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Entering orbit…
Habitability
8/ 100
Uninhabitable
GJ 876 d rendered in TerraForge from its measured mass, orbit and star

GJ 876 d

Also written GJ 876d

Sub-Neptune15 light years awayFound 2005
Mass
6.83 × Earth
Radius
2.51 × Earth
Gravity
1.08 g
Surface (est.)
653 K
Year
46.6 hrs
From Earth
15.3 ly
Mass
6.83 × Earth
Radius
2.51 × Earth
Gravity
1.08 g
Surface (est.)
653 K
Year
46.6 hrs
From Earth
15.3 ly

GJ 876 system

Type
M-type (Red Dwarf)
Spectral type
M2.5V
Temperature
3,294 K
Radius
0.3 ☉
Mass
0.32 ☉
Planets
4
Luminosity
0.0124 ☉

Published by the NASA Exoplanet Archive. This world’s temperature and habitability score are computed from this star’s measured luminosity, not from a stand-in for its class.

GJ 876 d0.021 AUYou are here6.8 M⊕
GJ 876 c0.13 AU227 M⊕
GJ 876 b0.21 AU723 M⊕
GJ 876 e0.33 AU15 M⊕
Habitable zone

Star and planets drawn to scale: the star is 13 times as wide as GJ 876 d, so only its edge is in view. Orbits spaced as in the star system view.

Estimated: this planet doesn't transit its star, so its size is inferred, not measured

GJ 876 d was, when it was found in 2005, by far the lightest planet known around a normal star other than the Sun: a few times Earth's mass, far lighter than the gas giants that dominated early discoveries. It orbits GJ 876, a red dwarf about 15 light-years away, in under two days, the innermost of the star's four planets.

Surface. No solid surface - a hydrogen-helium envelope wrapped around a rocky core (a sub-Neptune)

Sub-Neptune
Life unlikely — too hot for liquid waterMagnetic FieldLow Activity
GJ 876 d is a sub-Neptune: a 6.8x Earth core wrapped in a deep hydrogen-helium envelope, with no solid surface to stand on.
Sub-Neptunes are the most common planets Kepler found, though our own solar system has none. Too light to be a giant, they still hold on to a thick envelope of light gas over a rocky or icy core.
Classification Criteria:
✓ Mass: 6.8x Earth (under 20x, below the giants)✓ A hydrogen-helium envelope over the core

Reading GJ 876 d’s atmosphere

Dominant elements: O, Si, Fe, N, C, HThis is what a telescope would measure while GJ 876 d crosses in front of its star: starlight passes through the atmosphere and spreads into a rainbow band. Each dark line marks a wavelength the atmosphere absorbed.Every element absorbs its own signature wavelengths — hydrogen always darkens the same two spots, calcium another pair, and so on. Astronomers match the lines in a real spectrum against a library of these signatures to say what an atmosphere is made of, light-years away, without ever visiting. A deeper line generally means more of that element is present.GJ 876 d sits about 15 light-years away, found by the radial velocity method. Most catalogued distances trace back to parallax: as Earth orbits the Sun, a nearby star appears to shift very slightly against the distant background stars, and the size of that shift gives its distance directly.
Further reading
ExoplanetsSara Seager (editor)The field's standard reference: how planets are found, how they form, and what their atmospheres are made of.The Planet FactoryElizabeth TaskerHow planets are made, and why so few of them come out like Earth.Exoplanets: Diamond Worlds, Super Earths, Pulsar PlanetsMichael Summers & James TrefilThe strange end of the catalogue — carbon worlds, molten rock, planets orbiting dead stars.The Little Book of ExoplanetsJoshua WinnHow these worlds are found, and what a measurement can honestly tell us.
As an Amazon Associate I earn from qualifying purchases — if you buy through these links, TerraForge may receive a small commission at no extra cost to you.
Habitability ScoreUninhabitable
8
Contributing Factors
temperature4
Scorching - hot enough to melt lead, though rock stays solid; no chance for life
atmosphere30

Deep hydrogen-helium envelope with photochemical haze

Hydrogen-rich atmosphere typical of gas giants
water15
No solid surface — any water is locked in a supercritical mantle and hot ice, not oceans
magnetic Field100
Magnetic field provides radiation protection
geology60
Some geological activity provides slow nutrient cycling
Organic Chemistry80
Core organic elements (CHNO) present
rotation65
Slow rotation - significant day/night temperature variations, but life can adapt

Physical Properties

Mass
6.83x Earth (4.08e+25 kg)
Radius
1.90x Earth (12,088 km)
This planet: 1.90× Earth radiusEarth: 1× (baseline)
Surface Gravity
1.90x Earth (18.6 m/s²)
Surface Temperature
653 K (379 °C / 715 °F)
Rotation Period
46.6 hours/day (slower than Earth)
This planet: 47 h/dayEarth: 24 h/day

Time Dilation

Clocks run 6.7 seconds per year slower than deep space — about 9 minutes over a lifetime

Elemental Composition

How TerraForge built this world

GJ 876 d, from the NASA Exoplanet Archive (found 2005). Measured: 6.83 Earth masses, 0.0208 AU, a 3294 K M-type star. Composition inferred from a estimated bulk density of 0.43x Earth's, modeled rather than measured since this isn't a transiting planet, too low for bare rock at this size, suggesting a rocky core under roughly a 4% hydrogen/helium envelope by mass, with a day locked to its 1.9-day year, as anything orbiting this close would be.

A dark ringed planet hanging over the horizon of a rocky moon, its rings lit by a distant sun

New: Stand on Your Moons 🌒

A planet's surface view isn't the only ground any more. Press the surface button on a planet with moons and it asks where to stand: the planet itself, or any moon still in orbit, each tagged with what it became — ocean, ice, carbon, volcanic. Every moon gets its own ground and sky from its own physics: airless rock under a black sky, ice cracked by geysers over a hidden sea, sulfur plains shaking with eruptions, or orange haze you can't see the sun through. And hanging over the horizon, there's the planet you built.

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