The largest TRAPPIST-1 planet, at the cold edge of the habitable zone

TRAPPIST-1g

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

TRAPPIST-1g is the sixth of seven planets orbiting the ultracool red dwarf TRAPPIST-1, about 40 light-years away. It is the largest planet in the system, a little bigger and heavier than Earth, and it circles its star every 12.4 days.

It receives only about a quarter of the energy Earth gets from the Sun, placing it near the outer, colder edge of the habitable zone. Whether it is a frozen world, an ocean planet beneath ice or something else entirely is unknown: its atmosphere has not been detected and its surface has never been seen.

Key facts

Star: TRAPPIST-1M8V ultracool red dwarf
Distance: 40.5 light-years
Radius: 1.13 × Earth
Mass: 1.32 × Earth
Orbital period: 12.4 days
Equilibrium temperature: 197 K (-76 °C)Before any greenhouse warming
Starlight received: 0.25× Earth's
Surface gravity: ≈ 1.04 gIf rocky
Discovered: 2017Transit

Star: TRAPPIST-1 (M8V ultracool red dwarf)
Distance: About 40.5 light-years
Discovered: 2017, transit
Radius: About 1.13 Earth radii
Mass: About 1.32 Earth masses (from transit timing)
Orbital period: About 12.4 days
Starlight received: About 25% of what Earth gets from the Sun
Equilibrium temperature: About 197 K (−76 °C), assuming it reflects no light and has no atmosphere
Position: Sixth 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-1g's radius and mass are both well measured, thanks to its transits and to the gravitational interactions between the seven planets, which shift the timing of each transit by minutes.

Its density is slightly lower than Earth's and close to those of its siblings, suggesting broadly similar make-ups across the system. It could be a rocky world with a little less iron than Earth, or one with a significant layer of water or ice; present data cannot decide between them.

Its star

TRAPPIST-1 is an ultracool dwarf with about 9% of the Sun's mass, only slightly larger than Jupiter. It glows faintly, mostly in the infrared, and its frequent flares and starspots make it a demanding target for atmospheric studies. Because it is so dim, even TRAPPIST-1g, one of the outermost planets, orbits much closer to its star than Mercury does to the Sun.

Could it be habitable?

At the outer edge of the habitable zone, TRAPPIST-1g would need a substantial greenhouse atmosphere — perhaps a thick blanket of carbon dioxide — to warm its surface enough for liquid water. Without one, it would very likely be frozen.

Being farther from its active star, it may have been better shielded from atmospheric loss than the inner planets, and some formation models suggest the outer TRAPPIST-1 planets could be relatively rich in water. Those ideas remain untested. JWST has so far ruled out thick atmospheres only on the hot inner planets b and c and found no Earth-like atmosphere on d; TRAPPIST-1g itself has yet to be characterised in detail.

Orbit and climate

TRAPPIST-1g orbits about 0.047 astronomical units from its star — roughly 7 million kilometres — and is probably tidally locked. Its orbit is resonant with both neighbours: four orbits of TRAPPIST-1f take about as long as three of g, and three orbits of g match roughly two of TRAPPIST-1h. This finely balanced chain helps astronomers measure all seven planets' masses.

Discovery

TRAPPIST-1g was one of four planets announced in February 2017 by an international team led by Michaël Gillon, based on Spitzer Space Telescope observations and ground-based follow-up.

Future observations

TRAPPIST-1g's transits make it accessible to JWST, though its long-ish orbit means fewer transits per year than for the inner planets. Building up enough of them to detect or rule out an atmosphere will take time, and the star's activity must be modelled carefully.

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-1g

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-1g

TRAPPIST-1g is the largest planet in its system, yet it is only about 13% wider than Earth.
Its year lasts about 12.4 days — still shorter than two Earth weeks.
Surface gravity on TRAPPIST-1g is estimated to be close to Earth's, at roughly 1.04 times our own.
Its closest neighbours, f and h, would at times loom as visible discs in its sky, rather than the star-like points that Venus and Mars appear as from Earth.

Keep exploring

Sources