TRAPPIST-1 is the most thoroughly characterised planetary system outside our own. Seven roughly Earth-sized planets transit an ultracool dwarf star, which means their sizes were measured directly and their masses were extracted from the way they tug each other off schedule. Three - e, f and g - lie inside the conservative habitable zone.
Fly there in 3DAll seven planets are locked into a resonant chain: for every two orbits of h, g completes three, f four, e six, d nine, c fifteen and b twenty-four. Such a chain can only survive if the planets migrated inward together through a gas disk, and the pattern has held for billions of years since.
The whole system is more compact than Jupiter’s moons: every orbit fits inside 0.07 AU, about a sixth of Mercury’s distance from the Sun.
Standing on TRAPPIST-1e, the neighbouring planets would appear larger in the sky than our Moon does from Earth.
JWST has so far found no thick atmosphere on b or c. The habitable-zone planets are harder to measure and the results are not yet in.
| Spectral type | M8 V |
| Surface temperature | 2,566 K |
| Mass | 0.0898 solar masses |
| Radius | 0.1192 solar radii |
| Luminosity | 0.000553 times the Sun |
| Apparent magnitude | 18.8 |
| Age | about 7.6 billion years |
| Distance from the Sun | 40.7 light-years (12.5 parsecs) |
| Position (J2000) | RA 346.6224°, Dec -5.0414° |
| Constellation | Aquarius |
| Planet | Year | Orbit | Radius | Mass | Found |
|---|---|---|---|---|---|
| TRAPPIST-1b | 36.3 hours | 0.0115 AU | 1.116 R⊕ | 1.374 M⊕ | Transit, 2016 |
| TRAPPIST-1c | 2.42 days | 0.0158 AU | 1.097 R⊕ | 1.308 M⊕ | Transit, 2016 |
| TRAPPIST-1d | 4.05 days | 0.0223 AU | 0.788 R⊕ | 0.388 M⊕ | Transit, 2016 |
| TRAPPIST-1e | 6.10 days | 0.0293 AU | 0.92 R⊕ | 0.692 M⊕ | Transit, 2017 |
| TRAPPIST-1f | 9.21 days | 0.0385 AU | 1.045 R⊕ | 1.039 M⊕ | Transit, 2017 |
| TRAPPIST-1g | 12.4 days | 0.0468 AU | 1.129 R⊕ | 1.321 M⊕ | Transit, 2017 |
| TRAPPIST-1h | 18.8 days | 0.0619 AU | 0.755 R⊕ | 0.326 M⊕ | Transit, 2017 |
Values are the NASA Exoplanet Archive’s composite parameters. A radius marked estimated comes from a mass–radius relation; a mass marked minimum is M sin i. Planet sizes and colours in the 3D view are renderings: none of these worlds has been photographed as more than a point of light.
For a star of 2,566 K, 0.00055 times as bright as the Sun, the habitable zone, where starlight could keep water liquid on a rocky surface, runs from about 0.025 to 0.050 AU (approximate: the star lies just outside the range the formula was fitted to). 3 of its 7 known planets orbit inside it. The zone says how much starlight arrives, not whether a planet has water or air. The edges follow Kopparapu et al. (2014), as the 3D view draws them.
The light you see left 41 years ago, within living memory.