Confirmed planetTRAPPIST-1 system
TRAPPIST-1 d
The smallest world in the chain, sitting close to the inner edge of the habitable zone: cool enough to be interesting, hot enough to be uncertain.
- Orbital period
- 4.0492 days
- Distance from the star
- 0.0223 AU · 3.34 million km
- Radius
- 0.788 Earth radii
- Mass
- 0.388 Earth masses
- Equilibrium temperature
- About 288 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
The smallest of the seven
With a radius of 0.79 Earth radii and a mass of 0.39 Earth masses, TRAPPIST-1 d is the smallest planet in the system and one of the least massive rocky exoplanets ever measured. Its density is consistent with a rocky, perhaps partially differentiated body.
It orbits once every 4.05 days, closer to the star than any member of our Solar System orbits the Sun. Yet because the star is so faint, the heat it receives is only modestly above Earth's.
At the inner edge of the temperate zone
TRAPPIST-1 d's equilibrium temperature is around 288 K, close to Earth's global mean. That places it near the inner edge of the habitable zone: if it has little or no atmosphere, its surface may be hot and dry, while a thin atmosphere could still permit liquid water in places.
The uncertainty is real. Small differences in reflectivity, cloud cover, or greenhouse gases swing the answer from "hospitable" to "sterile desert," so astronomers treat d as a boundary case rather than a confirmed habitable world.
What we don't know
No atmosphere has been detected on TRAPPIST-1 d, and its geology, rotation, and surface composition are unmeasured. Future transit spectroscopy may show whether it holds any gas at all, and how much of any early atmosphere has been lost to the star.
- Does it have an atmosphere, and of what composition?
- Is surface water possible, or has it been lost?
- How did such a small, close planet form and survive?
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 |
Values compiled from the NASA Exoplanet Archive and the papers listed above.
Worth knowing
It is the smallest planet in the TRAPPIST-1 system.
Its year lasts about 4 Earth days.
It sits right at the inner edge of the habitable zone.