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Entering orbit…
Habitability
15/ 100
Uninhabitable
GJ 251 b rendered in TerraForge from its measured mass, orbit and star

GJ 251 b

Also written GJ 251b

Sub-Neptune18 light years awayFound 2020
Mass
3.85 × Earth
Radius
1.79 × Earth
Gravity
1.20 g
Surface (est.)
350 K
Year
14.2 days
From Earth
18.2 ly
Mass
3.85 × Earth
Radius
1.79 × Earth
Gravity
1.20 g
Surface (est.)
350 K
Year
14.2 days
From Earth
18.2 ly

GJ 251 system

Type
M-type (Red Dwarf)
Spectral type
M3.0 V
Temperature
3,342 K
Radius
0.36 ☉
Mass
0.35 ☉
Planets
2
Luminosity
0.0155 ☉

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 251 b0.081 AUYou are here3.9 M⊕
GJ 251 c0.20 AU3.9 M⊕
Habitable zone

Star and planets drawn to scale: the star is 22 times as wide as GJ 251 b, 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 251 b, from the NASA Exoplanet Archive (found 2020). Measured: 3.85 Earth masses, 0.0808 AU, a 3342 K M-type star. Composition inferred from a estimated bulk density of 0.67x 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 1% hydrogen/helium envelope by mass, with a day locked to its 14.2-day year, as anything orbiting this close would be.

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

Sub-Neptune
Life unlikely — too hot for liquid waterLow Activity
GJ 251 b is a sub-Neptune: a 3.9x 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: 3.9x Earth (under 20x, below the giants)✓ A hydrogen-helium envelope over the core

Reading GJ 251 b’s atmosphere

Dominant elements: O, Si, Fe, N, C, HThis is what a telescope would measure while GJ 251 b 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 251 b sits about 18 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
15
Contributing Factors
temperature32
Hot - near boiling, only heat-loving microbes would thrive
atmosphere18

Deep hydrogen-helium envelope with photochemical haze

Hydrogen-rich atmosphere typical of gas giants (partially stripped by slow rotation)
water15
No solid surface — any water is locked in a supercritical mantle and hot ice, not oceans
magnetic Field30
No magnetic field - vulnerable to solar radiation
geology60
Some geological activity provides slow nutrient cycling
Organic Chemistry80
Core organic elements (CHNO) present
rotation30
Very slow rotation - days and nights each last several Earth days, bringing extreme temperature swings

Physical Properties

Mass
3.85x Earth (2.30e+25 kg)
Radius
1.57x Earth (9,985 km)
This planet: 1.57× Earth radiusEarth: 1× (baseline)
Surface Gravity
1.57x Earth (15.4 m/s²)
Surface Temperature
350 K (77 °C / 171 °F)
Rotation Period
340.8 hours/day (slower than Earth)
This planet: 341 h/dayEarth: 24 h/day

Elemental Composition

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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