Extrasolar discoveries

Among exoplanets, a review in 2015 came to the conclusion that Kepler-62f, Kepler-186f and Kepler-442b were likely the best candidates for being potentially habitable. These are at a distance of 1200, 490 and 1,120 light-years away, respectively. Of these, Kepler-186f is similar in size to Earth with a 1.2-Earth-radius measure, and it is located towards the outer edge of the habitable zone around its red dwarf star. Among nearest terrestrial exoplanet candidates, Tau Ceti e is 11.9 light-years away. It is in the inner edge of its solar system's habitable zone, giving it an estimated average surface temperature of 68 °C (154 °F).

Studies that have attempted to estimate the number of terrestrial planets within the circumstellar habitable zone tend to reflect the availability of scientific data. A 2013 study by Ravi Kumar Kopparapu put η_e, the fraction of stars with planets in the CHZ, at 0.48, meaning that there may be roughly 95–180 billion habitable planets in the Milky Way. However, this is merely a statistical prediction; only a small fraction of these possible planets have yet been discovered.

Previous studies have been more conservative. In 2011, Seth Borenstein concluded that there are roughly 500 million habitable planets in the Milky Way. NASA's Jet Propulsion Laboratory 2011 study, based on observations from the Kepler mission, raised the number somewhat, estimating that about "1.4 to 2.7 percent" of all stars of spectral class F, G, and K are expected to have planets in their CHZs.

Early findingsedit

The first discoveries of extrasolar planets in the CHZ occurred just a few years after the first extrasolar planets were discovered. However these early detections were all gas giant sized, and many in eccentric orbits. Despite this, studies indicate the possibility of large, Earth-like moons around these planets supporting liquid water. One of the first discoveries was 70 Virginis b, a gas giant initially nicknamed "Goldilocks" due to it being neither "too hot" nor "too cold". Later study revealed temperatures analogous to Venus, ruling out any potential for liquid water. 16 Cygni Bb, also discovered in 1996, has an extremely eccentric orbit that spends only part of its time in the CHZ, such an orbit would causes extreme seasonal effects. In spite of this, simulations have suggested that a sufficiently large companion could support surface water year-round.

Gliese 876 b, discovered in 1998, and Gliese 876 c, discovered in 2001, are both gas giants discovered in the habitable zone around Gliese 876 that may also have large moons. Another gas giant, Upsilon Andromedae d was discovered in 1999 orbiting Upsilon Andromidae's habitable zone.

Announced on April 4, 2001, HD 28185 b is a gas giant found to orbit entirely within its star's circumstellar habitable zone and has a low orbital eccentricity, comparable to that of Mars in the Solar System. Tidal interactions suggest it could harbor habitable Earth-mass satellites in orbit around it for many billions of years, though it is unclear whether such satellites could form in the first place.

HD 69830 d, a gas giant with 17 times the mass of Earth, was found in 2006 orbiting within the circumstellar habitable zone of HD 69830, 41 light years away from Earth. The following year, 55 Cancri f was discovered within the CHZ of its host star 55 Cancri A. Hypothetical satellites with sufficient mass and composition are thought to be able to support liquid water at their surfaces.

Though, in theory, such giant planets could possess moons, the technology did not exist to detect moons around them, and no extrasolar moons had been discovered. Planets within the zone with the potential for solid surfaces were therefore of much higher interest.

Habitable super-Earthsedit

The 2007 discovery of Gliese 581 c, the first super-Earth in the circumstellar habitable zone, created significant interest in the system by the scientific community, although the planet was later found to have extreme surface conditions that may resemble Venus. Gliese 581 d, another planet in the same system and thought to be a better candidate for habitability, was also announced in 2007. Its existence was later disconfirmed in 2014, but only for short time. As of 2015, the planet has no newer disconfirmations. Gliese 581 g, yet another planet thought to have been discovered in the circumstellar habitable zone of the system, was considered to be more habitable than both Gliese 581 c and d. However, its existence was also disconfirmed in 2014, and astronomers are divided about its existence.

Discovered in August 2011, HD 85512 b was initially speculated to be habitable, but the new circumstellar habitable zone criteria devised by Kopparapu et al. in 2013 place the planet outside the circumstellar habitable zone.

Kepler-22 b, discovered in December 2011 by the Kepler space probe, is the first transiting exoplanet discovered around a Sun-like star. With a radius 2.4 times that of Earth, Kepler-22b has been predicted by some to be an ocean planet. Gliese 667 Cc, discovered in 2011 but announced in 2012, is a super-Earth orbiting in the circumstellar habitable zone of Gliese 667 C. It is one of the most Earth-like planet known.

Gliese 163 c, discovered in September 2012 in orbit around the red dwarf Gliese 163 is located 49 light years from Earth. The planet has 6.9 Earth masses and 1.8–2.4 Earth radii, and with its close orbit receives 40 percent more stellar radiation than Earth, leading to surface temperatures of about 60° C. HD 40307 g, a candidate planet tentatively discovered in November 2012, is in the circumstellar habitable zone of HD 40307. In December 2012, Tau Ceti e and Tau Ceti f were found in the circumstellar habitable zone of Tau Ceti, a Sun-like star 12 light years away. Although more massive than Earth, they are among the least massive planets found to date orbiting in the habitable zone; however, Tau Ceti f, like HD 85512 b, did not fit the new circumstellar habitable zone criteria established by the 2013 Kopparapu study. It is now considered as uninhabitable.

