A small world on the warm edge of TRAPPIST-1's habitable zone

TRAPPIST-1d

Orbits TRAPPIST-1 · 40.5 light-years · discovered 2016 (transit)

TRAPPIST-1d is the third of seven Earth-sized planets circling TRAPPIST-1, an ultracool red dwarf about 40 light-years away in the constellation Aquarius. It is the smallest of the planets near the habitable zone, with roughly 40% of Earth's mass, and it receives slightly more energy from its star than Earth does from the Sun.

That position puts it close to the warm, inner edge of the habitable zone, where a planet risks losing its water to a runaway greenhouse. In 2025, observations with the James Webb Space Telescope found no sign of an Earth-like atmosphere, narrowing the possibilities but not ending the story.

Key facts

Star: TRAPPIST-1M8V ultracool red dwarf
Distance: 40.5 light-years
Radius: 0.79 × Earth
Mass: 0.39 × Earth
Orbital period: 4.05 days
Equilibrium temperature: 286 K (13 °C)Before any greenhouse warming
Starlight received: 1.11× Earth's
Surface gravity: ≈ 0.62 gIf rocky
Discovered: 2016Transit

Star: TRAPPIST-1 (M8V ultracool red dwarf)
Distance: About 40.5 light-years
Discovered: 2016, transit (confirmed and refined in 2017)
Radius: About 0.79 Earth radii
Mass: About 0.39 Earth masses (from transit timing)
Orbital period: About 4.05 days
Starlight received: About 1.1 times what Earth gets from the Sun
Equilibrium temperature: About 286 K (13 °C), assuming it reflects no light and has no atmosphere
Position: Third 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

Because TRAPPIST-1d crosses the face of its star every four days, astronomers can measure its size from the dip in starlight it causes. Its mass comes from transit-timing variations — tiny shifts in when each planet transits, caused by the gravitational tugs of its neighbours.

Together these give a density consistent with a rocky world, slightly less dense than Earth. Like its siblings, it may have a little less iron or a little more lightweight material than our planet, but its surface has never been seen.

Its star

TRAPPIST-1 is barely a star at all: it has about 9% of the Sun's mass and is only slightly larger than Jupiter. It shines mostly in the infrared and is thought to be several billion years old. It is also magnetically active, producing frequent flares and bright and dark spots that complicate efforts to study its planets' atmospheres.

Could it be habitable?

TRAPPIST-1d sits near the inner boundary of the habitable zone, and whether it falls inside or outside depends on the climate model used. With slightly more energy than Earth, an atmosphere rich in water vapour could trap enough heat to boil away any oceans. On the other hand, a tidally locked world with reflective clouds on its dayside might stay temperate.

JWST transmission spectra published in 2025 were essentially flat, ruling out a clear, hydrogen-rich atmosphere and showing no detectable water, methane or carbon dioxide. The remaining options include no atmosphere at all, a very thin one like Mars's, or an atmosphere hidden beneath high-altitude clouds.

Orbit and climate

TRAPPIST-1d orbits about 0.022 astronomical units from its star — roughly 3.3 million kilometres. It is almost certainly tidally locked, with a permanent dayside and night side. Its orbit is locked into a chain of resonances with its neighbours: it completes three orbits for every two of TRAPPIST-1e.

Discovery

A Belgian-led team under Michaël Gillon announced three planets around TRAPPIST-1 in 2016, using the small TRAPPIST telescope at ESO's La Silla Observatory. The signal attributed to the third planet was uncertain at first. Intensive monitoring with NASA's Spitzer Space Telescope, reported in 2017, pinned down TRAPPIST-1d's true orbit and revealed that the system contains seven planets.

Future observations

Further JWST observations of the TRAPPIST-1 planets continue, with researchers working to separate the planets' signals from the effects of starspots and flares. For TRAPPIST-1d, more transits could tighten the limits on clouds or a thin atmosphere.

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-1d

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-1d

All seven TRAPPIST-1 planets would fit comfortably inside the orbit of Mercury.
TRAPPIST-1d has only about 40% of Earth's mass — less than Venus, though still several times more than Mars.
From its surface, neighbouring planets could sometimes appear larger in the sky than the Moon does from Earth.
TRAPPIST stands for TRAnsiting Planets and PlanetesImals Small Telescope — a nod to the Trappist beers of the team's native Belgium.

Keep exploring

Sources