmosphere that would allow warmer temperatures at lower altitudes. It is even possible that interior heating by radioactive decay would be sufficient to make the surface as warm as the Earth, but theory suggests that the surface may be completely covered by a very deep ocean.

Kapteyn b is a super-Earth orbiting within the habitable zone of Kapteyn's Star, which is 13 light-years away and is 11 billion years old.

The star Kepler-62 has five planets, two of which are the right distance from the star to have liquid water and potentially sustain life.

Kepler-62f is only 40 percent larger than Earth, making it the exoplanet closest to the size of our planet known in the habitable zone of another star. Kepler-62e orbits on the inner edge of the habitable zone and is roughly 60 percent larger than Earth. Both are assumed to be rocky planets, but since the star is 1200 light-years away, it's hard to be sure.

This large rocky planets is one of two known to be orbiting the star Kepler 69, which is similar to our sun. It's believed to be in the star's habitable zone.

It's 70% more massive than the Earth and has a 242-day orbit, similar to that of Venus.

NASA announced its discovery on 18 April 2013, along with the two Earth-like planets of Kepler 62.

Kepler (other results)

Among the 1,235 possible extrasolar planet candidates detected by NASA's planet-hunting Kepler space telescope during its first four months of operation, 54 are orbiting in the parent star's habitable 'Goldilocks' zone where liquid water could exist. Five of these are near Earth-size, and the remaining 49 habitable zone candidates range from twice the size of Earth to larger than Jupiter.

Proxima Centauri b, the nearest known exoplanet, is in the habitable zone of its host star, and might contain liquid water. More details about the planet's physical conditions are needed for a proper evaluation of its habitability.

In February 2017, seven planets were discovered in the star system TRAPPIST-1, previously unknown to host any planets. Three of the discovered planets, called TRAPPIST-1e, TRAPPIST-1f and TRAPPIST-1g, are candidates for liquid water. They are all located towards the outer system (with the closest to the star, TRAPPIST-1e, being within or slightly outside the habitable zone), making them cool planets. TRAPPIST-1e and f are probably tidally locked planets, and if liquid water exists in them, it is located in their respective terminator lines. However, if the planets support a thick enough atmosphere to transfer heat to the sides facing away from the star, much larger portions of them may be habitable.

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List of extrasolar candidates for liquid water

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Extraterrestrial liquid water in the Solar System is likely uncommon, although it has been hypothesized to exist in some of its moons, and to have formerly existed on Mars and Venus. Extrasolar liquid water has not yet been found to exist.

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The list of exoplanets detected by the Kepler space telescope contains bodies with a wide variety of properties, with significant ranges in orbital distances, masses, radii, composition, habitability, and host star type. As of January 2015, the Kepler space telescope and its follow-up observations have detected 2,300 confirmed planets, including hot Jupiters, super-Earths, circumbinary planets, and planets located in the circumstellar habitable zones of their host stars. In addition, Kepler has detected over 3,601 unconfirmed planet candidates and 2,165 eclipsing binary stars.

In addition to detecting planets itself, Kepler has also uncovered the properties of three previously known extrasolar planets. Public Kepler data has also been used by groups independent of NASA, such as the Planet Hunters citizen-science project, to detect several planets orbiting stars collectively known as Kepler Objects of Interest.

Kepler, launched on March 7, 2009, was designed to observe a fixed portion of the sky in visible light and measure the light curves of the various stars in its field of view, looking for planets crossing in front of their host stars via the transit method. Since the launch of the spacecraft, though, both the Kepler team at NASA and independent researchers have found new ways of detecting planets, including the use of the transit timing variation method and relativistic beaming. In addition, gravitational microlensing has been proposed as a method of using Kepler to detect compact objects, such as white dwarfs, neutron stars, and black holes. Kepler has also measured the reflected light from some planets already known, discovering planets undetectable with the transit method as well as improving knowledge of the characteristics of planets already discovered.

On February 26, 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. The exoplanets were found using a statistical technique called " verification by multiplicity". 95% of the discovered exoplanets were smaller than Neptune and four, including Kepler-296f, were less than 2 1/2 the size of Earth and were in habitable zones where surface temperatures are suitable for liquid water.

Histogram of Exoplanets by size - the gold bars represent Kepler's latest newly verified exoplanets (May 10, 2016).

Histogram of Exoplanet Discoveries - gold bar displays new planets "verified by multiplicity" (May 10, 2016).

On May 10, 2016, NASA announced that the Kepler mission has verified 1,284 new planets. Based on some of the planet's sizes, about 550 could potentially be rocky planets. Nine of these orbit in their stars' habitable zone.

All exoplanets discovered lie in one of the three northern constellations of Cygnus, Lyra and Draco, which contain Kepler's photometer's field of view.

Planet type

circumbinary planet

planet orbits a single star in a multiple star system

planet has a circumbinary orbit in a system with more than 2 stars

Kepler community planet[clarification needed ]

None of the above

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List of exoplanets discovered using the Kepler space telescope

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The list of exoplanets detected by the Kepler space telescope contains bodies with a wide variety of properties, with significant ranges in orbital distances, masses, radii, composition, habitability, and host star type. As of January 2015, the Kepler space telescope and its follow-up observations have detected 2,300 confirmed planets, including hot Jupiters, super-Earths, circumbinary planets, and planets located in the circumstellar habitable zones of their host stars.

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Kepler-438b (also known by its Kepler Object of Interest designation KOI-3284.01) is a confirmed near-Earth-sized exoplanet. It is likely rocky. It orbits on the inner edge of the habitable zone of a red dwarf, Kepler-438, about 640 light-years (196 parsecs) from Earth in the constellation Lyra. It receives 1.4 times our solar flux. The planet was discovered by NASA's Kepler spacecraft using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured. NASA announced the confirmation of the exoplanet on 6 January 2015.

Mass, radius and temperature

Kepler-438b is an Earth-sized planet, an exoplanet that has a mass and radius close to that of Earth. It has a radius of 1.12 R⊕, and an unknown mass. It has an equilibrium temperature of 276 K (3 °C; 37 °F), close to that of Earth.

