TRAPPIST-1e
Habitability Score
One of seven rocky planets orbiting the ultra-cool red dwarf TRAPPIST-1, planet e sits squarely in the habitable zone and is considered the most likely of the system's worlds to support liquid water — it receives almost exactly the same amount of energy from its star as Earth does from the Sun.
Overview
In 2017, astronomers announced the discovery of seven Earth-sized rocky planets orbiting the star TRAPPIST-1, about 39.5 light-years away. Three of these planets — d, e, and f — fall within the habitable zone. Of these, TRAPPIST-1e is widely considered the prime candidate for habitability because its energy balance most closely resembles Earth's.
The TRAPPIST-1 System
TRAPPIST-1 is an ultra-cool red dwarf star only slightly larger than Jupiter, with roughly 8% of our Sun's mass. Despite being much cooler and dimmer, it hosts a compact system of rocky planets that transit (pass in front of) the star regularly — making them highly accessible to atmospheric characterization by telescopes.
All seven planets orbit closer to TRAPPIST-1 than Mercury orbits our Sun, but because the star is so faint, the inner planets receive just enough heat for potentially habitable conditions.
Why TRAPPIST-1e Stands Out
- Receives ~91% of Earth's insolation — nearly identical energy input
- Size: 0.92 Earth radii — essentially Earth-sized
- Mass: ~0.77 Earth masses — consistent with a rocky composition
- Orbital period: 6.1 Earth days
- Equilibrium temperature: ~251 K (−22 °C) — would be warm enough with a greenhouse atmosphere
Density measurements suggest TRAPPIST-1e may have a thin atmosphere and surface water. Some models show it could maintain a global mean temperature above 0 °C with modest CO₂ concentrations.
What the James Webb Space Telescope Is Finding
The James Webb Space Telescope (JWST) has been systematically observing the TRAPPIST-1 planets. Early results for TRAPPIST-1b and c suggest those inner planets likely lack significant atmospheres, but 1e, 1f, and 1g remain under active study. Results are expected over the next few years and could directly reveal whether an atmosphere exists — and what it contains.
Challenges
- Tidal locking is likely (orbital period of 6.1 days)
- Stellar flares from TRAPPIST-1 are common and can be intense
- The system is 40 light-years away — no direct missions are feasible with current technology
Why the Whole System Matters
With three potentially habitable planets in one system, TRAPPIST-1 is a natural laboratory for comparative planetology around red dwarfs. If life thrives on a tidally locked ocean world, it might exist on trillions of similar planets throughout the Milky Way.