Confirmed planetTRAPPIST-1 system
TRAPPIST-1 b
The innermost member of a seven-planet chain: a scorched, roughly Earth-sized rock whose dayside temperature has now been measured directly by the James Webb Space Telescope.
- Orbital period
- 1.5109 days
- Distance from the star
- 0.0115 AU · 1.73 million km
- Radius
- 1.116 Earth radii
- Mass
- 1.374 Earth masses
- Equilibrium temperature
- About 400 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 scorched innermost world
TRAPPIST-1 b orbits just 0.0115 AU from its star, completing a full year in only 1.51 Earth days. At that distance it receives several times more stellar energy than Earth does, and its equilibrium temperature is around 400 K. Tidal forces almost certainly lock its rotation, so one hemisphere faces the star permanently.
The planet is about 11% larger than Earth in radius and 37% more massive. Its size and mass imply a rocky, roughly Earth-like density, though that tells us little about its actual surface, which no telescope has imaged.
What JWST measured
In 2023, the James Webb Space Telescope observed TRAPPIST-1 b by watching it pass behind its star, isolating the planet's own infrared glow. The measured dayside brightness ruled out a thick, heat-redistributing atmosphere and is consistent with a bare, dark rock surface.
That was the first thermal detection of a rocky exoplanet this small. It showed that the innermost TRAPPIST-1 world, at least, does not hold the kind of dense atmosphere a cooler planet might retain.
The limits of what we know
Everything measured so far comes from transits, transit-timing variations, and infrared eclipse observations. We know the planet's size and mass well, but not its geology, magnetic field, or the presence of a thin atmosphere. A hot, airless rock and a world with a very thin gas layer can look similar in current data.
The day-night temperature contrast is expected to be extreme. Without an atmosphere to carry heat, the dayside may bake while the nightside cools toward space, though the exact numbers depend on the surface and any tenuous gas.
Open questions
- Does it retain any atmosphere at all, even a very thin one?
- Is its surface bare rock, or has volcanism reshaped it?
- How different are its day and night sides in temperature?
- Can follow-up observations detect its composition?
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 | Greene et al., Thermal emission from TRAPPIST-1 b using JWST | Nature 618, 39 | DOI |
Values compiled from the NASA Exoplanet Archive and the papers listed above.
Worth knowing
Its year lasts just 1.51 Earth days.
It is likely locked, with one side always facing the star.
JWST measured its heat directly in 2023.