The planet orbits a ( M-type) red dwarf star named Kepler-438. The star has a mass of 0.54 M☉ and a radius of 0.52 R☉, both lower than those of the Sun by almost half. It has a surface temperature of 3748 K and is estimated to be about 4.4 billion years old, only 200 million years younger than the Sun and the Sun has a surface temperature of 5778 K.

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 14.467. Therefore, it is too dim to be seen with the naked eye.

Orbit and possible moons

Kepler-438b orbits its parent star once every 35.2 days. It is likely tidally locked due to its close distance to its star. A search for exomoons by the Hunt for Exomoons with Kepler project around Kepler-438b placed a maximum mass on a hypothetical moon at 29% that of the planet.

The planet was announced as orbiting within the habitable zone of Kepler-438, a region where liquid water could exist on the surface of the planet. However it has been found that this planet is subjected to powerful radiation activity from its parent star every 100 days, much more violent storms than the stellar flares emitted by the Sun and which would be capable of sterilizing life on Earth.

Researchers at the University of Warwick say that Kepler-438b is not habitable due to the large amount of radiation it receives. The question of what makes a planet habitable is much more complex than having a planet located at the right distance from its host star so that water can be liquid on its surface: various geophysical and geodynamical aspects, the radiation, and the host stars plasma environment can influence the evolution of planets and life, if it originated. The planet is more likely to resemble a larger and cooler version of Venus.

In 2009, NASA's Kepler spacecraft was completing observing stars on its photometer, the instrument it uses to detect transit events, in which a planet crosses in front of and dims its host star for a brief and roughly regular period of time. In this last test, Kepler observed stars in the Kepler Input Catalog, including Kepler-62; the preliminary light curves were sent to the Kepler science team for analysis, who chose obvious planetary companions from the bunch for follow-up at observatories. Observations for the potential exoplanet candidates took place between 13 May 2009 and 17 March 2012. After observing the respective transits, which for Kepler-438b occurred roughly every 35 days (its orbital period), it was eventually concluded that a planetary body was responsible for the periodic 35-day transits. The discovery, along with the planetary systems of the stars Kepler-442, Kepler-440 and Kepler-443 were announced on January 6, 2015.

At nearly 460 light-years (140 pc) distant, Kepler-438b is too far from Earth for either current telescopes, or even the next generation of planned telescopes, to accurately determine its mass or whether it has an atmosphere. The Kepler spacecraft can only focus on a small, fixed region of the sky, but the next generation of planet-hunting space telescopes, such as TESS and CHEOPS, will have more flexibility. Exoplanetary systems, with stars less distant than Kepler 438, can then be studied in tandem with the upcoming James Webb Space Telescope and ground-based observatories like the future Square Kilometer Array.

Coordinates: 18h 46m 35.000s, +41° 57′ 03.93″

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Kepler-438b (also known by its Kepler Object of Interest designation KOI-3284.01) is a confirmed near-Earth-sized exoplanet. It is likely rocky. It orbits on the inner edge of the habitable zone of a red dwarf, Kepler-438, about 640 light-years (196 parsecs) from Earth in the constellation Lyra. It receives 1.4 times our solar flux.

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The planet orbits its host star at a distance of about 0.409 AU (61.2 million km; 38.0 million mi) with an orbital period of roughly 112.3 days. It has a mass of around 2.3 and has a radius of about 1.34 times that of Earth. It is one of the more promising candidates for potential habitability, as its parent star is at least 40% less massive than the Sun - thus, it can have a lifespan of about 30 billion years.

The planet was discovered by NASA's Kepler spacecraft using the transit method, in which it measures the dimming effect that a planet causes as it crosses in front of its star. NASA announced the confirmation of the exoplanet on 6 January 2015.

Mass, radius, and temperature

Kepler-442b is a super-Earth, an exoplanet with a mass and radius bigger than Earth's but smaller than the ice giants Uranus and Neptune. It has an equilibrium temperature of 233 K (−40 °C; −40 °F). It has a radius of 1.34 . Because of its radius, it is likely to be a rocky planet with a solid surface. The mass of the exoplanet is estimated to be 2.36 . The surface gravity on Kepler-442b would be 30% stronger than Earth, assuming a rocky composition similar to that of Earth.

The planet orbits a ( K-type) star named Kepler-442. The star has a mass of 0.61 M☉ and a radius of 0.60 R☉. It has a temperature of 4,402 K (4,129 °C; 7,464 °F) and is around 2.9 billion years old, with some uncertainty. In comparison, the Sun is 4.6 billion years old and has a temperature of 5,778 K (5,505 °C; 9,941 °F). The star is somewhat metal-poor, with a metallicity (Fe/H) of −0.37, or 43% of the solar amount. Its luminosity () is 12% that of the Sun.

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 14.76. Therefore, it is too dim to be seen with the naked eye.

Kepler-442b orbits its host star with an orbital period of 112 days. It has an orbital radius of about 0.4 AU (60 million km; 37 million mi) (slightly larger than the distance of Mercury from the Sun, which is approximately 0.38 AU (57 million km; 35 million mi)). It receives about 70% of Earth's sunlight from the Sun.

The planet is in the habitable zone of its star, a region where liquid water could exist on the planet's surface. It is one of the most Earth-like planets yet found in size and temperature. It is just outside the zone (around 0.362 AU (54.2 million km; 33.7 million mi)) in which tidal forces from its host star would be enough to fully tidally lock it. As of July 2018, Kepler-442b was considered the most habitable non-tidally-locked exoplanet discovered.

K-type main-sequence stars are smaller than the Sun and live longer, remaining on the main sequence for 18 to 34 billion years compared to the Sun's estimated lifespan of 10 billion years. Despite these properties, the small M-type and K-type stars can threaten life. Because of their high stellar activity at the beginning of their lives, they emit strong solar winds. The duration of this period is inversely linked to the size of the star. However, because of the uncertainty of the age of Kepler-442, it is likely it may have passed this stage, making Kepler-442b potentially more suitable for habitability.

Because Kepler-442b is closer to its star than Earth is to the Sun, the planet will probably rotate much more slowly than Earth; its day could be weeks or months long (see Tidal effects on rotation rate, axial tilt, and orbit). This is reflected in its orbital distance, just outside of the point where the tidal interactions from its star would be strong enough to tidally lock it. Kepler-442b's axial tilt (obliquity) is likely tiny, in which case it would not have tilt-induced seasons as Earth and Mars do. Its orbit is probably close to circular (eccentricity 0.04), so it will also lack eccentricity-induced seasonal changes like Mars.

