Stars · exoplanets

TRAPPIST-1: seven Earth-sized planets round one small star

TRAPPIST-1, a small orange-red star, with two of its seven planets close by

TRAPPIST-1 is a star barely larger than Jupiter, 40.7 light-years from the Sun, and seven planets of about Earth's size go round it, every one of them closer to it than Mercury is to the Sun. It is one of the best places we know to ask whether a small rocky planet can keep an atmosphere, and in Exo-View you can fly there and see all seven.

Fly to TRAPPIST-1 See the Milky Way

Distance: 40.7 light-years, in the constellation Aquarius
Kind: Ultra-cool red dwarf, spectral type M8
Size: 0.12 times the Sun's width, just over Jupiter's
Surface temperature: About 2,570 K
Planets: Seven, b to h; e, f and g lie in the habitable zone

Planet	Width (Earth = 1)	Orbit (days)	From the star (AU)
b	1.12	1.5	0.0115
c	1.10	2.4	0.0158
d	0.79	4.0	0.0223
e	0.92	6.1	0.0293
f	1.05	9.2	0.0385
g	1.13	12.4	0.0469
h	0.76	18.8	0.0619

The sizes are measured as each planet crosses its star's face; the distances are from the periods. Mercury is 0.39 AU from the Sun.

How they were found

The star is named for the TRAPPIST telescope in Chile, which in 2016 saw it dim three times, as three planets crossed in front of it. A year later, with the Spitzer space telescope, the count rose to seven. Each time a planet passes in front of the star, its dip in the light tells us the planet's size.

A crowded sky

The whole system would fit inside Mercury's orbit. The planets are so close to each other that, from the surface of one, the others would hang in the sky larger than our Moon does. Their orbits are locked in a chain, each period close to a simple ratio of the next, which shows they formed further out and moved in together. Each is probably locked with one face to the star, a permanent day side and a permanent night side.

Could any of them have air?

A red dwarf is a rough neighbour: it flares often and floods its planets with ultraviolet and X-rays that can strip away an atmosphere. The James Webb Space Telescope has looked at the two innermost planets, b and c, and found no thick atmosphere on either. Astronomers are now watching the cooler planets, in the habitable zone, where air is more likely to survive. Whether any of them has air or water is not yet known.

In Exo-View

The key below flies from the Sun to TRAPPIST-1. Its seven planets are drawn in their real orbits, with sizes and periods from the table; their surfaces are an illustration made from those figures, since no one has a picture of a planet so far away, and the plates say so.