TRAPPIST-1

About 40 light years from here there is a small, dim, red star called TRAPPIST-1. It is only a little wider than Jupiter.

Seven planets go round it. All seven are about the size of the Earth, and all are probably made of rock.

They are packed in close. The nearest takes a day and a half to go round the star. The furthest takes under 19 days. All seven orbits would fit inside the orbit of Mercury with a great deal of room to spare.

Being so close to their star would roast them, except that this star is so faint. Three of the planets get about the right amount of warmth for water to stay liquid, if they have air to hold it in.

Nobody knows yet whether they do. Telescopes are trying to find out now.

No telescope can show what these planets look like. Any picture you see of them, including ours, is an informed guess.

TRAPPIST-1 is a red dwarf star in the constellation Aquarius, about 40 light years away. It is far too faint to see without a large telescope. It has 9 per cent of the Sun’s mass, a surface at about 2,300 °C against the Sun’s 5,500 °C, and gives out less than a thousandth of the Sun’s light.

It is named after the small Belgian telescope in Chile that found its first planets in 2016. In February 2017, after three weeks of nearly unbroken watching with the Spitzer space telescope, the count stood at seven.

The planets. All seven transit the star, so their sizes are measured from how much light each blocks. They are lettered b to h, outwards.

Planet	Year (Earth days)	Distance from star (million km)	Width (Earth = 1)	Starlight received (Earth = 1)
b	1.5	1.7	1.12	4.2
c	2.4	2.4	1.10	2.2
d	4.0	3.3	0.79	1.1
e	6.1	4.4	0.92	0.65
f	9.2	5.8	1.05	0.37
g	12.4	7.0	1.13	0.25
h	18.8	9.3	0.76	0.14

For comparison, Mercury is 58 million km from the Sun, and Mars receives 0.43 of the sunlight that the Earth does.

In step. The years are not random. In the time planet h goes round twice, g goes round 3 times, f 4 times, e 6, d 9, c 15 and b 24. The planets are locked in a chain of resonances, each tugging its neighbours at regular moments. This is useful. The tugs make each transit arrive a little early or late, and from the size of those shifts the masses of all seven planets have been worked out, to within a few per cent. No other small planets outside the Solar System have been weighed so well.

The masses run from about a third of the Earth’s to nearly one and a half times. Put together with the sizes, they give densities a little lower than the Earth’s. That fits rock, perhaps with less iron than the Earth has or with a share of water. It rules out thick blankets of hydrogen like Neptune’s.

Could any be like home? Planets e, f and g lie in the habitable zone, where a planet with a suitable atmosphere could keep liquid water on its surface. That is a statement about starlight and distance, and no more. Three things make these planets different from the Earth.

• They are so close to the star that tides have very probably locked them, with one face always in daylight and the other always dark, as the Moon keeps one face to the Earth.
• Red dwarfs throw out strong flares and far more ultraviolet and X-rays, for their size, than the Sun. Over billions of years that could strip a planet’s air away.
• The star is old, about 7.6 billion years, give or take 2 billion. There has been plenty of time for air to be lost, or for life to begin.

What has been measured so far. The Webb telescope has taken the temperature of the day sides of the two inner planets. Planet b is at about 230 °C and planet c at about 110 °C. Those are the temperatures expected of bare rock with little or no air to carry heat round to the night side. For planet e, in the habitable zone, the first observations, published in 2025, could not tell a planet with air from one without. More are being made.

The view. From any one of these planets, the others would at times show as discs in the sky, some as large as the Moon looks from the Earth. That much follows from the measured sizes and distances. Their colours, clouds and surfaces are unknown, and anything the 3D view shows of them is labelled as a guess.