One review essay in 2015 concluded that Kepler-442b, Kepler-186f, and Kepler-62f were likely the best candidates for being potentially habitable planets. Also, according to an index developed in 2015, Kepler-442b is even more likely to be habitable than a hypothetical "Earth twin" with physical and orbital parameters matching those of Earth. Going by this index, Earth has a rating of 0.829, but Kepler-442b has a rating of 0.836. The actual habitability is uncertain because Kepler-442b's atmosphere and surface are unknown. The paper introducing the habitability index clarifies that a higher-than-Earth value "does not mean these planets are "more habitable" than Earth".

In 2009, NASA's Kepler spacecraft was completing observing stars on its photometer, the instrument it uses to detect transit events when a planet crosses in front of and dims its host star for a brief and roughly regular period. In this last test, Kepler observed 50,000 stars in the Kepler Input Catalog, including Kepler-442; the telescope sent the preliminary light curves to the Kepler science team for analysis, who chose prominent planetary companions from the bunch for follow-up at observatories. Observations for the potential exoplanet candidates took place between 13 May 2009 and 17 March 2012. After observing the respective transits, which for Kepler-442b occurred roughly every 113 days (its orbital period), the scientists eventually concluded that a planetary body was responsible for the periodic 113-day transits. The discovery, along with the unique planetary systems of the stars Kepler-438 and Kepler-440, was announced on 6 January 2015.

At nearly 370 parsecs (1,200 ly) distant, Kepler-442b is too remote and its star too far for current telescopes or the next generation of planned telescopes to determine its mass or whether it has an atmosphere. The Kepler spacecraft focused on a single small region of the sky. Still, next-generation planet-hunting space telescopes, such as TESS and CHEOPS, will examine nearby stars throughout the sky.

The James Webb Space Telescope and future large ground-based telescopes can then study nearby stars with planets to analyze atmospheres, determine masses, and infer compositions. Additionally, the Square Kilometer Array would significantly improve radio observations over the Arecibo Observatory and Green Bank Telescope.

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The planet orbits its host star at a distance of about 0.409 AU (61.2 million km; 38.0 million mi) with an orbital period of roughly 112.3 days. It has a mass of around 2.3 and has a radius of about 1.34 times that of Earth.

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Gliese 667 Cc (also known as GJ 667Cc, HR 6426Cc, or HD 156384Cc) is an exoplanet orbiting within the habitable zone of the red dwarf star Gliese 667 C, which is a member of the Gliese 667 triple star system, approximately 23.62 light-years (6.8 parsecs, or about 217,000,000,000,000 km) away in the constellation of Scorpius. The exoplanet was found by using the radial velocity method, from radial-velocity measurements via observation of Doppler shifts in the spectrum of the planet's parent star.

Mass, radius and temperature

Gliese 667 Cc is a super-Earth, an exoplanet with a mass and radius greater than that of Earth, but smaller than that of the giant planets Uranus and Neptune. It is heavier than Earth with a minimum mass of about 3.7 Earth masses. The equilibrium temperature of Gliese 667 Cc is estimated to be 277.4 K (4.3 °C; 39.6 °F). It is expected to have a radius of around 1.5 R⊕, dependent upon its composition.

The planet orbits a red dwarf ( M-type) star named Gliese 667 C, orbited by a total of two planets. The star is part of a trinary star system, with Gliese 667 A and B both being more massive than the smaller companion. Gilese 667 C has a mass of 0.31 M☉ and a radius of 0.42 R☉. It has a temperature of 3700 K, but its age is poorly constrained, estimates place it greater than 2 billion years old. In comparison, the Sun is 4.6 billion years old and has a surface temperature of 5778 K. This star is radiating only 1.4% of the Sun's luminosity from its outer atmosphere. It is known to have a system of two planets: claims have been made for up to seven, but these may be in error due to failure to account for correlated noise in the radial velocity data. Since red dwarfs emit little ultraviolet light, the planets likely receive minimal amounts of ultraviolet radiation.

Gliese 667 Cc is the second confirmed planet out from Gliese 667 C, orbiting along the middle of the habitable zone. From its surface, the star would have an angular diameter of 1.24 degrees and would appear to be 2.3 times the visual diameter of our Sun as it appears from the surface of the Earth. Gliese 667 C would have a visual area 5.4 times greater than that of the Sun but would still only occupy 0.003 percent of Gliese 667 Cc's sky sphere or 0.006 percent of the visible sky when directly overhead.

The apparent magnitude of the star is 10.25, giving it an absolute magnitude of about 11.03. It is too dim to be seen from Earth with the naked eye, and even smaller telescopes cannot resolve it against the brighter light from Gliese 667 A and B.

The orbit of Gliese 667Cc has a semi-major axis of 0.1251 astronomical units, making its year 28.155 Earth-days long. Based on its host star's bolometric luminosity, GJ 667 Cc would receive 90% of the light Earth does; however, a good part of that electromagnetic radiation would be in the invisible infrared part of the spectrum.

Based on black body temperature calculation, GJ 667 Cc should absorb similar but slightly more overall electromagnetic radiation than Earth, making it a little bit warmer (277.4 K (4.3 °C; 39.6 °F)) and consequently placing it slightly closer to the "hot" inner edge of the habitable zone than Earth (254.3 K (−18.8 °C; −1.9 °F)). According to PHL, Gliese 667 Cc is (as of July 2018) the fourth most Earth-like exoplanet located in the conservative habitable zone of its parent star.

Its host star is a red dwarf, with about a third as much mass as the Sun. As a result, stars like Gliese 667 C have the ability to live up to 100-150 billion years, 10-15 times longer than the Sun will live.

The planet is likely tidally locked, with one side of its hemisphere permanently facing towards the star, while the opposite side is shrouded in eternal darkness. However, between these two intense areas, there would be a sliver of habitability - called the terminator line, where the temperatures may be suitable (about 273 K (0 °C; 32 °F)) for liquid water to exist. Additionally, a much larger portion of the planet may be habitable if it supports a thick enough atmosphere to transfer heat to the side facing away from the star.

