Create your free account

Enter your email and we'll send a 6-digit code — no password needed.
We only keep your email and the username you choose. We never share or sell your email, and we don't send marketing — we use it only to email you a sign-in code. Read our privacy policy.
Entering orbit…
Habitability
21/ 100
Extremely Harsh

Reading K2-18 c’s atmosphere

Dominant elements: H, O, Si, Fe, N, CThis is what a telescope would measure while K2-18 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.K2-18 c sits about 124 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.

K2-18 c

BarrenExtremely Harsh124 light years awayFound 2017

K2-18 c, from the NASA Exoplanet Archive (found 2017). Measured: 7.51 Earth masses, 0.06 AU, a 3457 K M-type star. Composition inferred from a estimated bulk density of 0.40x Earth's, modeled rather than measured since this isn't a transiting planet, larger than even a pure water/ice world of this mass, suggesting a water-rich interior under roughly a 1% hydrogen/helium envelope by mass, with a day locked to its 9.0-day year, as anything orbiting this close would be. Note: TerraForge comes out at 475 K against the archive's 363 K, because it models stars by spectral class, and K2-18 is not an average M-type.

Host star

K2-18

Type
M-type (Red Dwarf)
Spectral type
M2.5 V
Temperature
3,457 K
Radius
0.411 ☉
Mass
0.359 ☉
Known planets
2, including this one
Luminosity
0.0253 ☉

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.

Barren

Extreme Activity
Why is this a Barren?Your planet has an unusual composition that doesn't fit standard classifications, or lacks the elements needed for complex geological processes.
Barren worlds can form from uncommon element combinations or may be remnants of larger bodies that lost their atmospheres and volatile materials.
Classification Criteria:
Composition doesn't match standard planet typesMass: 7.5x EarthTemperature: 475K
Further reading
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.Envisioning ExoplanetsMichael CarrollWhat these worlds might look like from the ground, painted from the same measurements this page renders.Imagined LifeJames Trefil & Michael SummersWhat could live on a world like this one: ice shells, supergravity, tidally locked twilight.
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 ScoreExtremely Harsh
21
Contributing Factors
temperature15
Extremely hot - hostile to known life forms
atmosphere42
Nitrogen atmosphere provides protection but not breathable (partially stripped by slow rotation)
water62
Abundant water - likely as steam
magnetic Field30
No magnetic field - vulnerable to solar radiation
geology40
Extreme volcanic activity may be hazardous
Organic Chemistry80
Core organic elements (CHNO) present
rotation30
Very slow rotation - extreme temperature swings between eternal day and night sides

Physical Properties

Mass
7.51x Earth (4.48e+25 kg)
Radius
1.96x Earth (12,476 km)
This planet: 1.96× Earth radiusEarth: 1× (baseline)
Surface Gravity
1.96x Earth (19.2 m/s²)
Surface Temperature
475 K (202 °C / 396 °F)
Rotation Period
215.0 hours/day (slower than Earth)
This planet: 215 h/dayEarth: 24 h/day

Orbital Environment

Star Type
M-type (Red Dwarf)
Distance from Star
0.06 AU (8.98 million km)
Orbital Period
10 days

Atmosphere

Thin CO₂ and nitrogen atmosphere

Elemental Composition

H - Hydrogen
55.0%
O - Oxygen
29.7%
Si - Silicon
5.9%

Surface Characteristics

Barren rocky surface with unusual elemental composition
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.

Create your free account

Enter your email and we'll send a 6-digit code — no password needed.
We only keep your email and the username you choose. We never share or sell your email, and we don't send marketing — we use it only to email you a sign-in code. Read our privacy policy.

Send feedback

We're still building out the best experience we can — and we'd love your input. Tell us about features you enjoy, ideas you'd like to see, or pain points where something feels off, and we'll know where to look next.

Type
Idea
Bug
Pain point
Other