TRAPPIST-1 d
TRAPPIST-1 d is a terrestrial planet orbiting TRAPPIST-1 in the constellation Aquarius. It lies about 41 light-years from Earth and was discovered in 2016 using the transit method. It orbits within the habitable zone of its star — the region where liquid water could exist on a planet's surface.
How Big Is TRAPPIST-1 d?
TRAPPIST-1 d has a radius of 0.79 times that of Earth. Its mass is 0.39 times that of Earth, giving it a density of 4.37 g/cm³ — between that of rocky and gaseous planets.
Is TRAPPIST-1 d in the Habitable Zone?
TRAPPIST-1 d orbits within the optimistic habitable zone of TRAPPIST-1 — the broader region where liquid water might be possible under favorable atmospheric conditions.
Habitable zone of TRAPPIST-1: 0.019–0.053 AU (conservative: 0.025–0.050 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
See the full interactive habitable-zone view in the Exoplanet Explorer app ›
Temperature on TRAPPIST-1 d
TRAPPIST-1 d receives 1.12 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on TRAPPIST-1 d — one full orbit around TRAPPIST-1 — lasts 4.05 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.022 AU — closer to its star than Mercury is to the Sun.
How Was TRAPPIST-1 d Discovered?
TRAPPIST-1 d was discovered in 2016 using the transit method, with observations from La Silla Observatory.
The transit method watches a star for the tiny, regular dip in brightness that occurs when a planet crosses in front of it. The depth and timing of these dips reveal the planet's size and orbital period.
How Far Away Is TRAPPIST-1 d?
TRAPPIST-1 d is 40.5 light-years (12.4 parsecs) from Earth. Light arriving here tonight left the planet around the year 1986. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 712,800 years to make the journey.
Earth Similarity Index
The Earth Similarity Index (ESI) scores how physically similar a planet is to Earth, from 0 to 1, based on radius, density, escape velocity and surface temperature. TRAPPIST-1 d scores 0.91, ranking #5 of 5,565 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: TRAPPIST-1
TRAPPIST-1
- Surface temperature
- 2,566 K (Sun: 5,772 K)
- Mass
- 0.09 M☉
- Radius
- 0.12 R☉
- Luminosity
- 0.0006 L☉
The TRAPPIST-1 Planetary System
TRAPPIST-1 d is one of 7 known planets in the TRAPPIST-1 system. Its siblings:
- TRAPPIST-1 b (Terrestrial)
- TRAPPIST-1 c (Terrestrial)
- TRAPPIST-1 e (Terrestrial)
- TRAPPIST-1 f (Terrestrial)
- TRAPPIST-1 g (Terrestrial)
- TRAPPIST-1 h (Terrestrial)
TRAPPIST-1 d — Complete Data
| Radius | 0.788 Earth radii (0.070 Jupiter radii) |
|---|---|
| Mass | 0.39 Earth masses (0.001 Jupiter masses) |
| Density | 4.37 g/cm³ (Earth: 5.51) |
| Orbital period | 4.05 days |
| Orbital distance | 0.022 AU |
| Stellar irradiation | 1.12× Earth |
| Earth Similarity Index | 0.91 |
| Distance from Earth | 40.5 light-years (12.4 parsecs) |
| Constellation | Aquarius |
| Discovery method | Transit |
| Discovery facility | La Silla Observatory |
| Discovery year | 2016 |
Data: NASA Exoplanet Archive, last updated 2022-11-14. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About TRAPPIST-1 d
Is TRAPPIST-1 d habitable?
TRAPPIST-1 d orbits within the habitable zone of TRAPPIST-1, the region where liquid water could exist on a planet's surface. Whether it is actually habitable depends on its atmosphere and composition, which remain unknown.
How far away is TRAPPIST-1 d?
TRAPPIST-1 d is about 41 light-years from Earth in the constellation Aquarius. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 712,800 years to get there.
How big is TRAPPIST-1 d compared to Earth?
TRAPPIST-1 d has 0.79 times the radius of Earth and about 0.39 times its mass.
How long is a year on TRAPPIST-1 d?
One orbit around TRAPPIST-1 takes 4.0 Earth days — short enough that 90 of its years would fit into one Earth year.
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