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 , with a small uncertainty of about . Transit-timing variations and advanced computer simulations helped constrain the planet's mass, which turned out to be , or about 15% less massive than
Venus. 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 (late
M-type)
ultracool dwarf star named
TRAPPIST-1. The star has a mass of 0.089 —near the boundary between a
brown dwarf and low-mass star—and a radius of 0.121 . It has a temperature of and is 7.6 billion years old. In comparison, the
Sun is 4.6 billion years old and has a temperature of . 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 () 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 6.099 Earth days (~146 hours) to complete. It orbits at a distance of , 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 . 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==