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Planet profiles

TRAPPIST-1 c

Earth Like · 40 light-years away · Last reviewed October 14, 2026

TRAPPIST-1 c rendered in TerraForge from its measured mass, orbit and star
TerraForge's rendering of TRAPPIST-1 c, generated by our physics engine from its measured mass, size, orbit and star. An illustration of what the science suggests, not a photograph.
TerraForge classification
Earth Like
Host star
TRAPPIST-1 (M8.0 V)
Distance
40 light-years
Orbit
2.42 days
Mass
1.31 × Earth
Radius
1.1 × Earth
Discovered
2016, transit

Figures from the NASA Exoplanet Archive. The classification is TerraForge’s, from its physics model.

TRAPPIST-1 c is the second of seven Earth-sized planets around TRAPPIST-1, a small, cool red dwarf about 40 light-years away. It orbits every 2.42 days and receives about twice the light Earth gets from the Sun, roughly what Venus receives. That made it a natural test of whether a Venus-like planet around a red dwarf can keep a thick atmosphere.

It was discovered in 2016 with its inner neighbour TRAPPIST-1 b, by a team led by Michaël Gillon, from the small dips in starlight as the planets crossed the star. Five more planets followed in 2017. Measurements of how the planets' gravity shifts the timing of each other's transits, analysed by a team led by Eric Agol in 2021, put its mass at about 1.31 Earth masses, with a radius about 1.1 times Earth's.

In 2023 a team led by Sebastian Zieba used the James Webb Space Telescope to measure heat from its day side, at about 380 kelvin. That is too hot for a thick atmosphere rich in carbon dioxide, which would spread heat around the planet. A Venus-like atmosphere with sulfuric acid clouds was also disfavoured. A thin atmosphere or bare rock both fit. In 2025 a team led by Michaël Gillon measured how TRAPPIST-1 c's brightness changes around its orbit with Webb. They found its day side cooler than TRAPPIST-1 b's, at about 370 kelvin, and ruled out thick atmospheres with strong greenhouse warming. TRAPPIST-1 c may still hold a thin atmosphere mostly of oxygen, or be airless with a more reflective surface than b. Zieba's team suggested that the lack of a thick atmosphere points to the planet forming with relatively little water.

Sources

  1. Temperate Earth-sized planets transiting a nearby ultracool dwarf star — arXiv (Nature)
  2. Refining the transit timing and photometric analysis of TRAPPIST-1: Masses, radii, densities, dynamics, and ephemerides — arXiv (The Planetary Science Journal)
  3. No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c — arXiv (Nature)
  4. First JWST thermal phase curves of temperate terrestrial exoplanets reveal no thick atmosphere around TRAPPIST-1 b and c — arXiv
A banded gas giant with four moons in different colors: a yellow sulfur moon, two rust-red rock moons and a blue water moon

New: Choose What Your Moons Are Made Of 🌙

Every moon used to be grey rock, white ice or blue ocean, decided for you. Now each one has a "Made of" choice in the builder's Moons tab — silicate rock, iron, water ice, carbon, sulfur, or nitrogen and methane ice — and the physics decides what that becomes. Put water ice too close to the star and it boils off to bare rock. Put sulfur where a neighbouring moon keeps it tidally flexed and it erupts in Io's yellows. Give a Titan-sized moon nitrogen ice far from the sun and it wraps itself in haze. Hang a water moon about the mass of Earth off a giant planet in the habitable zone and it can hold air and open seas — scored for life on the same scale as a planet, with a day as long as its orbit. A tint slider nudges the colour, but only as far as that material could really go.

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