TRAPPIST-1 b
TRAPPIST-1 b is a terrestrial planet orbiting TRAPPIST-1 in the constellation Aquarius. It was discovered in 2016 using the transit method.
How Big Is TRAPPIST-1 b?
TRAPPIST-1 b has a radius of 1.12 times that of Earth. Its mass is 1.4 times that of Earth, giving it a density of 5.44 g/cm³ — comparable to rocky planets like Earth (5.51 g/cm³).
Is TRAPPIST-1 b in the Habitable Zone?
TRAPPIST-1 b orbits inside the inner edge of the habitable zone of TRAPPIST-1. So close to its star, surface conditions are far too hot for liquid water.
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 b
TRAPPIST-1 b receives 4.15 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on TRAPPIST-1 b — one full orbit around TRAPPIST-1 — lasts just 36.3 hours, shorter than Mercury's 88-day year. It orbits at an average distance of 0.012 AU — closer to its star than Mercury is to the Sun.
How Was TRAPPIST-1 b Discovered?
TRAPPIST-1 b 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.
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 b scores 0.57, ranking #173 of 5,568 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 b is one of 7 known planets in the TRAPPIST-1 system. Its siblings:
- TRAPPIST-1 c (Terrestrial)
- TRAPPIST-1 d (Terrestrial)
- TRAPPIST-1 e (Terrestrial)
- TRAPPIST-1 f (Terrestrial)
- TRAPPIST-1 g (Terrestrial)
- TRAPPIST-1 h (Terrestrial)
TRAPPIST-1 b — Complete Data
| Radius | 1.116 Earth radii (0.100 Jupiter radii) |
|---|---|
| Mass | 1.37 Earth masses (0.004 Jupiter masses) |
| Density | 5.44 g/cm³ (Earth: 5.51) |
| Orbital period | 1.51 days |
| Orbital distance | 0.012 AU |
| Stellar irradiation | 4.15× Earth |
| Earth Similarity Index | 0.57 |
| 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 b
Is TRAPPIST-1 b habitable?
No — TRAPPIST-1 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How big is TRAPPIST-1 b compared to Earth?
TRAPPIST-1 b has 1.12 times the radius of Earth and about 1.4 times its mass.
How long is a year on TRAPPIST-1 b?
One orbit around TRAPPIST-1 takes 1.5 Earth days — short enough that 242 of its years would fit into one Earth year.
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