Artist conception of KOI-4878.01, a potentially habitable exoplanet
A habitable exoplanet is a theoretical type of planet which has liquid water and other potential key ingredients needed to support life. 55 potentially habitable extrasolar planets have been found as of March 2020 - one being about the size of Mars, 20 being about the size of Earth, and 34 being larger than Earth (dubbed a super-Earth). Another key feature of a habitable exoplanet are the planets being located in the habitable zone of their parent star (though it is theoretically possible for life to exist outside of a habitable zone).
Characteristics[]
Mass[]
At least for any Earth-like life to exist, a habitable exoplanet would need to have a mass somewhere between 0.1 and 10 Earth masses.
Type[]
The planet would likely have to be a rocky planet like Earth in order to be habitable, although it has been theorized by scientists like Carl Sagan and Stephen Hawking that life may be able to develop in the atmospheres of gas giants. Nonetheless, no planet on the potentially habitable list is a gas giant, although there have been a few gas giants found orbiting within the habitable zone of their parent stars.
Temperature[]
A comfortable temperature for an alien lifeform could vary based on many different factors, but among the exoplanets deemed habitable so far, a range of 182 K (-91 °C; -132.1 °F) for Kepler-186f and 285 K (11.8 °C; 53.3 °F) for Tau Ceti e can be found.
Radius[]
The radius would need to be somewhere between 0.5 and 2.5 Earth radii.
Host star[]
G-type stars would be needed for a planet to be the most similar to Earth, but F, K, and M stars should also be capable of hosting habitable exoplanets. It is speculated that K-type main-sequence stars would provide necessary conditions for a superhabitable planet, which are hypothetical planets more capable of hosting life than Earth.
M-type stars (red dwarfs) are considered possible hosts of habitable exoplanets, even flare stars like Proxima Centauri, although flare stars would greatly reduce the chances of a planet being habitable.