TRAPPIST-1h is an

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

orbiting around the ultra-cool dwarf star

TRAPPIST-1 , - ! style="background-color: #FFFFC0; text-align: center;" colspan="2", Characteristics , - style="vertical-align:top" ! style="text-align:left" , Evolutionary stage , Main sequence , - style="vertical-align:top" ! style="text-ali ...

, located away from Earth in the constellation Aquarius. It was one of four new exoplanets to be discovered orbiting the star in 2017 using observations from the

Spitzer Space Telescope The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003, that was deactivated when operations ended on 30 January 2020. Spitzer was the third space telescope dedicate ...

. In the following years, more studies were able to refine its physical parameters. The outermost known planet in its system, it is roughly one third the mass of

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

, and about 76% as large. Its relatively low density indicates that it is likely water-rich, like several other planets in the system.

Physical characteristics

Mass, radius, and temperature

TRAPPIST-1h has a radius of , a mass of , and about 57% Earth's surface gravity. It was initially estimated to have a density of 3.97 g/cm³, similar to that of Mars. Given this density, about ≤5% of its mass may be water, likely in the form of a thick ice shell, since it only receives about 13% of the stellar flux that Earth does. It has an equilibrium temperature of , similar to that of Earth's south pole.

Host star

TRAPPIST-1h orbits the ultracool dwarf star TRAPPIST-1. It is 0.121 and 0.089 , with a temperature of 2,511 K and an age between 3 and 8 billion years. For comparison, the Sun has a temperature of 5,778 K and is about 4.5 billion years old. TRAPPIST-1 is also very dim, with about 0.0005 times the

luminosity of the Sun The solar luminosity () is a unit of radiant flux ( power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxies and other celestial objects in terms of the output of the Sun. One nominal s ...

. The star's

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

, or how bright it appears from Earth's perspective, is 18.8. Therefore, it is too dim to be seen with the naked eye.

Orbit

Despite it being the most distant known planet in its system, TRAPPIST-1h orbits its host star with an orbital period of 18.868 days and an orbital radius of about 0.0619 AU. This is even smaller than Mercury's orbit around the Sun (which is about 0.38 AU).

Stable liquid water

Although TRAPPIST-1h's orbit falls near its star's frost line, it could harbor liquid water under an H₂-rich atmosphere, either primordial or resulting from continuous

outgassing Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation (whic ...

combined with

internal heating Internal heat is the heat source from the interior of celestial objects, such as stars, brown dwarfs, planets, moons, dwarf planets, and (in the early history of the Solar System) even asteroids such as Vesta, resulting from contraction caused ...

, although existence of such atmosphere was strongly disfavored by observations in 2021 and 2022. If ice-covered, it could also potentially harbor a subsurface ocean by way of

tidal heating Tidal heating (also known as tidal working or tidal flexing) occurs through the tidal friction processes: orbital and rotational energy is dissipated as heat in either (or both) the surface ocean or interior of a planet or satellite. When an objec ...

, which could lead to

cryovolcanism A cryovolcano (sometimes informally referred to as an ice volcano) is a type of volcano that erupts gases and volatile material such as liquid water, ammonia, and hydrocarbons. The erupted material is collectively referred to as ''cryolava''; ...

in the form of erupting

geyser A geyser (, ) is a spring with an intermittent water discharge ejected turbulently and accompanied by steam. The formation of geysers is fairly rare and is caused by particular hydrogeological conditions that exist only in a few places on Ea ...

References