10 exoplanets where can exist extraterrestrial life

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10 exoplanets where can exist extraterrestrial life

Variousmag
Friday, December 6, 2019
In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.

1. Proxima Centauri b

Proxima Centauri b (also called Proxima b or Alpha Centauri Cb) is an exoplanet orbiting in the habitable zone of the red dwarf star Proxima Centauri, which is the closest star to the Sun and part of a triple star system. It is located about 4.2 light-years (1.3 parsecs, 40 trillion km, or 25 trillion miles) from Earth in the constellation of Centaurus, making it the closest known exoplanet to the Solar System. Proxima Centauri b orbits the star at a distance of roughly 0.05 AU (7,500,000 km; 4,600,000 mi) with an orbital period of approximately 11.2 Earth days, and has an estimated mass of at least 1.3 times that of the Earth. Its habitability has not been established, though it is unlikely to be habitable since the planet is subject to stellar wind pressures of more than 2,000 times those experienced by Earth from the solar wind.
The discovery of the planet was announced in August 2016 by the European Southern Observatory. The planet was found using the radial velocity method, where periodic Doppler shifts of spectral lines of the host star suggest an orbiting object. From these readings, the radial velocity of the parent star relative to the Earth is varying with an amplitude of about 1.4 metres (4.5 feet) per second. According to Guillem Anglada‐Escudé, its proximity to Earth offers an opportunity for robotic exploration of the planet with the Starshot project or, at least, "in the coming centuries". Without the inclination of its orbit known, the exact mass of Proxima Centauri b is unknown. If its orbit is nearly edge-on, it would have a mass of 1.27+0.19−0.17 Earth masses. Statistically, there is a roughly 90% chance that the planet's mass is less than 3 M⊕ (Earth masses).

In May 2019, a paper presenting recent Spitzer Space Telescope data concluded that Proxima Centauri b does not transit its sun relative to Earth, and attributed previous transit detections to correlated noise.

2. Gliese 667 Cc

Gliese 667 Cc (also known as GJ 667Cc) 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. 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).
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.

3. Kepler-442b

Kepler-442b is a confirmed near-Earth-sized exoplanet, likely rocky, orbiting within the habitable zone of the K-type main-sequence star Kepler-442, about 1,206 light-years (370 parsecs), from Earth in the constellation Lyra. 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. Kepler-442b is a super-Earth, an exoplanet with a mass and radius bigger than that of Earth, but smaller than that of the ice giants Uranus and Neptune. It has an equilibrium temperature of 233 K (−40 °F). The planet was announced as being located within the habitable zone of its star, a region where liquid water could exist on the surface of the planet. It was described as being one of the most Earth-like planets, in terms of size and temperature, yet found. It is outside of the zone (around 0.02 AU) where tidal forces from its host star would be enough to 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 15 to 30 billion years compared to the Sun's estimated 10 billion. Despite these properties, the small M-type and K-type stars can pose a threat to 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 it 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 very small, in which case it would not have tilt-induced seasons as Earth and Mars do. Its orbit is probably close to circular so it will also lack eccentricity-induced seasonal changes like those of Mars.
One review essay in 2015 concluded that Kepler-442b, along with the exoplanets Kepler-186f and Kepler-62f, were likely the best candidates for being potentially habitable planets.

4. Wolf 1061c

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 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'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.

5. Kepler-1229b

Kepler-1229b is a confirmed super-Earth exoplanet, likely rocky, orbiting within the habitable zone of the red dwarf star Kepler-1229, located about 870 light years (267 parsecs from Earth in the constellation of Cygnus. It was discovered in 2016 by the Kepler space telescope. 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. Kepler-1229b is likely a rocky super-Earth, an exoplanet with a radius and mass bigger than Earth, but smaller than that of the gas giants Neptune and Uranus. It has an equilibrium temperature of 213 K (−60 °C; −76 °F).
The exoplanet, along with eight others, was announced to be orbiting in 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. Its host star is a red dwarf, with about half as much mass than the Sun does. As a result, stars like Kepler-1229 have the ability to live up to 50–60 billion years, 5–6 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 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.

6. Kapteyn b

Kapteyn b is an exoplanet that orbits within the habitable zone of the red subdwarf Kapteyn's star, located approximately 12.8 light-years (3.92 pc) from Earth. Kapteyn b is within the estimated habitable zone of its star. It was the closest suspected potentially habitable exoplanet to the Solar System other than Tau Ceti e up until 2016, when Proxima Centauri b at 4.22 light-years was confirmed.
It was pushed into fourth when Ross 128 b was confirmed in 2017. However, later research had cast doubt on the existence of Kapteyn b, suggesting the signal is consistent with stellar activity rather than a planet. This doubt, however, has been refuted by the original discovery team. The system itself is estimated to be 11 billion years old, substantially older than the Solar System. Kapteyn b is a super-Earth, a planet that has a radius and mass bigger than that of Earth, but smaller than that of the ice giants Uranus and Neptune. It has a surface temperature of 205 K (−68 °C; −91 °F).