However, in a 2013 paper, it was revealed that Gliese 667 Cc is subject to tidal heating 300 times that of Earth. This in part is due to its small eccentric orbit around the host star. Because of this, the chances of habitability may be lower than originally estimated.

Gliese 667 Cc was first announced in a pre-print made public on 21 November 2011 by the European Southern Observatory's High Accuracy Radial Velocity Planet Searcher (HARPS) group using the radial velocity method (Doppler method). The announcement of a refereed journal report came on 2 February 2012 by researchers at the University of Göttingen and the Carnegie Institution for Science and backing up the ESO HARPS group discovery.

Gliese 667 Cc features in the story "The Audience" by Sean McMullen in the June 2015 issue of Analog Science Fiction and Fact. In the Alien vs. Predator franchise, Gliese 667 Cc was the first planet to be terraformed, being done so by the Weyland Corporation in 2039. It is also mentioned in the 2015 novel Not Alone by Craig A. Falconer. This planet is also featured in Arkwright.

Coordinates: 17h 18m 57.16483s, −34° 59′ 23.1416″

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Gliese 667 Cc (also known as GJ 667Cc, HR 6426Cc, or HD 156384Cc) is an exoplanet orbiting within the habitable zone of the red dwarf star Gliese 667 C, which is a member of the Gliese 667 triple star system, approximately 23.62 light-years (6.8 parsecs, or about 217,000,000,000,000 km) away in the constellation of Scorpius.

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Kepler-186f (also known by its Kepler object of interest designation KOI-571.05) is an exoplanet orbiting the red dwarf Kepler-186, about 582 light-years (178.5 parsecs, or nearly km) from the Earth. It is the first planet with a radius similar to Earth's to be discovered in the habitable zone of another star. NASA's Kepler space telescope detected it using the transit method, along with four additional planets orbiting much closer to the star (all modestly larger than Earth). Analysis of three years of data was required to find its signal. The results were presented initially at a conference on 19 March 2014 and some details were reported in the media at the time. The public announcement was on 17 April 2014, followed by publication in .

Mass, radius and temperature

The only physical property directly derivable from the observations (besides the orbital period) is the ratio of the radius of the planet to that of the central star, which follows from the amount of occultation of stellar light during a transit. This ratio was measured to be 0.021, giving a planetary radius of 1.11±0.14 times that of Earth, The planet is about 11% larger in radius than Earth (between 4.5% smaller and 26.5% larger), giving a volume about 1.37 times that of Earth (between 0.87 and 2.03 times as large).[citation needed ]

A very wide range of possible masses can be calculated by combining the radius with densities derived from the possible types of matter from which planets can be made. For example, it could be a rocky terrestrial planet or a lower density ocean planet with a thick atmosphere. A massive hydrogen/ helium (H/He) atmosphere is thought to be unlikely in a planet with a radius below 1.5 R⊕. Planets with a radius of more than 1.5 times that of Earth tend to accumulate the thick atmospheres which make them less likely to be habitable. Red dwarfs emit a much stronger extreme ultraviolet (XUV) flux when young than later in life. The planet's primordial atmosphere would have been subjected to elevated photoevaporation during that period, which would probably have largely removed any H/He-rich envelope through hydrodynamic mass loss.

Mass estimates range from 0.32 M⊕ for a pure water/ice composition to 3.77 M⊕ if made up entirely of iron (both implausible extremes). For a body with radius 1.11 R⊕, a composition similar to that of Earth (i.e., 1/3 iron, 2/3 silicate rock) yields a mass of 1.44 M⊕, taking into account the higher density due to the higher average pressure compared to Earth.[citation needed ]

The estimated equilibrium temperature for Kepler-186f, which is the surface temperature without an atmosphere, is said to be around 188 K (−85 °C; −121 °F), somewhat colder than the equilibrium temperature of Mars.

The planet orbits Kepler-186, an M-type star which has a total of five planets. The star has a mass of 0.54 M☉ and a radius of 0.52 R☉. It has a temperature of 3755 K and is about 4 billion years old, about 600 million years younger than the Sun, which is 4.6 billion years old and has a temperature of 5778 K.

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 14.62. This is too dim to be seen with the naked eye, which can only see objects with a magnitude up to at least 6 or lower.

Kepler-186f orbits its star with about 5% of the Sun's luminosity with an orbital period of 129.9 days and an orbital radius of about 0.40 times that of Earth's (compared to 0.39 AU for Mercury). The habitable zone for this system is estimated conservatively to extend over distances receiving from 88% to 25% of Earth's illumination (from 0.23 to 0.46 AU). Kepler-186f receives about 32%, placing it within the conservative zone but near the outer edge, similar to the position of Mars in our Solar System.

Kepler-186f's location within the habitable zone does not ensure it is habitable; this is also dependent on its atmospheric characteristics, which are unknown. However, Kepler-186f is too distant for its atmosphere to be analyzed by existing telescopes (e.g., NESSI) or next-generation instruments such as the James Webb Space Telescope. A simple climate model - in which the planet's inventory of volatiles is restricted to nitrogen, carbon dioxide and water, and clouds are not accounted for - suggests that the planet's surface temperature would be above 273 K (0 °C; 32 °F) if at least 0.5 to 5 bars of CO ₂ is present in its atmosphere, for assumed N ₂ partial pressures ranging from 10 bar to zero, respectively.

The star hosts four other planets discovered so far, although Kepler-186 b, c, d, and e (in order of increasing orbital radius), being too close to their star, are considered too hot to have liquid water. The four innermost planets are probably tidally locked, but Kepler-186f is in a higher orbit, where the star's tidal effects are much weaker, so the time could have been insufficient for its spin to slow down significantly. Because of the very slow evolution of red dwarfs, the age of the Kepler-186 system was poorly constrained, although it is likely to be greater than a few billion years. Recent results have placed the age at around 4 billion years. The chance that it is tidally locked is approximately 50%. Since it is closer to its star than Earth is to the Sun, it will probably rotate much more slowly than Earth; its day could be weeks or months long (see Tidal effects on rotation rate, axial tilt and orbit).