Near Earth-sized planets and Solar analogsedit

Recent discoveries have uncovered planets that are thought to be similar in size or mass to Earth. "Earth-sized" ranges are typically defined by mass. The lower range used in many definitions of the super-Earth class is 1.9 Earth masses; likewise, sub-Earths range up to the size of Venus (~0.815 Earth masses). An upper limit of 1.5 Earth radii is also considered, given that above 1.5 R_⊕ the average planet density rapidly decreases with increasing radius, indicating these planets have a significant fraction of volatiles by volume overlying a rocky core. A genuinely Earth-like planet – an Earth analog or "Earth twin" – would need to meet many conditions beyond size and mass; such properties are not observable using current technology.

A solar analog (or "solar twin") is a star that resembles the Sun. To date, no solar twin with an exact match as that of the Sun has been found. However, some stars are nearly identical to the Sun and are considered solar twins. An exact solar twin would be a G2V star with a 5,778 K temperature, be 4.6 billion years old, with the correct metallicity and a 0.1% solar luminosity variation. Stars with an age of 4.6 billion years are at the most stable state. Proper metallicity and size are also critical to low luminosity variation.

Using data collected by NASA's Kepler Space observatory and the W. M. Keck Observatory, scientists have estimated that 22% of solar-type stars in the Milky Way galaxy have Earth-sized planets in their habitable zone.

On 7 January 2013, astronomers from the Kepler team announced the discovery of Kepler-69c (formerly KOI-172.02), an Earth-size exoplanet candidate (1.7 times the radius of Earth) orbiting Kepler-69, a star similar to our Sun, in the CHZ and expected to offer habitable conditions. The discovery of two planets orbiting in the habitable zone of Kepler-62, by the Kepler team was announced on April 19, 2013. The planets, named Kepler-62e and Kepler-62f, are likely solid planets with sizes 1.6 and 1.4 times the radius of Earth, respectively.

With a radius estimated at 1.1 Earth, Kepler-186f, discovery announced in April 2014, is the closest yet size to Earth of an exoplanet confirmed by the transit method though its mass remains unknown and its parent star is not a Solar analog.

Kapteyn b, discovered in June 2014 is a possible rocky world of about 4.8 Earth masses and about 1.5 earth radii was found orbiting the habitable zone of the red subdwarf Kapteyn's Star, 12.8 light-years away.

On 6 January 2015, NASA announced the 1000th confirmed exoplanet discovered by the Kepler Space Telescope. Three of the newly confirmed exoplanets were found to orbit within habitable zones of their related stars: two of the three, Kepler-438b and Kepler-442b, are near-Earth-size and likely rocky; the third, Kepler-440b, is a super-Earth. Howewer, Kepler-438b is found to be a subject of powerful flares, so it is now considered uninhabitable. 16 January, K2-3d a planet of 1.5 Earth radii was found orbiting within the habitable zone of K2-3, receiving 1.4 times the intensity of visible light as Earth.

Kepler-452b, announced on 23 July 2015 is 50% bigger than Earth, likely rocky and takes approximately 385 Earth days to orbit the habitable zone of its G-class (solar analog) star Kepler-452.

The discovery of a system of three tidally-locked planets orbiting the habitable zone of an ultracool dwarf star, TRAPPIST-1, was announced in May 2016. The discovery is considered significant because it dramatically increases the possibility of smaller, cooler, more numerous and closer stars possessing habitable planets.

Two potentially habitable planets, discovered by the K2 mission in July 2016 orbiting around the M dwarf K2-72 around 227 light year from the Sun: K2-72c and K2-72e are both of similar size to Earth and receive similar amounts of stellar radiation.

Announced on the 20 April 2017, LHS 1140b is a super-dense super-Earth 39 light years away, 6.6 times Earth's mass and 1.4 times radius, its star 15% the mass of the Sun but with much less observable stellar flare activity than most M dwarfs. The planet is one of few observable by both transit and radial velocity that's mass is confirmed with an atmosphere may be studied.

Discovered by radial velocity in June 2017, with approximately three times the mass of Earth, Luyten b orbits within the habitable zone of Luyten's Star just 12.2 light-years away.

At 11 light-years away, a second closest planet, Ross 128 b, was announced in November 2017 following a decade's radial velocity study of relatively "quiet" red dwarf star Ross 128. At 1.35 Earth's mass is it roughly Earth-sized and likely rocky in composition.

Discovered in March 2018, K2-155d is about 1.64 time the radius of Earth, is likely rocky and orbits in the habitable zone of its red dwarf star 203 light years away.

One of the earliest discoveries by the Transiting Exoplanet Survey Satellite (TESS) announced July 31, 2019 is a Super Earth planet GJ 357 d orbiting the outer edge of a red dwarf 31 light years away.

K2-18b is an exoplanet 124 light-years away, orbiting in the habitable zone of the K2-18, a red dwarf. This planet is significant for water vapour found in its atmosphere; this was announced on September 17, 2019.

In September 2020, astronomers identified 24 superhabitable planet (planets better than Earth) contenders, from among more than 4000 confirmed exoplanets at present, based on astrophysical parameters, as well as the natural history of known life forms on the Earth.

Notable exoplanets – Kepler Space Telescope
Confirmed small exoplanets in habitable zones. (Kepler-62e, Kepler-62f, Kepler-186f, Kepler-296e, Kepler-296f, Kepler-438b, Kepler-440b, Kepler-442b) (Kepler Space Telescope; January 6, 2015).