Which TRAPPIST-1 planets are in the habitable zone?

Planets e, f and g lie within the star's habitable zone, and planet d sits close to its inner edge. That is why the system is often described as having three or four potentially habitable planets.

Do the TRAPPIST-1 planets have atmospheres?

JWST measurements show that the two innermost planets, b and c, have little or no thick atmosphere. Observations of the habitable-zone planets, especially e, are under way, and so far they have only ruled out a thick hydrogen-rich atmosphere.

How far away is TRAPPIST-1?

It is about 40 light-years from Earth, in the constellation Aquarius. That is close in galactic terms, but far too distant to reach with current spacecraft.

Could there be life on TRAPPIST-1 planets?

No evidence of life has been found. Life as we know it would need liquid water and probably an atmosphere, and it is still unknown whether the habitable-zone planets have kept atmospheres despite their active star.

Can you see TRAPPIST-1 with a telescope?

It is very faint in visible light, around magnitude 18.8, so it needs a large telescope. The planets themselves cannot be seen directly; they are detected from the shadows they cast as they pass in front of the star.

Seven Earth-sized worlds around a tiny, cool star

TRAPPIST-1

Seven-planet system · ≈ 40 ly

TRAPPIST-1 is a small, cool star about 40 light-years away in the constellation Aquarius. It is barely larger than Jupiter, yet it hosts seven rocky planets of roughly Earth's size, the largest such family known around a single star. All seven orbit closer to their star than Mercury does to the Sun, and three or four of them lie in the zone where liquid water could exist on the surface.

Because the planets regularly pass in front of their star, astronomers can measure their sizes, masses and densities with unusual precision. That has made TRAPPIST-1 one of the most important targets for the James Webb Space Telescope, which is testing whether rocky planets around red dwarfs can hold on to atmospheres.

Key facts

Distance: ≈ 40 light-years
Star type: M8 ultracool red dwarf
Star mass: ≈ 0.09 solar massesAbout 94 times Jupiter's mass
Star radius: ≈ 0.12 × the SunOnly slightly larger than Jupiter
Star temperature: ≈ 2,570 KLess than half the Sun's
Planets: 7 knownb, c, d, e, f, g and h
Orbital periods: 1.5 to 18.8 days
Habitable-zone planets: e, f and gd lies near the inner edge
Age: ≈ 7–8 billion yearsOlder than the Solar System

Discovery

The star itself was catalogued in 1999 by an infrared sky survey, but its planets were unknown until a team led by Michaël Gillon of the University of Liège began monitoring it with TRAPPIST, a small robotic telescope at ESO's La Silla Observatory in Chile. In 2016 they announced three planets, detected from the tiny dips in brightness as each one passed across the star's face. The system is named after the telescope.

Follow-up observations, above all a nearly continuous 20-day stare by NASA's Spitzer Space Telescope, showed that some of those dips actually came from other planets. In February 2017 the team announced that TRAPPIST-1 hosts seven transiting planets, a result that drew worldwide attention because so many potentially habitable worlds had never been found around one star.

Seven Earth-sized worlds

The planets, labelled b to h outwards from the star, range from about three-quarters of Earth's diameter to about 1.13 times. Their masses run from about a third of Earth's to about 1.4 times. Precise measurements suggest they all have broadly similar rocky compositions, slightly less dense than Earth, which could mean a little less iron or more lightweight material such as water.

The whole system is remarkably compact. The innermost planet, b, completes an orbit in about 1.5 days, while the outermost, h, takes about 19 days. All seven would fit well inside Mercury's orbit around the Sun. From the surface of one planet, its neighbours would often appear larger in the sky than the Moon does from Earth.

A resonant chain

The planets' orbital periods are linked by simple ratios, such as eight orbits of b for every five of c, forming a chain of resonances that runs through all seven. Such a delicate arrangement suggests the planets formed further out and migrated inward together, and that the system has not been disturbed since.