Kepler-186f's axial tilt (obliquity) is likely very small, in which case it would not have tilt-induced seasons like Earth's. Its orbit is probably close to circular, so it will also lack eccentricity-induced seasonal changes like those of Mars. However, the axial tilt could be larger (about 23 degrees) if another undetected nontransiting planet orbits between it and Kepler-186e; planetary formation simulations have shown that the presence of at least one additional planet in this region is likely. If such a planet exists, it cannot be much more massive than Earth as it would then cause orbital instabilities.

One review assay in 2015 concluded that Kepler-186f, along with the exoplanets Kepler-442b and Kepler-62f, were likely the best candidates for being potentially habitable planets.

In June 2018, studies suggest that Kepler-186f may have seasons and a climate similar to those on Earth.

As part of the SETI Institute's search for extraterrestrial intelligence, the Allen Telescope Array had listened for radio emissions from the Kepler-186 system for about a month as of 17 April 2014. No signals attributable to extraterrestrial technology were found in that interval, however to be detectable such transmissions, if radiated in all directions equally and thus not preferentially towards the Earth, would need to be at least 10 times as strong as those from Arecibo Observatory. Another search, undertaken at the crowdsourcing project SETILive, reports inconclusive but optimistic-looking signs in the radio noise from the Allen Array observations. The more well known Seti@Home search does not cover any object in the Kepler field of view. Another follow-up survey using the Green Bank Telescope has not reviewed Kepler 186f. Given the interstellar distance of 490 light-years (151 pc), the signals would have left the planet many years ago.

At nearly 582 light-years (178.5 pc) distant, Kepler-186f is too far and its star too faint for current telescopes or the next generation of planned telescopes to determine its mass or whether it has an atmosphere. However, the discovery of Kepler-186f demonstrates conclusively that there are other Earth-sized planets in habitable zones. The Kepler spacecraft focused on a single small region of the sky but next-generation planet-hunting space telescopes, such as TESS and CHEOPS, will examine nearby stars throughout the sky. Nearby stars with planets can then be studied by the upcoming James Webb Space Telescope and future large ground-based telescopes to analyze atmospheres, determine masses and infer compositions. Additionally the Square Kilometer Array would significantly improve radio observations over the Arecibo Observatory and Green Bank Telescope.

As the Kepler telescope observational campaign proceeded, an initially identified system was entered in the Kepler Input Catalog (KIC), and then progressed as a candidate host of planets to a Kepler Object of Interest (KOI). Thus, Kepler 186 started as KIC 8120608 and then was identified as KOI 571. Kepler 186f was mentioned when known as KOI-571-05 or KOI-571.05 or using similar nomenclatures in 2013 in various discussions and publications before its full confirmation.

The nearest-to-Earth-size planet in a habitable zone previously known was Kepler-62f with 1.4 Earth radii. Kepler-186f orbits an M-dwarf star, while Kepler-62f orbits a K-type star. A study of atmospheric evolution in Earth-size planets in habitable zones of G-Stars (a class containing the Sun, but not Kepler-186) suggested that 0.8-1.15 R ⊕ is the size range for planets small enough to lose their initial accreted hydrogen envelope but large enough to retain an outgassed secondary atmosphere such as Earth's.

Coordinates: 19h 54m 36.651s, +43° 57′ 18.06″

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Kepler-186f (also known by its Kepler object of interest designation KOI-571.05) is an exoplanet orbiting the red dwarf Kepler-186, about 582 light-years (178.5 parsecs, or nearly km) from the Earth. It is the first planet with a radius similar to Earth's to be discovered in the habitable zone of another star.

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Kepler-62f (also known by its Kepler Object of Interest designation KOI-701.04 and as Lekan's planet) is a super-Earth exoplanet orbiting within the habitable zone of the star Kepler-62, the outermost of five such planets discovered around the star by NASA's Kepler spacecraft. It is located about 990 light-years (300 parsecs) from Earth in the constellation of Lyra.

Kepler-62f orbits its star at a distance of 0.718 AU (107,400,000 km; 66,700,000 mi) from its host star with an orbital period of roughly 267.3 days, has a mass at least 2.8 times that of Earth, and has a radius of around 1.41 times that of Earth. It is one of the more promising candidates for potential habitability, as its parent star is a relatively quiet star, and has less mass than the Sun - thus it can live up to a span of about 30 billion years or so. Due to its mass, Kepler-62f is likely a terrestrial or ocean-covered planet. However, key components of the exoplanet still need to be assessed to determine habitability; such as its atmosphere if one exists, since it lies within the outer part of its host star's habitable zone.

The discovery of the exoplanet (along with Kepler-62e) was announced in April 2013 by NASA as part of the Kepler spacecraft data release. The exoplanet was found by using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured. According to scientists, it is a potential candidate to search for extraterrestrial life, and was chosen as one of the targets to study by the Search for Extraterrestrial Intelligence (SETI) program.

Mass, radius and temperature

Kepler-62f is a super-Earth, an exoplanet with a radius and mass bigger than Earth, but smaller than that of the ice giants Neptune and Uranus. It has an equilibrium temperature of 208 K (−65 °C; −85 °F), close to that of Mars. It has a radius of 1.4 , placing it below the radius of ≥1.6 R⊕ where it would otherwise be a mini-Neptune with a volatile composition, with no solid surface. Due to its radius, it is likely a rocky planet. However, the mass isn't constrained yet, estimates place an upper limit of <35 , the real mass is expected to be significantly lower than this. The true value is likely around 2.8 M⊕, assuming a rocky composition.

The planet orbits a ( K-type) star named Kepler-62, orbited by a total of five known planets. The star has a mass of 0.69 and a radius of 0.64 . It has a temperature of 4925 K and is 7 billion years old. In comparison, the Sun is 4.6 billion years old and has a temperature of 5778 K. The star is somewhat metal-poor, with a metallicity ([Fe/H]) of −0.37, or 42% of the solar amount. Its luminosity () is 21% that of the Sun.

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 13.65. Therefore, it is too dim to be seen with the naked eye.

