TRAPPIST-1

Star

TRAPPIST-1 — an ultracool red dwarf (M8V) about 40 light-years away in the constellation Aquarius. Only slightly larger than Jupiter (~0.12 solar radii) and just ~9% of the Sun's mass, it is so cool (~2566 K) and faint (~0.05% of the Sun's luminosity) that it glows deep red and radiates mostly in the infrared. It hosts seven confirmed Earth-sized planets — more than any other known system — three of them in the habitable zone.

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Key facts

Mass

1.79 × 10²⁹ kg

0.0898 M☉

Radius

82,927 km

12% of the Sun's radius

Surface temperature

2,566 K (2293 °C)

Spectral type

M8V (ultracool red dwarf)

Science

Physical

Mass: 1.79 × 10²⁹ kg · 0.0898 M☉
Radius: 82,927 km · 12% of the Sun's radius
Density: 74,767 kg/m³ · 75× water
Spectral type: M8V (ultracool red dwarf)

Rotation

Equatorial period: 3.3 days
Note: Rotates about once every 3.3 days. Like most red dwarfs it is a flare star: sudden magnetic outbursts bathe its close-in planets in X-rays and ultraviolet — a real hazard to any atmospheres, and one of the open questions about the system's habitability.

Composition

By mass: A fully convective ball of hydrogen and helium — so cool (~2566 K) and faint (~0.05% of the Sun's luminosity) that most of its light is infrared. At about 0.12 solar radii it is only slightly larger than Jupiter, near the boundary between the smallest stars and brown dwarfs.

Temperature

Surface: 2,566 K (2293 °C)

Radiation & temperature

Luminosity: 2.12 × 10²³ W · 0.000553 L☉
Surface temperature: 2,566 K (2293 °C)
Peak wavelength: 1129 nm · near-infrared (mostly invisible to the eye)

Discovery & history

Name: Named TRAPPIST-1 as the first planet-hosting star of the TRAPPIST survey — the TRAnsiting Planets and PlanetesImals Small Telescope, itself a nod to Belgium's Trappist breweries. Its planets are lettered b through h, from the innermost outward.

Cataloged in 1999 as the faint red star 2MASS J23062928−0502285 and noted only as an ultracool dwarf. It became one of the most studied stars in the sky in 2016–2017, when the Belgian-led TRAPPIST telescope at ESO's La Silla Observatory and NASA's Spitzer Space Telescope found seven Earth-sized planets transiting it — the largest family of terrestrial worlds known around any single star.

Images

TRAPPIST-1 (right) beside the Sun — the red dwarf is only slightly larger than Jupiter. Artist's impression, not a photograph — ESO (CC BY 4.0) · Wikimedia Commons

TRAPPIST-1 with its seven planets — artist's impression, not a photograph — ESO / NASA (CC BY 4.0) · Wikimedia Commons

Images from open archives · credit shown per image