TRAPPIST-1

TRAPPIST-1 is a cold red dwarf star, with a surface temperature of about 2,566 K, in the constellation Aquarius. It has a planetary system of seven known planets. And is slightly larger than Jupiter and has a mass of about 9% of that of the Sun.

## System

The star has a planetary system of seven known exoplanets. Observations in 2016 from the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) at La Silla Observatory in Chile and numerous other telescopes led to the discovery of two terrestrial planets in orbit around TRAPPIST-1. In 2017, further analysis of the original observations identified five more planets. These seven planets take between about 1.5 days and 19 days to orbit around the star. The planets are likely tidally locked to TRAPPIST-1, such that one side of each planet always faces the star, leading to permanent day on one side and permanent night on the other.

As many as four of the planets – designated *d*, *e*, *f* and *g* – orbit at distances where temperatures are suitable for the existence of liquid water, and are thus potentially hospitable to life. There is no evidence of an atmosphere on any of the planets and it is unclear whether radiation emissions from TRAPPIST-1 would allow for one. The planets have low densities; they may consist of large amounts of volatile materials.

#### TRAPPIST-1b

**TRAPPIST-1b** is about 37% more massive than Earth and about 12% larger. Observations published in 2018 suggested that the atmosphere of TRAPPIST-1b may be very hot and potentially rich in CO2, although the presence of an atmosphere could not be confirmed. More recent modeling studies have suggested the planet is too hot to allow the formation of sulfuric acid clouds, such as are found on Venus, the hottest planet in the Solar System. Observations by the James Webb Space Telescope announced in 2023 suggest that the planet does not have any significant atmosphere. Its albedo is very low making it dark in color.

#### TRAPPIST-1c

**TRAPPIST-1c**, also designated as **2MASS J23062928-0502285 c**, is a mainly rocky, Venus-like exoplanet orbiting around the ultracool dwarf star TRAPPIST-1, located 40.7 light-years (12.5 parsecs) away from Earth in the constellation Aquarius. It is the third most massive and third largest planet of the system, with about 131% the mass and 110% the radius of Earth. Its density indicates a primarily rocky composition, but it is unknown whether it has an atmosphere.

#### TRAPPIST-1d

**TRAPPIST-1d**, also designated as **2MASS J23062928-0502285 d**, is a small exoplanet (about 40% the mass of the Earth), which orbits on the inner edge of the habitable zone of the ultracool dwarf star TRAPPIST-1, located 40.7 light-years (12.5 parsecs) 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. The first signs of the planet were announced in 2016, but it wasn't until the following years that more information concerning the probable nature of the planet was obtained. TRAPPIST-1d is the second-least massive planet of the system and is likely to have a compact hydrogen-poor atmosphere similar to Venus, Earth, or Mars. It receives just 4.3% more sunlight than Earth, placing it on the inner edge of the habitable zone. It has about <5% of its mass as a volatile layer, which could consist of atmosphere, oceans, and/or ice layers. A 2018 study by the University of Washington concluded that TRAPPIST-1d might be a Venus-like exoplanet with an uninhabitable atmosphere. The planet is an eyeball planet candidate.

#### TRAPPIST-1e

**TRAPPIST-1e**, also designated as **2MASS J23062928-0502285 e**, is a rocky, close-to-Earth-sized exoplanet orbiting within the habitable zone around the ultracool 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 found the exoplanet by using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured.

The exoplanet was one of seven exoplanets discovered orbiting the star using observations from the Spitzer Space Telescope. Three of the seven (*e*, *f*, and *g*) are in the habitable zone or the "goldilocks zone". TRAPPIST-1e is similar to Earth's mass, radius, density, gravity, temperature, and stellar flux. It is also confirmed to not have a cloud-free hydrogen-dominated atmosphere, meaning 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.

#### TRAPPIST-1f

**TRAPPIST-1f**, also designated as **2MASS J23062928-0502285 f**, is an exoplanet, likely rocky, orbiting within the habitable zone around the ultracool dwarf star TRAPPIST-1, located 40.7 light-years (12.5 parsecs) 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 four new exoplanets to be discovered orbiting the star in 2017 using observations from the Spitzer Space Telescope.

The planet is likely tidally locked, and has been depicted as an eyeball planet in artistic impressions by NASA.

#### TRAPPIST-1g

**TRAPPIST-1g**, also designated as **2MASS J23062928-0502285 g** and **K2-112 g**, is an exoplanet orbiting around the ultra-cool dwarf star TRAPPIST-1, located 40.7 light-years (12.5 parsecs) away from Earth in the constellation Aquarius. It was one of four new exoplanets to be discovered orbiting the star in 2017 using observations from the Spitzer Space Telescope. The exoplanet is within the optimistic habitable zone of its host star. It 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.

The second most distant known planet in its system, TRAPPIST-1g is a planet somewhat larger than Earth and with a similar density, meaning it is likely a rocky planet.

#### TRAPPIST-1h

**TRAPPIST-1h**, also designated as **2MASS J23062928-0502285 h**, is an exoplanet orbiting around the ultra-cool dwarf star TRAPPIST-1, located 40.7 light-years (12.5 parsecs) away from Earth in the constellation Aquarius. It was one of four new exoplanets to be discovered orbiting the star in 2017 using observations from the Spitzer Space Telescope. In the following years, more studies were able to refine its physical parameters.

The outermost known planet in its system, it is roughly one third the mass of Earth, and about 76% as large. Its relatively low density indicates that it is likely water-rich, like several other planets in the system.