Confirmed planetTRAPPIST-1 system
TRAPPIST-1 f
A cool, Earth-sized world toward the outer side of the habitable zone, where a thicker greenhouse atmosphere could still keep water liquid.
- Orbital period
- 9.2067 days
- Distance from the star
- 0.0385 AU · 5.76 million km
- Radius
- 1.045 Earth radii
- Mass
- 1.039 Earth masses
- Equilibrium temperature
- About 219 K without an atmosphere
- Rotation
- Likely tidally locked
- Discovery
- 2017 · transit
- Host star
- M8V ultracool dwarf · about 0.09 solar masses
- Distance from the Sun
- 40.7 light-years
A cool Earth-sized world
TRAPPIST-1 f is nearly the same size and mass as Earth, orbiting once every 9.2 days at the outer edge of the habitable zone. Its equilibrium temperature is around 219 K, colder than Earth, so habitability would depend on a substantial greenhouse effect to warm the surface.
It receives roughly a third of the solar energy Earth does. Without an atmosphere it would be frozen, but a thick carbon-dioxide or other greenhouse layer could, in principle, keep surface water liquid in places.
The outer temperate zone
The outer habitable zone is forgiving in one way, planets there are less exposed to the star's flares and radiation, but demanding in another: they need more atmospheric warming to stay temperate. TRAPPIST-1 f is a key test of whether such outer-edge worlds can hold thick atmospheres.
Its likely synchronous rotation adds another layer of uncertainty. If locked, its climate would be organised around a permanent dayside and nightside, and habitability would depend on how well winds and oceans transport heat.
What we don't know
No atmosphere has been detected, and the planet's surface is unknown. Future observations will try to determine whether f holds a greenhouse atmosphere and whether its climate could support liquid water.
- Does it have a thick enough atmosphere to stay warm?
- Is any surface water liquid, frozen, or absent?
- How does its climate behave if the rotation is locked?
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 nearly Earth's twin in size and mass.
It lies at the outer edge of the habitable zone.
Its year lasts about 9.2 Earth days.