Kepler-62f orbits its host star every 267.29 days at a semi-major axis distance of about 0.718 astronomical units (107,400,000 km, 66,700,000 mi), which is roughly the same as Venus's semi-major axis from the Sun. Compared to Earth, this is about seven-tenths of the distance from it to the Sun. Kepler-62f is estimated to receive about 41% of the amount of sunlight that Earth does from the Sun, which is comparable to Mars, which receives 43%.

Given the planet's age (7 ± 4 billion years), irradiance (0.41 ± 0.05 times Earth's) and radius (1.41 ± 0.07 times Earth's), a rocky (silicate-iron) composition with the addition of a possibly substantial amount of water is considered plausible. A modeling study indicates it is likely that a great majority of planets in its size range are completely covered by ocean. If its density is the same as Earth's, its mass would be 1.41 ³ or 2.80 times Earth's. The planet has the potential for hosting a moon according to a study of tidal effects on potentially habitable planets. The planet may be the only habitable-zone candidate which would avoid desiccation by irradiation from the host star at its current location.

Although Kepler-62f may be an ocean-covered planet possessing rock and water at the surface, it is the farthest out from its star, so without a supplementary amount of carbon dioxide (), it may be a planet covered entirely in ice. In order for Kepler-62f to sustain an Earth-like climate (with an average temperature of around 284-290 K (11-17 °C; 52-62 °F), at least 5 bars (4.9 atm) of carbon dioxide would have to be present in the planet's atmosphere.

Because it is the outermost planet of its star system, the effects of tidal evolution from the inner planets and the host star on Kepler-62f are not likely to have had significant outcomes over its lifetime. The axial tilt is likely to have been unchanged, and thus, the planet may have an axial tilt (anywhere from 14°-30°) and rotational period somewhat similar to Earth. This can further make the planet more sustainable for habitability, as it would be able to transfer heat to the night side, instead of it being a planet with its surface being half water and half ice. K-type stars like Kepler-62 can live for approximately 20-40 billion years, 2 to 4 times longer than the estimated lifetime of our Sun. The low stellar activity of orange dwarfs like Kepler-62, creates a relatively benign radiation environment for planets orbiting in their habitable zones, increasing their potential habitability.

One review assay in 2015 concluded that Kepler-62f, along with the exoplanets Kepler-186f and Kepler-442b, were likely the best candidates for being potentially habitable planets.

On 13 May 2016, researchers at University of California, Los Angeles (UCLA) announced that they had found various scenarios that allow the exoplanet to be habitable. They tested several simulations based on Kepler-62f having an atmosphere that ranges in thickness from the same as Earth's all the way up to 12 times thicker than our planet's, various concentrations of carbon dioxide in its atmosphere, ranging from the same amount as is in the Earth's atmosphere up to 2,500 times that level and several different possible configurations for its orbital path.

In June 2018, studies suggest that Kepler-62f may have seasons and a climate similar to those on Earth.

There is another possibility. Kepler-62f and Kepler-62e are very close to each other. It has been estimated the trip from one to another planet would take only 52 days. So, the panspermia is quite possible.

Discovery, follow-up studies & SETI target

NASA's Kepler spacecraft observed stars in the Kepler Input Catalog, including Kepler-62, between 13 May 2009 and 17 March 2012. The software pipeline that searched for periodic dip in the stellar brightness, the sign of a planetary transit of the star, initially found three planets around Kepler-62, including Kepler-62e. Due to a bug in the software pipeline, the planet 62f was missed. Eric Agol, a Professor of Astronomy at the University of Washington, discovered three additional transits that had been missed by the pipeline, which occurred every 267 days, and with a more detailed analysis the Kepler team concluded that a fourth planetary body, 62f, was responsible for the periodic 267-day transits. The discovery, along with the planetary system of the star Kepler-69 were announced on April 18, 2013.

On 9 May 2013, a congressional hearing by two U.S. House of Representatives subcommittees discussed " Exoplanet Discoveries: Have We Found Other Earths?," prompted by the discovery of exoplanet Kepler-62f, along with Kepler-62e and Kepler-69c. A related special issue of the journal Science, published earlier, described the discovery of the exoplanets.

Kepler-62f and the other Kepler-62 exoplanets are being specially targeted as part of the Search for Extraterrestrial Intelligence (SETI) search programs. They will scan the areas for any signals that may represent technological life in the system. Given the interstellar distance of 990 light-years (300 pc), the signals would have left the planet that many years ago. As of 2016, no such signals have been found.

At nearly 300 parsecs (990 ly) distant, Kepler-62f is too remote and its star too far for current telescopes or the next generation of planned telescopes to determine its mass or whether it has an atmosphere. The Kepler spacecraft focused on a single small region of the sky but next-generation planet-hunting space telescopes, such as TESS and CHEOPS, will examine nearby stars throughout the sky.

Nearby stars with planets can then be studied by the upcoming James Webb Space Telescope and future large ground-based telescopes to analyze atmospheres, determine masses and infer compositions. Additionally the Square Kilometer Array would significantly improve radio observations over the Arecibo Observatory and Green Bank Telescope.

Coordinates: 18h 52m 51.06019s, +45° 20′ 59.507″

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Kepler-62f (also known by its Kepler Object of Interest designation KOI-701.04 and as Lekan's planet) is a super-Earth exoplanet orbiting within the habitable zone of the star Kepler-62, the outermost of five such planets discovered around the star by NASA's Kepler spacecraft. It is located about 990 light-years (300 parsecs) from Earth in the constellation of Lyra.

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Kepler-452b (a planet sometimes quoted to be an Earth 2.0 or Earth's Cousin based on its characteristics; also known by its Kepler Object of Interest designation KOI-7016.01) is an exoplanet orbiting the Sun-like star Kepler-452 about 1,402 light-years (430 pc) from Earth in the constellation Cygnus. It was identified by the Kepler space telescope, and its discovery was announced by NASA on 23 July 2015. However, a study in 2018 by Mullally et al. implied that statistically, Kepler-452b has not been proven to exist and must still be considered a candidate. It is the first potentially rocky super-Earth planet discovered orbiting within the habitable zone of a star very similar to the Sun.

The planet is about 1,400 light-years away from the Solar System. At the speed of the New Horizons spacecraft, about 59,000 km/h (37,000 mph), it would take approximately 26 million years to get there.

