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Entering orbit…
Habitability
9/ 100
Uninhabitable
Kepler-138 b rendered in TerraForge from its measured mass, orbit and star

Kepler-138 b

Also written Kepler-138b, Kepler 138b, Kepler 138 b

Barren218 light years awayFound 2014
Mass
0.07 × Earth
Radius
0.64 × Earth
Gravity
0.17 g
Surface (est.)
503 K
Year
10.3 days
From Earth
218 ly
Mass
0.07 × Earth
Radius
0.64 × Earth
Gravity
0.17 g
Surface (est.)
503 K
Year
10.3 days
From Earth
218 ly

Kepler-138 system

Type
K-type (Orange)
Spectral type
M1 V
Temperature
3,841 K
Radius
0.535 ☉
Mass
0.535 ☉
Planets
4
Luminosity
0.0560 ☉

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.

Kepler-138 b0.075 AUYou are here0.1 M⊕
Kepler-138 c0.091 AU2.3 M⊕
Kepler-138 d0.13 AU2.1 M⊕
Kepler-138 e0.18 AU0.4 M⊕
Habitable zone

Star and planets drawn to scale: the star is 91 times as wide as Kepler-138 b, so only its edge is in view. Orbits spaced as in the star system view.

Kepler-138 b, from the NASA Exoplanet Archive (found 2014). Measured: 0.07 Earth masses, 0.0753 AU, a 3841 K K-type star. Composition inferred from a measured bulk density of 0.27x Earth's, larger than even a pure water/ice world of this mass, suggesting a water-rich interior under roughly a 0% hydrogen/helium envelope by mass, with a day locked to its 10.3-day year, as anything orbiting this close would be.

Surface. Barren rocky surface with unusual elemental composition

Barren
None Activity
Kepler-138 b 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: 0.1x EarthTemperature: 503K

Reading Kepler-138 b’s atmosphere

Dominant elements: H, O, Si, Fe, N, CThis is what a telescope would measure while Kepler-138 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.Kepler-138 b sits about 218 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.
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.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 ScoreUninhabitable
9
Contributing Factors
temperature9
Extremely hot - hostile to known life forms
atmosphere42

Thin CO₂ and nitrogen atmosphere

Nitrogen atmosphere provides protection but not breathable (partially stripped by slow rotation)
water20
Abundant water - likely as steam (low gravity lets water escape to space)
magnetic Field30
No magnetic field - vulnerable to solar radiation
geology30
Geologically dead - no nutrient recycling
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
0.10x Earth (5.97e+23 kg)
Radius
0.46x Earth (2,957 km)
This planet: 0.46× Earth radiusEarth: 1× (baseline)
Surface Gravity
0.46x Earth (4.6 m/s²)
Surface Temperature
503 K (230 °C / 446 °F)
Rotation Period
247.2 hours/day (slower than Earth)
This planet: 247 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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