An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not then recognized as such. The first confirmation of the detection occurred in 1992. A different planet, first detected in 1988, was confirmed in 2003. According to statistics from the NASA Exoplanet Archive, As of 8 August 2024, there are 5,743 confirmed exoplanets in 4,286 planetary systems, with 961 systems having more than one planet. The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and to give more insight into their traits, such as their composition, environmental conditions, and potential for life.
Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked body possesses synchronous rotation, the object takes just as long to rotate around its own axis as it does to revolve around its partner. For example, the same side of the Moon always faces Earth, although there is some variability because the Moon's orbit is not perfectly circular. Usually, only the satellite is tidally locked to the larger body. However, if both the difference in mass between the two bodies and the distance between them are relatively small, each may be tidally locked to the other; this is the case for Pluto and Charon, as well as for Eris and Dysnomia. Alternative names for the tidal locking process are gravitational locking, captured rotation, and spin–orbit locking.
In astronomy and astrobiology, the habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure. The bounds of the HZ are based on Earth's position in the Solar System and the amount of radiant energy it receives from the Sun. Due to the importance of liquid water to Earth's biosphere, the nature of the HZ and the objects within it may be instrumental in determining the scope and distribution of planets capable of supporting Earth-like extraterrestrial life and intelligence.
Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and maintain environments hospitable to life. Life may be generated directly on a planet or satellite endogenously or be transferred to it from another body, through a hypothetical process known as panspermia. Environments do not need to contain life to be considered habitable nor are accepted habitable zones (HZ) the only areas in which life might arise.
An ocean world, ocean planet or water world is a type of planet that contains a substantial amount of water in the form of oceans, as part of its hydrosphere, either beneath the surface, as subsurface oceans, or on the surface, potentially submerging all dry land. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid or thalassogen, such as lava, ammonia or hydrocarbons. The study of extraterrestrial oceans is referred to as planetary oceanography.
Gliese 581c is an exoplanet orbiting within the Gliese 581 system. It is the second planet discovered in the system and the third in order from the star. With a mass about 6.8 times that of the Earth, it is classified as a super-Earth.
The habitability of natural satellites is the potential of moons to provide habitats for life, though it is not an indicator that they harbor it. Natural satellites are expected to outnumber planets by a large margin and the study of their habitability is therefore important to astrobiology and the search for extraterrestrial life. There are, nevertheless, significant environmental variables specific to moons.
A lava planet is a type of terrestrial planet, with a surface mostly or entirely covered by molten lava. Situations where such planets could exist include a young terrestrial planet just after its formation, a planet that has recently suffered a large collision event, or a planet orbiting very close to its star, causing intense irradiation and tidal forces to melt its surface.
The theorized habitability of red dwarf systems is determined by a large number of factors. Modern evidence suggests that planets in red dwarf systems are unlikely to be habitable, due to their low stellar flux, high probability of tidal locking, likely lack of magnetospheres and atmospheres, and the high stellar variation such planets would experience. However, the sheer number and longevity of red dwarfs could provide ample opportunity to realize any small possibility of habitability.
The discovery of extrasolar Earth-sized planets has encouraged research into their potential for habitability. One of the generally agreed requirements for a life-sustaining planet is a mobile, fractured lithosphere cyclically recycled into a vigorously convecting mantle, in a process commonly known as plate tectonics. Plate tectonics provide a means of geochemical regulation of atmospheric particulates, as well as removal of carbon from the atmosphere. This prevents a “runaway greenhouse” effect that can result in inhospitable surface temperatures and vaporization of liquid surface water. Planetary scientists have not reached a consensus on whether Earth-like exoplanets have plate tectonics, but it is widely thought that the likelihood of plate tectonics on an Earth-like exoplanet is a function of planetary radius, initial temperature upon coalescence, insolation, and presence or absence of liquid-phase surface water.
TRAPPIST-1f, also designated as 2MASS J23062928-0502285 f, is an exoplanet, likely rocky, orbiting within the habitable zone around the ultracool dwarf star TRAPPIST-1, located 40.7 light-years away from Earth in the constellation of Aquarius. 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.
