Artist's impression of TRAPPIST-1e from 2018, depicted here as a tidally locked planet with a liquid ocean. The actual appearance of the exoplanet is currently unknown, but based on its density it is likely not entirely covered in water.
|Discovered by||Spitzer Space Telescope|
|Discovery date||22 February 2017|
|0.02928285 (± 3.4e-07) AU|
|6.099043 (± 0.000015) d|
−0.42 5.65g cm−3
|Temperature||246.1 ± 3.5 K (−27.05 ± 3.50 °C; −16.69 ± 6.30 °F)|
TRAPPIST-1e, also designated as 2MASS J23062928-0502285 e, is a solid, almost Earth-sized exoplanet orbiting within the habitable zone around the ultracool dwarf star TRAPPIST-1 approximately 40 light-years (12.1 parsecs, or nearly ×1014 3.7336km) away from Earth in the constellation of Aquarius. The exoplanet was found by using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured.
It was one of seven new exoplanets to be discovered orbiting the star using observations from the Spitzer Space Telescope. Three of the seven (e, f, and g) are in the habitable zone. TRAPPIST-1e is very similar to Earth, with just about the same mass, radius, density, gravity, temperature, and stellar flux. It is also confirmed to have a compact atmosphere like the terrestrial planets in our solar system. In November 2018, researchers determined that of the seven exoplanets in the multiplanetary system, TRAPPIST-1e is the one with the greatest chance of being an Earth-like ocean world and the one most worthy of further study with regard to habitability. According to the Planetary Habitability Catalogue, Trappist 1-e is the most potentially habitable exoplanet discovered so far, with an Earth Similarity Index of 0.87.
Mass, radius, 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 0.910 R⊕, with a very small uncertainty of about 166–172 km. Transit-timing variations and advanced computer simulations helped constrain the planet's mass, which turned out to be 0.772 M⊕. 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. TRAPPIST-1e is unusual in its system as it is the only planet with a pure rock-iron composition, and the only one with a higher density than Earth (TRAPPIST-1c also appears to be just about entirely rock, but it has an extremely thick atmosphere that makes it less dense than TRAPPIST-1e). It has a density of 5.65 g/cm3, about 1.024 times Earth's density of 5.51 g/cm3. The higher density of TRAPPIST-1e implies a very Earth-like composition and a solid rocky surface. This is also unusual among the TRAPPIST-1 planets, as most are completely covered in either a thick steam atmosphere, a global liquid ocean, or an ice shell. TRAPPIST-1e has 93% the surface gravity of Earth, the second highest in the system. Its radius and mass are also the third least among the TRAPPIST-1 planets.
The planet has a calculated equilibrium temperature of 246.1 K (−27.1 °C; −16.7 °F), given an albedo of 0. For a more realistic Earth-like albedo of 0.3, it would have an equilibrium temperature of 225 K (−48 °C; −55 °F).
TRAPPIST-1e orbits its host star quite closely. One full revolution around TRAPPIST-1 only takes 6.099 Earth days (~146 hours) to complete. It orbits at a distance of 0.02928285 AU, or just under 3% the separation between Earth and the Sun. For comparison, the closest planet in our Solar System, Mercury, takes 88 days to orbit the Sun at a distance of 0.38 AU. Despite its close proximity to its host star, TRAPPIST-1e only gets about 60% the starlight that Earth gets from the Sun, due to the extremely low luminosity of its host.
The planet orbits an (late M-type) ultracool dwarf star named TRAPPIST-1. The star has a mass of 0.089 M☉ and a radius of 0.121 R☉. It has a temperature of 2516 K and is anywhere between 3 and 8 billion years old. In comparison, the Sun is 4.6 billion years old and has a temperature of 5778 K. 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 too dim to be seen with the naked eye.
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 very likely rocky. Its host star is a red ultracool 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 ability to live up to 4–5 trillion years, 400–500 times longer than the Sun will live. Because of this ability to live for long periods of time, it is likely TRAPPIST-1 will be one of the last remaining stars when the Universe is much older than it is now, when the gas needed to form new stars will be exhausted, and the remaining ones begin to die off.
The planet is very likely tidally locked, with one side of its hemisphere permanently facing towards the star, while the opposite side is shrouded in eternal darkness. However, between these two intense areas, there would be a sliver of habitability – called the terminator line, where the temperatures may be suitable (about 273 K (0 °C; 32 °F)) for liquid water to exist. Additionally, a much larger portion of the planet may be habitable if it supports a thick enough atmosphere to transfer heat to the side facing away from the star.
More detailed studies of TRAPPIST-1e and the other TRAPPIST-1 planets released in 2018 determined that the planet is one of the most Earth-like worlds found. It is extremely similar to Earth physically, with 91% the radius, 77% the mass, 102.4% the density (5.65 g/cm3), and 93% the surface gravity. TRAPPIST-1e is confirmed to be a terrestrial planet with a solid, rocky surface. It is cool enough for liquid water to pool on the surface, but not too cold for it to freeze like on TRAPPIST-1f, g, and h. The planet receives a stellar flux 0.604 times that of Earth, about a third lower than that of Earth but significantly more than that of Mars. Its equilibrium temperature ranges from 225 K (−48 °C; −55 °F) to 246.1 K (−27.1 °C; −16.7 °F), depending on how much light the planet reflects into space. Both of these are between those of Earth and Mars as well. TRAPPIST-1e is confirmed to have a compact, hydrogen-free atmosphere like those of our Solar System's rocky planets, further raising the chances of habitability. Hydrogen is a powerful greenhouse gas, so if there was enough to be easily detected, it would mean that the surface of TRAPPIST-1e would be completely inhospitable. Since such an atmosphere is not present, it raises the chances for the planet to have a more Earth-like atmosphere instead.
TRAPPIST-1e is set to be one of the first science targets of the James Webb Space Telescope, which will be able to better analyze the planet's atmosphere and search for the chemical signs of life.
- List of extrasolar candidates for liquid water
- List of potentially habitable exoplanets
- List of transiting exoplanets
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