TRAPPIST-1e is a rocky, close-to-Earth-sized exoplanet orbiting within the habitable zone around the ultra-cool red dwarf star TRAPPIST-1, located 40.7 light-years (12.5 parsecs; 385 trillion kilometers; 239 trillion miles) away from Earth in the constellation of Aquarius. Astronomers used the transit method to find the exoplanet, a method that measures the dimming of a star when a planet crosses in front of it. The planet was one of seven planets discovered orbiting the star using observations from the Spitzer Space Telescope. Three of seven planets (e, f, and g) are in the habitable zone. TRAPPIST-1e is similar to Earth's mass, radius, density, gravity, temperature, and stellar flux. It is also confirmed that TRAPPIST-1e lacks a cloud-free hydrogen-dominated atmosphere, meaning that if the planet has an atmosphere it is more likely to have a compact atmosphere like the terrestrial planets in the Solar System. In November 2018, researchers determined that of the seven exoplanets in the multi-planetary system, TRAPPIST-1e has the best chance of being an Earth-like ocean planet, and the one most worthy of further study regarding habitability. According to the Habitable Exoplanets Catalog, TRAPPIST-1e is among the best potentially habitable exoplanets discovered. The most recent observation in 2025 was unable to conclude with confidence if there was an atmosphere or not, though it could rule out certain atmosphere scenarios.
Physical characteristics
Mass, radius, density, composition and temperature TRAPPIST-1e was detected with the transit method, where the planet blocked a small percentage of its host star's light when passing between it and Earth. This allowed scientists to accurately determine the planet's radius at 0.920 R🜨, with a small uncertainty of about 83 km (52 mi). Transit-timing variations and advanced computer simulations helped constrain the planet's mass, which turned out to be 0.692 M🜨, or about 15% less massive than Venus. TRAPPIST-1e has 82% the surface gravity of Earth, the third lowest in the system. Its radius and mass are also the third least among the TRAPPIST-1 planets. With both the radius and mass of TRAPPIST-1e determined with low error margins, scientists could accurately calculate the planet's density, surface gravity, and composition. Initial density estimates in 2018 suggested it has a density of 5.65 g/cm3, about 1.024 times Earth's density of 5.51 g/cm3. TRAPPIST-1e appeared to be unusual in its system, as it was the only planet with a density consistent with a pure rock-iron composition, and the only one with a higher density than Earth (TRAPPIST-1c also appeared to be entirely rock, but with a lower density than TRAPPIST-1e). The higher density of TRAPPIST-1e implies an Earth-like composition and a solid rocky surface. This also appeared to be unusual among the TRAPPIST-1 planets, as most were thought to have densities consistent with being completely covered in either a thick steam/hot CO2 atmosphere, a global liquid ocean, or an ice shell. However, refined estimates show that all planets in the system have similar densities, consistent with rocky compositions, with TRAPPIST-1e having a somewhat lower but still Earth-like bulk density. The planet has a calculated equilibrium temperature of 246.1 K (−27.1 °C; −16.7 °F) given an albedo of 0, also known as its "blackbody" temperature. For a more realistic Earth-like albedo, however, this provides an unrealistic picture of the surface temperature of the planet. Earth's equilibrium temperature is 255 K; it is Earth's greenhouse gases that raise its surface temperatures to the levels we experience. If TRAPPIST-1e has a thick atmosphere, its surface could be much warmer than its equilibrium temperature.
Host star
The planet orbits an ultracool dwarf star named TRAPPIST-1. The star has a mass of 0.09 M☉ and a radius of 0.12 R☉, hear the boundary between that of a brown dwarf and low-mass star. It has a temperature of 2,566 K (2,293 °C; 4,159 °F) and is 7.6 billion years old. In comparison, the Sun is 4.6 billion years old and has a temperature of 5,778 K (5,505 °C; 9,941 °F). The star is metal-rich, with a metallicity ([Fe/H]) of 0.04, or 109% the solar amount. This is particularly odd as such low-mass stars near the boundary between brown dwarfs and hydrogen-fusing stars should be expected to have considerably less metal content than the Sun. Its luminosity (L☉) is 0.0522% of that of the Sun. The star's apparent magnitude, or how bright it appears from Earth's perspective, is 18.8. Therefore, it is far too dim to be seen with the naked eye.
Orbit TRAPPIST-1e orbits its host star quite closely. One full revolution around TRAPPIST-1 takes only 6.10 days (~146 hours) to complete. It orbits at a distance of 0.0293 AU (4.38 million km; 2.72 million mi), or just under 3% the separation between Earth and the Sun. For comparison, the closest planet in the Solar System, Mercury, takes 88 days to orbit the Sun at a distance of 0.38 AU (57 million km; 35 million mi). It is in 3:2 resonance with TRAPPIST-1f and 3:2 resonance with TRAPPIST-1d. Despite its close proximity to its host star, TRAPPIST-1e gets only about 60% the starlight that Earth gets from the Sun due to the low luminosity of its star. The star would cover an angular diameter of about 2.17 degrees from the surface of the planet, and so would appear about four times larger than the Sun does from Earth.
Atmosphere Transit observations with James Webb Space Telescope suggested no clear answer about the existence of an atmosphere, but it did rule out many atmosphere scenarios. See the "Habitability" studies below.
Habitability
The exoplanet was announced to be orbiting within the habitable zone of its parent star, the region where, with the correct conditions and atmospheric properties, liquid water may exist on the surface of the planet. TRAPPIST-1e has a radius of around 0.91 R🜨, so it is likely a rocky planet. Its host star is an ultracool red dwarf, with only about 8% of the mass of the Sun (close to the boundary between brown dwarfs and hydrogen-fusing stars). As a result, stars like TRAPPIST-1 have the potential to remain stable for up to 12 trillion years, which is over 2,000 times longer than the Sun. Because of this ability to live for such a long period of time, it is likely TRAPPIST-1 will be one of the last remaining stars in the Universe, when the gas needed to form new stars will be exhausted, and the existing stars begin to die off.
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