TRAPPIST-1e, also designated as 2MASS J23062928-0502285 e, is a rocky, close-to-Earth-sized exoplanet orbiting within the habitable zone around the ultracool dwarf star TRAPPIST-1, located 40.7 light-years away from Earth in the constellation of Aquarius. Astronomers used the transit method to find the exoplanet, a method that measures the dimming of a star when a planet crosses in front of it.
LHS 1140 b is an exoplanet orbiting within the conservative habitable zone of the red dwarf LHS 1140. Discovered in 2017 by the MEarth Project, LHS 1140 b is about 5.6 times the mass of Earth and about 70% larger in radius, putting it within the super-Earth category of planets. It was initially thought to be a dense rocky planet, but refined measurements of its mass and radius have found a lower density, indicating that it is likely an ocean world with 9-19% of its mass composed of water. LHS 1140 b orbits entirely within the star's habitable zone and gets 43% the incident flux of Earth. The planet is 49 light-years away and transits its star, making it an excellent candidate for atmospheric studies with ground-based and/or space telescopes.
Ross 128 b is a confirmed Earth-sized exoplanet, likely rocky, that is orbiting within the inner habitable zone of the red dwarf star Ross 128, at a distance of around 11 light-years from Earth. The exoplanet was found using a decade's worth of radial velocity data using the European Southern Observatory's HARPS spectrograph at the La Silla Observatory in Chile. Ross 128 b is the nearest exoplanet around a quiet red dwarf, and is considered one of the best candidates for habitability. The planet is only 35% more massive than Earth, receives only 38% more starlight, and is expected to be a temperature suitable for liquid water to exist on the surface, if it has an atmosphere.
Kepler-1652b is a super-Earth exoplanet, orbiting within the habitable zone of the red dwarf Kepler-1652 about 822 light-years away in the Cygnus constellation. Discovered by NASA's Kepler spacecraft, Kepler-1652b was first announced as a candidate in 2013, but wasn't validated until four years later in 2017. It is a potential super-Earth with 160% Earth's radius. The planet orbits well within the habitable zone of its system, the region where liquid water can exist on a planet's surface. The planet is an eyeball planet candidate.
LHS 3844 b, formally named Kua'kua, is an exoplanet orbiting the red dwarf LHS 3844, about 48.5 light-years away in the constellation Indus, discovered using the Transiting Exoplanet Survey Satellite. It orbits its parent star once every 11 hours, and its radius is 1.32 times that of Earth. It has a low albedo, indicating that its surface may resemble that of the Moon or Mercury. LHS 3844 b probably does not have an atmosphere as almost no heat goes to its night side, and it has a dayside temperature of 1,040 K. The presence of cloudy atmosphere with cloud tops above pressure level of 0.1 bar cannot be excluded though.
Kepler-1649c is an Earth-sized exoplanet, likely rocky, orbiting within the habitable zone of the red dwarf star Kepler-1649, the outermost planet of the planetary system discovered by Kepler’s space telescope. It is located about 301 light-years (92 pc) away from Earth, in the constellation of Cygnus.
Kepler-737b is a super-Earth exoplanet 669 light years away. There is a chance it could be on the inner edge of the habitable zone.
LHS 475 b is a terrestrial planet orbiting the star LHS 475 which is about 40.7 light years away, in the constellation of Octans. It was the first extrasolar planet to be confirmed by the James Webb Space Telescope. It completes an orbit every 2 days and is 99% the diameter of Earth. It is also one of the most similar-to-Earth exoplanets discovered, in terms of radius.
HD 63433 d is a confirmed exoplanet orbiting HD 63433, a Sun-like star located 73 light-years away in the constellation Gemini. Its radius is measured at around 1.1 R🜨, which makes it similar to the Earth in size. It was the third exoplanet to be discovered in orbit around this star; the other two, HD 63433 b and c, were discovered in 2020. Orbiting its star at a distance of 0.0503 astronomical units (7,520,000 km), HD 63433 d is the innermost planet orbiting HD 63433, and completes an orbit around it just every 4 days. Due to the proximity of its star, the planet is scorching hot, having a temperature estimated at 1260 °C at daytime. The proximity of its star also causes it to be tidally locked.
