Confirmed planetTRAPPIST-1 system
TRAPPIST-1 c
The second planet out: a hot, Earth-sized world where JWST found no thick carbon-dioxide atmosphere, narrowing what kind of planet it can be.
- Orbital period
- 2.4219 days
- Distance from the star
- 0.0158 AU · 2.36 million km
- Radius
- 1.097 Earth radii
- Mass
- 1.308 Earth masses
- Equilibrium temperature
- About 342 K without an atmosphere
- Rotation
- Likely tidally locked
- Discovery
- 2016 · transit
- Host star
- M8V ultracool dwarf · about 0.09 solar masses
- Distance from the Sun
- 40.7 light-years
A hot, rocky sibling
TRAPPIST-1 c circles its star in 2.42 days, receiving enough energy for an equilibrium temperature around 342 K. It is almost identical in size to Earth and slightly more massive, with a density that points to a rocky interior.
Like its inner neighbour, it is expected to be tidally locked. That geometry, combined with the heat it receives, makes it a challenging place for a stable temperate climate, but its basic character remained unknown until JWST looked at it directly.
No thick carbon-dioxide blanket
In 2023, JWST observed TRAPPIST-1 c as it passed behind the star and found that a dense, Venus-like carbon-dioxide atmosphere could not explain the measured heat. The data instead favour a world with either no atmosphere or a very thin one.
That result matters beyond this one planet. Venus-scale atmospheres were a leading possibility for rocky exoplanets close to cool stars, and TRAPPIST-1 c showed that such a thick envelope can be ruled out with a single sensitive eclipse measurement.
The limits of what we know
We know the planet's radius and mass with reasonable precision, and we know it lacks a thick CO₂ atmosphere. But whether it is entirely airless, whether it has a thin secondary atmosphere, and what its surface looks like all remain open questions.
The outer members of the chain, which receive less heat, are better candidates for retaining atmospheres. Comparing c with those cooler worlds will help reveal how stellar radiation shapes the atmospheres of small planets around M dwarfs.
Open questions
- Does it have any atmosphere, however thin?
- Has its surface been stripped bare by the star's radiation?
- How does its geology compare with the larger outer 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 |
| 2023 | Zieba et al., No thick carbon dioxide atmosphere on TRAPPIST-1 c | Nature 620, 746 | DOI |
Values compiled from the NASA Exoplanet Archive and the papers listed above.
Worth knowing
Its year lasts 2.42 Earth days.
JWST ruled out a thick, Venus-like atmosphere in 2023.
It is nearly the same size as Earth but much hotter.