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Entering orbit…
Habitability
64/ 100
Habitable

Reading GJ 251 c’s atmosphere

Dominant elements: H, O, Si, Fe, N, CThis is what a telescope would measure while GJ 251 c 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 c 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.

GJ 251 c

Ice WorldHabitable18 light years awayFound 2025

GJ 251 c, from the NASA Exoplanet Archive (found 2025). Measured: 3.88 Earth masses, 0.196 AU, a 3342 K M-type star. Composition inferred from a estimated bulk density of 0.67x Earth's at this size, modeled rather than measured since this isn't a transiting planet, suggesting roughly 49% water/ice by mass, with an assumed 24-hour day — rotation cannot be measured for exoplanets.

Host star

GJ 251

Type
M-type (Red Dwarf)
Spectral type
M3.0 V
Temperature
3,342 K
Radius
0.36 ☉
Mass
0.35 ☉
Known planets
2, including this one
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.

Ice World

No life — too cold for liquid waterMagnetic FieldLow Activity
Why is this a Ice World?Your planet orbits at 0.20 AU from its star, resulting in frigid temperatures of 249K.
Ice worlds form beyond the frost line where temperatures are low enough for water and other volatiles to freeze solid. They may have subsurface oceans beneath thick ice shells.
Classification Criteria:
✓ Distance: 0.20 AU (>3 AU) or Temperature <200K✓ Temperature: 249K✓ Water content: 47.2% for ice formation
Further reading
Imagined LifeJames Trefil & Michael SummersWhat could live on a world like this one: ice shells, supergravity, tidally locked twilight.Envisioning ExoplanetsMichael CarrollWhat these worlds might look like from the ground, painted from the same measurements this page renders.The Little Book of ExoplanetsJoshua WinnHow these worlds are found, and what a measurement can honestly tell us.Exoplanets: Diamond Worlds, Super Earths, Pulsar PlanetsMichael Summers & James TrefilThe strange end of the catalogue — carbon worlds, molten rock, planets orbiting dead stars.
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Habitability ScoreHabitable
64
Contributing Factors
temperature40
Extreme temperature - very difficult for life
atmosphere70
Nitrogen atmosphere provides protection but not breathable
water80
Abundant water - mostly frozen
magnetic Field100
Magnetic field provides radiation protection
geology60
Some geological activity provides slow nutrient cycling
Organic Chemistry80
Core organic elements (CHNO) present
rotation100
Optimal rotation period - balanced day/night cycle, moderate weather, and good heat distribution

Physical Properties

Mass
3.88x Earth (2.32e+25 kg)
Radius
1.57x Earth (10,011 km)
This planet: 1.57× Earth radiusEarth: 1× (baseline)
Surface Gravity
1.57x Earth (15.4 m/s²)
Surface Temperature
249 K (-24 °C / -12 °F)
Rotation Period
24.0 hours/day
This planet: 24 h/dayEarth: 24 h/day

Orbital Environment

Star Type
M-type (Red Dwarf)
Distance from Star
0.20 AU (29.3 million km)
Orbital Period
58 days

Atmosphere

Thin nitrogen and methane atmosphere

Elemental Composition

H - Hydrogen
33.1%
O - Oxygen
30.6%
Si - Silicon
14.6%

Surface Characteristics

Frozen surface covered in water ice, possible subsurface ocean if tidal heating present
A binary star system's orrery: two suns close together, a red dashed dead-zone boundary, and a debris trail left by a destroyed world

New: Binary Star Systems 🌀

A system can now orbit two suns. Add a companion star when you build one — close, medium, or wide — and the physics decides what survives: a shared center of mass carves out a dead zone where gravity from both stars tears a world apart, leaving it as debris, and every surviving planet's orbit goes elliptical, moving fastest at closest approach and slowest out at aphelion. Push the separation to wide and the dead zone opens tens of AU across, wide enough to gut a system that would have been stable around one star alone. Stand on a world that made it through and you'll watch two suns cross the sky together.

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