Confirmed planetTRAPPIST-1 system
TRAPPIST-1 g
The largest world in the chain, orbiting at the cool outer edge of the habitable zone where a strong greenhouse would be needed for surface water.
- Orbital period
- 12.3524 days
- Distance from the star
- 0.0468 AU · 7.00 million km
- Radius
- 1.129 Earth radii
- Mass
- 1.321 Earth masses
- Equilibrium temperature
- About 199 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
The largest of the seven
TRAPPIST-1 g is the biggest planet in the system, about 13% larger than Earth in radius and 32% more massive. Its density is consistent with a rocky world, though some analyses allow a modest water or volatile fraction.
It orbits once every 12.35 days, the second-longest period in the chain. At 0.0468 AU it is still far closer to its star than Mercury is to the Sun, but the star's faintness keeps the planet cold.
Cold, unless wrapped in greenhouse gas
With an equilibrium temperature near 199 K, TRAPPIST-1 g would be frozen without an atmosphere. It sits at the outer edge of the habitable zone, so any temperate surface would require a thick greenhouse atmosphere to trap heat.
The planet's larger size may have helped it hold onto a more substantial atmosphere than the smaller inner worlds. That makes g an interesting candidate for a cold but potentially sheltered world, though no atmosphere has been detected.
What we don't know
Its atmosphere, surface temperature, and water content are all unmeasured. Future spectroscopy will look for greenhouse gases and try to decide whether g is an ice world, a cold desert, or something more temperate beneath a thick blanket.
- Does it hold a thick greenhouse atmosphere?
- Is its water locked in ice, or liquid under the surface?
- How does its climate distribute heat if it is tidally 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 the largest planet in the TRAPPIST-1 system.
It lies at the outer edge of the habitable zone.
Its year lasts about 12.4 Earth days.