The exoplanet was announced to be orbiting in 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. Kapteyn b has a radius range of 1.2–1.6 R⊕, so it is likely rocky. Its host star is a red subdwarf, with a little more than a quarter as much mass then the Sun does. As a result, stars like Kapteyn's Star have the ability to live up to 100–200 billion years, 10–20 times longer than the Sun will live. Another crucial factor in habitability is temperature and atmospheric properties. The estimated equilibrium temperature for Kapteyn b is around 205 K (−68 °C; −91 °F), too cold to support liquid water on the surface. Without the proper greenhouse gases in its atmosphere (if it has one), it is likely to be a planet covered in ice. However, if it has enough CO2 in its atmosphere, the surface temperature may rise enough to have water exist in its liquid form on the surface.

7. Kepler-186f

Kepler-186f is an exoplanet orbiting the red dwarf Kepler-186, about 582 light-years (178.5 parsecs, or nearly 5.298×1015 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 Science. 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.
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 CO2 is present in its atmosphere, for assumed N2 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).

8. Luyten b

Luyten b is a confirmed exoplanet, likely rocky, orbiting within the habitable zone of the nearby red dwarf Luyten's Star. It is one of the most Earth-like planets ever found and is the fourth-closest potentially habitable exoplanet known, at a distance of 12.2 light-years. Only Proxima Centauri b and Barnard's Star b and Ross 128 b are closer. Discovered alongside GJ 273c in June 2017, Luyten b is a Super-Earth of around 3 times the mass of Earth and receives only 6% more starlight than Earth, making it one of the best candidates for habitability. In October 2017 and 2018, the nonprofit organization METI (Messaging Extraterrestrial Intelligence) sent a message containing dozens of short musical compositions and a scientific "tutorial" towards the planet in hopes of contacting any potential extraterrestrial civilizations.
Luyten b is a Super-Earth, meaning that it has a mass and/or radius greater than that of Earth, but less than that of Uranus or Neptune. Luyten b is one of the most Earth-like planets ever found. The first factor in its potential habitability is its distance from the host star. With an orbital radius of about 0.0911 AU, and given the star's low luminosity, Luyten b is orbiting well within the habitable zone. This is the region around a star where temperatures are just right for liquid water to pool on a planet's surface, given sufficient atmospheric pressure. Many other factors are also in favor of Luyten b's potential habitability. It's only about 2.9 times the mass of Earth, almost certainly making it a rocky planet.
Unlike many other potentially habitable exoplanets orbiting red dwarfs, like Proxima b and the TRAPPIST-1 planets, Luyten b has the advantage of orbiting a very quiet host. Luyten's Star has a very long rotational period of 118 days and is not prone to powerful solar flares. Strong enough flare events can strip the atmospheres of orbiting planets and eliminate their chances of habitability; a good example of this is Kepler-438b. However, with the low activity of its host, Luyten b is likely to retain any atmosphere for billions of years, potentially enabling the development of life as we know it. It has an Earth Similarity Index (ESI) value of 0.91, having the fourth-highest ESI of any confirmed planet

9. Teegarden c

Teegarden c is a candidate exoplanet found orbiting in the habitable zone of Teegarden's star, an M-type red dwarf star around 12 light years away from the Solar System. It orbits in a conservative habitable zone around its star. It is fourth closest potentially habitable exoplanet as of July 2019.
Teegarden c is the outermost planet of the system. It has orbital period of 11.4 days. The minimum mass of the planet is one Earth mass, and radius is probably Earth-like, suggesting an Earth-like composition, with iron core and rocky crust. Teegarden c probably has an ocean of water on surface, or ice because of temperatures. Teegarden c orbits in the conservative habitable zone. However, in this case the orbit also results in temperatures that are around −47°C. Consequently, Teegarden c may have frozen oceans.
It has an Earth Similarity Index (ESI) of 0.68. One positive factor for habitability is its star. Most red dwarfs emit strong flares, which can strip the atmosphere and eliminate habitability. A good example is Kepler-438b, which has an ESI score of 0.88, but because its sun is an active star it is likely uninhabitable. Another example is Proxima Centauri, the closest star to the sun. Teegarden's Star is inactive and quiet, making the planet possibly habitable. Other quiet red dwarfs with potentially habitable exoplanets are Ross 128 and Luyten's Star.

10. Kepler-62f

Kepler-62f 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 1,200 light-years (370 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.