Confirmed planetTRAPPIST-1 system
TRAPPIST-1 h
The outermost and coldest member of the chain: a small, frozen world whose orbit closes out the system's resonant structure.
- Orbital period
- 18.767 days
- Distance from the star
- 0.0619 AU · 9.26 million km
- Radius
- 0.755 Earth radii
- Mass
- 0.326 Earth masses
- Equilibrium temperature
- About 173 K without an atmosphere
- Rotation
- Unknown
- Discovery
- 2017 · transit
- Host star
- M8V ultracool dwarf · about 0.09 solar masses
- Distance from the Sun
- 40.7 light-years
The outer edge of the chain
TRAPPIST-1 h orbits once every 18.77 days, the longest period of the seven. Even so, its entire orbit would fit inside Mercury's. It is the smallest and least massive of the family, with a radius of 0.76 Earth radii and a mass of 0.33 Earth masses.
Its density is consistent with a rocky or icy body. The planet receives very little heat from the star, and its equilibrium temperature is around 173 K, well below the freezing point of water.
A frozen, airless world
No atmosphere has been detected on TRAPPIST-1 h. Without significant greenhouse warming, any water would be frozen solid, and the surface would be a cold, barren landscape more like the icy moons of the outer Solar System than a temperate planet.
Its slow, distant orbit helps anchor the resonant chain that links all seven planets. The same dynamical structure that keeps the system orderly also constrains how much the outer planet could have migrated after its formation.
What we don't know
The planet's composition, rotation, and the presence of any thin atmosphere remain unknown. Future observations may reveal whether h is purely rocky, partly icy, or holds a tenuous layer of gas.
- Is it rocky, icy, or a mixture?
- Does it retain even a thin atmosphere?
- How tightly is its orbit linked to the inner planets?
Key publications
| Year | Paper | Venue | Link |
|---|---|---|---|
| 2017 | Gillon et al., Seven temperate terrestrial planets around TRAPPIST-1 | Nature 542, 456 | DOI |
| 2021 | Agol et al., Refined masses, radii, and densities of the TRAPPIST-1 planets | Planetary Science Journal 2, 1 | DOI |
Values compiled from the NASA Exoplanet Archive and the papers listed above.
Worth knowing
It is the coldest and smallest of the seven planets.
Its year lasts about 18.8 Earth days.
Its whole orbit would fit inside Mercury's.