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Entering orbit…
Habitability
48/ 100
Marginal

Reading K2-9 b’s atmosphere

Dominant elements: H, O, Si, Fe, N, CThis is what a telescope would measure while K2-9 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.K2-9 b sits about 271 light-years away, found by the transit 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.

K2-9 b

Water-RichMarginal271 light years awayFound 2015

K2-9 b, from the NASA Exoplanet Archive (found 2015). Measured: 5.69 Earth masses, 0.091 AU, a 3390 K M-type star. Composition inferred from a measured bulk density of 0.50x Earth's at this size, suggesting roughly 90% water/ice by mass, with a day locked to its 18.4-day year, as anything orbiting this close would be.

Host star

K2-9

Type
M-type (Red Dwarf)
Spectral type
M2.5 V
Temperature
3,390 K
Radius
0.31 ☉
Mass
0.3 ☉
Known planets
1 (this one)
Luminosity
0.0120 ☉

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.

Water World

Life DetectedExtreme Activity
Why is this a Water World?Your planet has abundant oxygen (30.1%) and hydrogen (50.9%), creating extensive water coverage while in the habitable temperature zone.
Water worlds have deep global oceans covering most or all of their surface. They form when planets have high water content and orbit in the habitable zone.
Classification Criteria:
✓ Oxygen: 30.1% + Hydrogen: 50.9% (water-forming elements)✓ Temperature: 324K (habitable range)✓ Distance: 0.09 AU (habitable zone)
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.
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 ScoreMarginal
48
Contributing Factors
temperature70
Challenging but potentially habitable temperature
atmosphere60
Nitrogen-oxygen atmosphere - breathable and protective (partially stripped by slow rotation)
water100
Abundant water
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
5.69x Earth (3.40e+25 kg)
Radius
1.79x Earth (11,374 km)
This planet: 1.79× Earth radiusEarth: 1× (baseline)
Surface Gravity
1.79x Earth (17.5 m/s²)
Surface Temperature
324 K (51 °C / 124 °F)
Rotation Period
441.6 hours/day (slower than Earth)
This planet: 442 h/dayEarth: 24 h/day

Orbital Environment

Star Type
M-type (Red Dwarf)
Distance from Star
0.09 AU (13.6 million km)
Orbital Period
18 days

Atmosphere

Nitrogen and oxygen with water vapor, mild greenhouse effect

Elemental Composition

H - Hydrogen
50.9%
O - Oxygen
30.1%
Si - Silicon
7.7%

Surface Characteristics

Vast oceans covering most of the surface, scattered islands or archipelagos

Habitability

✓ Conditions suitable for life detectedThis planet exhibits the necessary conditions to support biological processes, including stable temperatures, liquid water, and protective magnetic fields.
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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