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.
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).
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
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.