Mass, radius and temperature

Kepler-452b has a probable mass five times that of Earth, and its surface gravity is twice Earth's, though calculations of mass for exoplanets are only rough estimates. If it is a terrestrial planet, it is most likely a super-Earth with many active volcanoes due to its higher mass and density. The clouds on the planet would be thick and misty, covering much of the surface as viewed from space.

The planet takes 385 Earth days to orbit its star. Its radius is 50% bigger than Earth's, and lies within the conservative habitable zone of its parent star. It has an equilibrium temperature of 265 K (−8 °C; 17 °F), a little warmer than Earth.

The host star, Kepler-452, is a G-type star that is about the same mass of the Sun, only 3.7% more massive and 11% larger. It has a surface temperature of 5757 K, nearly the same as the Sun, which has a surface temperature of 5778 K. The star's age is estimated to be about 6 billion years old, about 1.4 billion years older than the Sun, which is 4.6 billion years old. From the surface of Kepler-452b, its star would look almost identical to the Sun as viewed from the Earth.

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 13.426; therefore, it is too dim to be seen with the naked eye.

Kepler-452b orbits its host star with an orbital period of 385 days and an orbital radius of about 1.04 AU, nearly the same as Earth's (1 AU). Kepler-452b is most likely not tidally locked, and has a circular orbit. Its host star, Kepler-452, is about 20% more luminous than the Sun (L = 1.2 L☉).

It is not known if Kepler-452b is a rocky planet but based on its small radius, Kepler-452b is likely to be rocky. It is not clear if Kepler-452b offers habitable environments. It orbits a G2V-type star, like the Sun, which is 20% more luminous, with nearly the same temperature and mass. However, the star is six billion years old, making it 1.4 billion years older than the Sun. At this point in its star's evolution, Kepler-452b is receiving 10% more energy from its parent star than Earth is currently receiving from the Sun. If Kepler-452b is a rocky planet, it may be subject to a runaway greenhouse effect similar to that seen on Venus. However, due to the planet being 60% bigger than Earth, it is likely to have an estimated mass of 5 M⊕, which could allow it to hold on to any oceans it may have for a longer period, preventing Kepler-452b from succumbing to runaway greenhouse effect for another 500 million years. This in turn would be accompanied with the carbonate-silicate cycle being "buffered" extending its lifetime due to increased volcanic activity on Kepler-452b. This could allow any potential life on the surface to inhabit the planet for another 500-900 million years before the habitable zone is pushed out of Kepler-452b's orbit.

In 2009, NASA's Kepler spacecraft was observing stars on its photometer, the instrument it uses to detect transit events, in which a planet crosses in front of and dims its host star for a brief and roughly regular period of time. In this last test, Kepler observed stars in the Kepler Input Catalog, including Kepler-452; the preliminary light curves were sent to the Kepler science team for analysis, who chose obvious planetary companions from the bunch for follow-up by other telescopes. Observations for the potential exoplanet candidates took place between 13 May 2009 and 17 March 2012. Kepler-452b exhibited a transit which occurred roughly every 385 days, and it was eventually concluded that a planetary body was responsible. The discovery was announced by NASA on 23 July 2015.

At a distance of nearly 1,400 light-years (430 pc), Kepler-452b is too remote for current telescopes or the next generation of planned telescopes to determine its true mass or whether it has an atmosphere. The Kepler spacecraft focused on a single small region of the sky but next-generation planet-hunting space telescopes, such as TESS and CHEOPS, will examine nearby stars throughout the sky with follow up studies planned for these closer exoplanets by the upcoming James Webb Space Telescope and future large ground-based telescopes to analyze their atmospheres, determine masses and infer compositions.

SETI targeting

Scientists with the SETI (Search for Extraterrestrial Intelligence) Institute have already begun targeting Kepler-452b, the first near-Earth-size world found in the habitable zone of a Sun-like star. SETI Institute researchers are using the Allen Telescope Array, a collection of 6-meter (20 feet) telescopes in the Cascade Mountains of California, to scan for radio transmissions from Kepler-452b. As of July 2015, the array has scanned the exoplanet on over 2 billion frequency bands, with no result. The telescopes will continue to scan over a total of 9 billion channels, searching for alien radio signals.

Kepler-452b is 1,400 light-years (1.3×10 ²¹ centimetres; 89,000,000 astronomical units; 8.2×10 ¹⁵ miles) from Earth. The fastest current space craft, the New Horizons unmanned probe that passed Pluto in July 2015, travels at just 56,628 km/h (35,187 mph; 0.00037853 AU/h). At that speed, it would take a spacecraft about 26 million years to reach Kepler-452b from Earth, if it was going in that direction.

Ryan Weed from Positron Dynamics suggests that with a spacecraft accelerating at a constant 1 g up to a velocity of 0.99995 c and due to time dilation we could reach Kepler-452b in 12 years, although from Earth perspective 1,500 years would have passed.

A diagram of the orbit of Kepler-452b within the Kepler-452 system, as compared to the inner Solar System and Kepler-186 system, and their respective projected habitable zones.

This artist's concept compares Earth (left) to Kepler-452b, which is about 50% larger in diameter.

Comparison of Kepler-452b and related exoplanets with Earth

Coordinates: 19h 44m 00.89s, +44° 16′ 39.2″

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Kepler-452b (a planet sometimes quoted to be an Earth 2.0 or Earth's Cousin based on its characteristics; also known by its Kepler Object of Interest designation KOI-7016.01) is an exoplanet orbiting the Sun-like star Kepler-452 about 1,402 light-years (430 pc) from Earth in the constellation Cygnus.

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Wolf 1061c or WL 1061c is an exoplanet orbiting within the habitable zone of the red dwarf star Wolf 1061 in the constellation Ophiuchus, about 13.8 light years from Earth, making it the fifth closest known, potentially habitable, and confirmed exoplanet to Earth (after Proxima Centauri b, Ross 128 b, Luyten b and Tau Ceti e), yielding interest from astronomers. It is the second planet in order from its host star in a triple planetary system, and has an orbital period of 17.9 days. Wolf 1061c is classified as a super-Earth exoplanet as its estimated radius is greater than 1.5 R⊕.

