An Earth-mass world in TRAPPIST-1's cooler habitable zone

TRAPPIST-1f

Orbits TRAPPIST-1 · 40.5 light-years · discovered 2017 (transit)

TRAPPIST-1f is the fifth of seven planets orbiting the ultracool red dwarf TRAPPIST-1, about 40 light-years from Earth. Its radius and mass are both within a few per cent of Earth's, making it one of the most Earth-like exoplanets in bulk size known.

Size is where the measured resemblance ends, however. TRAPPIST-1f receives only around 37% of the energy Earth gets from the Sun, so without a strong greenhouse effect it would likely be frozen. Nothing is yet known about its atmosphere or surface, and its potential to host liquid water is an open question.

Key facts

Star: TRAPPIST-1M8V ultracool red dwarf
Distance: 40.5 light-years
Radius: 1.04 × Earth
Mass: 1.04 × Earth
Orbital period: 9.21 days
Equilibrium temperature: 218 K (-55 °C)Before any greenhouse warming
Starlight received: 0.37× Earth's
Surface gravity: ≈ 0.95 gIf rocky
Discovered: 2017Transit

Star: TRAPPIST-1 (M8V ultracool red dwarf)
Distance: About 40.5 light-years
Discovered: 2017, transit
Radius: About 1.05 Earth radii
Mass: About 1.04 Earth masses (from transit timing)
Orbital period: About 9.2 days
Starlight received: About 37% of what Earth gets from the Sun
Equilibrium temperature: About 218 K (−55 °C), assuming it reflects no light and has no atmosphere
Position: Fifth of seven known planets

Measured values follow the NASA Exoplanet Archive and discovery papers; many carry large uncertainties. The 3D globe and walkable surface on this page are an artist's impression generated from these numbers.

What we know

TRAPPIST-1f's radius comes from the depth of its transits, and its mass from the slight timing shifts the planets induce on each other. Both values are now known to within a few per cent, a precision rare for planets this small.

Its density, like that of its siblings, is a little lower than Earth's. That fits a rocky planet with somewhat less iron, or a rocky planet carrying a modest amount of water or ice. The two possibilities cannot yet be told apart.

Its star

TRAPPIST-1 has roughly 9% of the Sun's mass and is only slightly larger than Jupiter. It emits most of its energy in the infrared and is prone to flares. Because the star is so faint, its habitable zone lies extremely close in, and three or four of its planets fall inside or near it depending on the definition used.

Could it be habitable?

TRAPPIST-1f lies in the outer part of the habitable zone. To keep surface water liquid, it would probably need a thick atmosphere with a strong greenhouse effect, for instance one rich in carbon dioxide. Without that, it would more likely be an ice-covered world.

Some models suggest that, if it formed with plenty of water, TRAPPIST-1f could be an ocean planet beneath a frozen crust, or have open water near the point that permanently faces its star. These remain scenarios, not observations. JWST results for the inner TRAPPIST-1 planets have so far found no thick atmospheres, but the cooler outer planets may have had a better chance of keeping their volatiles.

Orbit and climate

The planet orbits roughly 0.039 astronomical units from its star — under 6 million kilometres — and completes a year in about 9.2 days. It is probably tidally locked. If so, the substellar point would be the warmest place on the planet and the most likely spot for any liquid water. Its orbit is in a 4:3 resonance with TRAPPIST-1g.

Discovery

TRAPPIST-1f was announced in February 2017 alongside planets e, g and h, after the Spitzer Space Telescope watched the star almost continuously for about three weeks. Later analysis of years of transit timings, published in 2021, produced the precise masses now used for all seven planets.

Future observations

Because the TRAPPIST-1 planets transit, JWST can in principle detect their atmospheres from starlight passing through them. For a cool world like TRAPPIST-1f, this requires stacking many transits and carefully correcting for the star's activity. Astronomers see it as an important target for future atmospheric searches.

About the 3D impression

Artist's impression — the real surface has never been seen. The globe and the surface you can walk on are generated from what is measured — size, the starlight it receives and the kind of star it orbits — using the same procedural engine as our imaginary worlds. Seas, ice, clouds and any plant-like colours are possibilities, not observations: around a red dwarf the daylight is redder, so the scene uses warmer skies and darker vegetation.

Walk on an imagined surface of TRAPPIST-1f

Earth-like exoplanets compared

Well-known potentially Earth-like exoplanets, nearest first
Planet	Distance	Radius	Year	Sunlight
Proxima Centauri b	4.2 ly	—	11.2 days	0.65× Earth
Ross 128 b	11.0 ly	—	9.87 days	1.38× Earth
Teegarden's Star b	12.5 ly	—	4.91 days	1.08× Earth
Gliese 667 Cc	23.6 ly	—	28.1 days	0.88× Earth
TRAPPIST-1d	40.5 ly	0.79 R⊕	4.05 days	1.11× Earth
TRAPPIST-1e	40.5 ly	0.92 R⊕	6.10 days	0.65× Earth
TRAPPIST-1f	40.5 ly	1.04 R⊕	9.21 days	0.37× Earth
TRAPPIST-1g	40.5 ly	1.13 R⊕	12.4 days	0.25× Earth
LHS 1140 b	48.9 ly	1.73 R⊕	24.7 days	0.43× Earth
TOI-700 d	101 ly	1.07 R⊕	37.4 days	0.87× Earth
Kepler-186f	580 ly	1.17 R⊕	129.9 days	0.30× Earth
Kepler-22b	635 ly	2.10 R⊕	289.9 days	1.01× Earth
Kepler-62f	980 ly	1.41 R⊕	267.3 days	0.41× Earth
Kepler-442b	1,194 ly	1.34 R⊕	112.3 days	0.70× Earth
Kepler-452b	1,800 ly	1.63 R⊕	384.8 days	1.10× Earth

Surprising facts about TRAPPIST-1f

TRAPPIST-1f's mass and radius are both within about 5% of Earth's — but it orbits its star in just over nine days.
Seen from TRAPPIST-1f, its neighbouring planets e and g would at times appear as visible discs rather than mere points of light.
The whole TRAPPIST-1 system, all seven planets included, would fit well within Mercury's orbit around the Sun.
TRAPPIST-1 is expected to keep shining for trillions of years, far longer than the Sun.

Keep exploring

Sources