TRAPPIST-1h


TRAPPIST-1h, also designated as 2MASS J23062928-0502285 h, is an exoplanet orbiting around the ultra-cool dwarf star TRAPPIST-1 39 light-years away from Earth in the constellation Aquarius. It was one of four new exoplanets to be discovered orbiting the star using observations from the Spitzer Space Telescope. Throughout 2017 and 2018, 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, and about 77% as large. Its relatively low density indicates that it is likely water-rich, like several other planets in the system.

Characteristics

Mass, radius, and temperature

TRAPPIST-1h has a radius of 0.773, a mass of 0.331, and about 56% Earth's surface gravity. It has a density of 3.97 g/cm3, extremely similar to that of Mars. Given this density, about ≤5% of its mass must be water, likely in the form of a thick ice shell, seeing as it only receives about 13% of the stellar flux that Earth does. It has an equilibrium temperature of 169 K, 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 2511 K and an age between 3 and 8 billion years. For comparison, the Sun has a temperature of 5778 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 star's apparent magnitude, 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.

Might host water

Although TRAPPIST-1h's orbit falls within its star's frost line it could harbor liquid water under an H2-rich atmosphere, either primordial or resulting from continuous outgassing combined with internal heating. It could also potentially harbor a subsurface ocean by way of tidal heating, which can lead to volcanic activity and the formation of geysers.