Mass, radius and temperature

Wolf 1061c is thought to be a rocky planet estimated to be a super-Earth exoplanet as its mass is about 4.3 times that of Earth and radius is over 1.5 which would give it a density either near or possibly higher than Earth. It has an estimated surface gravity of 1.6 times that on Earth.

In astronomical terms, the Wolf 1061 system is relatively close to Earth, at only 13.8 light years away.

The discovery was announced on 17 December 2015, following a study that used 10 years of archival spectra of the star Wolf 1061 using the HARPS spectrograph attached to the ESO 3.6 m Telescope at the European Southern Observatory at La Silla, Chile.

The planet has an equilibrium temperature of 223 K (−50 °C; −58 °F), slightly higher than that of Mars.

The planet orbits a ( M-type) star named Wolf 1061, orbited by a total of three planets. The star has a mass of 0.25 M☉ and a radius of 0.26 R☉. It has a temperature of 3380 K. The age is poorly constrained/unknown, but estimates would place it around a few billion years. In comparison, the Sun is 4.6 billion years old and has a surface temperature of 5778 K.

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 10.1. Therefore, it is too dim to be seen with the naked eye.

Wolf 1061c orbits its host star with less than 1% of the Sun's luminosity every 17.9 days at a distance of 0.08 AU (compared to Mercury which orbits at a distance of 0.38 AU).

The planet's orbital distance of 0.084 AU (assuming mild eccentricity) lies at the inner edge of its star's habitable zone, which extends from approximately 0.073 to 0.190 AU (for comparison, the habitable zone of the Sun is approximated at 0.5 to 3.0 AU for its different energy emission). Its host star is a red dwarf, with about a quarter as much mass as the Sun. As a result, stars like Wolf 1061 have the ability to burn up to 400-500 billion years, 40-50 times longer than the Sun will.

Because it is so close to the star, it is likely to be tidally locked, meaning one side permanently faces the star and the other side permanently faces away. Although this scenario could result in extreme temperature differences on the planet, the terminator line that separates the illuminated side and the dark side could potentially be habitable, as the temperature there could be suitable for liquid water to exist.[deprecated source ] Additionally, a much larger portion of the planet could also be habitable if it has a thick enough atmosphere to facilitate heat transfer away from the side facing the star.

Coordinates: 16h 30m 18.1s, −12° 39′ 45″

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Wolf 1061c or WL 1061c is an exoplanet orbiting within the habitable zone of the red dwarf star Wolf 1061 in the constellation Ophiuchus, about 13.8 light years from Earth, making it the fifth closest known, potentially habitable, and confirmed exoplanet to Earth (after Proxima Centauri b, Ross 128 b, Luyten b and Tau Ceti e), yielding interest from astronomers.

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The exoplanet was announced to be orbiting within the habitable zone of its parent star, the region where, with the correct conditions and atmospheric properties, liquid water may exist on the surface of the planet. TRAPPIST-1e has a radius of around 0.92 R⊕, so it is very likely rocky. Its host star is a red ultracool dwarf, with only about 8% of the mass of the Sun (close to the boundary between brown dwarfs and hydrogen-fusing stars). As a result, stars like TRAPPIST-1 have the ability to live up to 4–5 trillion years, 400–500 times longer than the Sun will live.[7] Because of this ability to live for long periods of time, it is likely TRAPPIST-1 will be one of the last remaining stars when the Universe is much older than it is now, when the gas needed to form new stars will be exhausted, and the remaining ones begin to die off.

The planet is very likely tidally locked, with one side of its hemisphere permanently facing towards the star, while the opposite side is shrouded in eternal darkness. However, between these two intense areas, there would be a sliver of habitability – called the terminator line, where the temperatures may be suitable (about 273 K (0 °C; 32 °F)) for liquid water to exist. Additionally, a much larger portion of the planet may be habitable if it supports a thick enough atmosphere to transfer heat to the side facing away from the star.

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Τον Φεβρουάριο του 2017 η NASA ανακοίνωσε σε συνέντευξη τύπου ότι το πλανητικό σύστημα αυτού του αστέρα αποτελείται από επτά πλανήτες παρόμοιους με τη Γη, τρεις από τους οποίους (TRAPPIST e, f και g) βρίσκονται στην κατοικήσιμη ζώνη.[24][25][26]. Τα μεγέθη των πλανητών κυμαίνονται ανάμεσα στο μέγεθος του Άρη και της Γης, [20]ενώ οι πυκνότητες τους παίρνουν τιμές από ~0.60 μέχρι ~1.17 της Γης (ρ⊕, 5.51 g/cm3) υποδεικνύοντας κυρίως βραχώδεις συνθέσεις. Οι αποστάσεις των πλανητών από τον TRAPPIST-1 είναι όλες εγγύτερες του Ερμή από τον Ήλιο, ως εκ τούτου οι αποστάσεις μεταξύ τους είναι σχετικά μικρές. Για παράδειγμα ο TRAPPIST-1b απέχει από τον TRAPPIST-1c 1,6 φορές όσο απέχει η Γη από τη Σελήνη.

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It is notable due to its predicted habitability and Earth-like properties, with anEarth Similarity Index of 0.77 and an orbit that places it on the hot inner edge of Tau Ceti's habitable zone. With an a minimum mass of 4.3 Earth masses, it is classified as a super-Earth. Due to its proximity to Tau Ceti (0.552 AU), it may be Venus-like with a very dense atmosphere causing on it a powerful greenhouse effect. If it possesses an Earth-like atmosphere, the surface temperature would be around 70°C (343 K). At this temperature range, the only Earth life that could exist on the surface would be thermophiles. Thus, to make Tau Ceti e able to support humans, methods of cooling it similar to the ones used to terraform Venus could be used, such as a reduction of atmosphere, an increase in albedo, solar shades, or artificial cooling systems in the atmosphere. Oceans and heat tolerant plants and animals are then introduced. The planet may be easy to terraform for the advanced humans that get there, which is feasible if even near-light speed spacecraft are developed, considering it's less than 12 light years away.

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Οι κατοικήσιμες ζώνε

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Οι κατοικήσιμες ζώνες του ηλιακού μας συστήματος και του ηλιακού συστήματος του Gliese