The habitable zone

Because TRAPPIST-1 is so faint, its habitable zone lies very close to the star. Planets e, f and g receive amounts of starlight similar to those of Earth and Mars, with d near the inner edge. Like other planets this close to their stars, they are probably tidally locked, keeping one face in permanent daylight.

An ultracool dwarf star

TRAPPIST-1 is an ultracool dwarf, a star only just massive enough to sustain hydrogen fusion in its core. With a surface temperature of about 2,570 K, it gives off most of its light in the infrared and shines with less than a thousandth of the Sun's brightness. To the human eye, sunlight on its planets would look dim and deep orange-red.

Small stars like this burn their fuel extremely slowly. TRAPPIST-1 is already older than the Sun and is expected to keep shining for trillions of years, far longer than the current age of the universe. The drawback is its activity: it produces frequent flares and strong ultraviolet and X-ray radiation, especially when young, which may have eroded its planets' atmospheres.

What JWST has found

In 2023 the James Webb Space Telescope measured the heat coming from the day side of TRAPPIST-1 b by watching the planet slip behind its star. The day side reached about 500 K, or roughly 230°C, close to what is expected for a bare, dark rock with no atmosphere to carry heat to the night side. Later in 2023 a similar measurement of TRAPPIST-1 c found about 380 K, ruling out a thick, carbon-dioxide-rich atmosphere like that of Venus.

These results do not completely exclude thin atmospheres, and further observations of planet b have shown that its data could also fit a hazy atmosphere rich in carbon dioxide. Still, the overall picture for the two innermost planets is of little or no thick atmosphere. Webb is now observing the cooler planets in the habitable zone, especially TRAPPIST-1 e. Early results have ruled out a thick hydrogen-rich atmosphere there, but whether it has a thinner, Earth-like one is not yet known.

Why TRAPPIST-1 matters

Red dwarfs make up roughly three-quarters of the stars in the Milky Way, and many host small rocky planets. If such planets can keep their atmospheres despite their stars' flares, they could be the most common habitable worlds in the galaxy. If they cannot, Earth-like conditions may be rarer than once hoped.

TRAPPIST-1 offers the best natural experiment for answering this question. Its seven planets share the same star and history but sit at different distances, so comparing them shows how starlight and radiation shape rocky worlds. Its closeness, the small size of its star and the frequent transits of its planets make it one of the few systems where such atmospheres can be studied today.

Surprising facts about TRAPPIST-1

All seven TRAPPIST-1 planets orbit closer to their star than Mercury is to the Sun.
A year on the innermost planet lasts only about 36 hours.
TRAPPIST-1 is not much bigger than Jupiter, but about 90 times more massive.
The star is expected to keep shining for trillions of years, long after the Sun has died.
The system is named after a 60-centimetre robotic telescope in Chile.

Frequently asked questions

Which TRAPPIST-1 planets are in the habitable zone?: Planets e, f and g lie within the star's habitable zone, and planet d sits close to its inner edge. That is why the system is often described as having three or four potentially habitable planets.
Do the TRAPPIST-1 planets have atmospheres?: JWST measurements show that the two innermost planets, b and c, have little or no thick atmosphere. Observations of the habitable-zone planets, especially e, are under way, and so far they have only ruled out a thick hydrogen-rich atmosphere.
How far away is TRAPPIST-1?: It is about 40 light-years from Earth, in the constellation Aquarius. That is close in galactic terms, but far too distant to reach with current spacecraft.
Could there be life on TRAPPIST-1 planets?: No evidence of life has been found. Life as we know it would need liquid water and probably an atmosphere, and it is still unknown whether the habitable-zone planets have kept atmospheres despite their active star.
Can you see TRAPPIST-1 with a telescope?: It is very faint in visible light, around magnitude 18.8, so it needs a large telescope. The planets themselves cannot be seen directly; they are detected from the shadows they cast as they pass in front of the star.

Keep exploring

The 3D views in Ember’s Universe are scientific visualisations: physically based where possible, compressed or exaggerated where the real scales would hide everything.

Sources