TRAPPIST-1
TRAPPIST-1 is a single ultracool red dwarf (M8V) in Aquarius, about 40 light-years away, orbited by seven confirmed Earth-sized planets — the most terrestrial-sized worlds known in any one system. All seven transit their star, so both their sizes and (through the gravitational tugs they exert on one another) their masses are measured, giving real densities. The planets are packed extraordinarily tightly: the whole system would fit well inside Mercury's orbit. Three — e, f and g — lie in the habitable zone, where a suitable atmosphere could allow liquid water, though no atmosphere or water has been confirmed on any of them.
Key facts
Science
Classification
- Type
- Single-star planetary system
- Central star
- TRAPPIST-1 — M8V ultracool red dwarf (~2566 K, ~0.12 R☉)
Planets
- Confirmed planets
- 7 · b, c, d, e, f, g, h — all Earth-sized, all transiting
- Habitable-zone candidates
- 3 · e, f and g — potentially habitable, not confirmed
- Compactness
- All within 0.062 AU · the whole system fits inside about a sixth of Mercury's orbit
Distance & age
- Distance
- 40.5 light-years · in the constellation Aquarius
- Age
- ~7.6 billion years · estimated — older than the Sun
How we know
- Note
- Planet count and parameters follow the NASA Exoplanet Archive (checked 2026-07-03): seven confirmed planets, each with a measured radius and mass (hence real density). Equilibrium temperatures are computed model values, not measurements. 'Habitable zone' means only that e, f and g receive starlight comparable to Earth's — no atmosphere, surface or water has been detected on any of them, and TRAPPIST-1 is a flare star whose radiation may strip atmospheres. On the map the system is placed at a usable distance; it truly lies about 40.5 light-years away.
Discovery & history
- Name
- Named for the TRAPPIST telescope (TRAnsiting Planets and PlanetesImals Small Telescope) that found its first planets; the star is the '1' of that survey and its planets are lettered b through h.
The faint red star was cataloged in 1999 but drew little notice until 2016–2017, when the Belgian-led TRAPPIST telescope and NASA's Spitzer Space Telescope revealed seven Earth-sized planets transiting it (Gillon et al., Nature). Their masses were later measured from the tiny gravitational tugs the tightly-packed planets exert on one another.
Images
Images from open archives · credit shown per image