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Entering orbit…
Habitability
50/ 100
Marginal
Kepler-138 e rendered in TerraForge from its measured mass, orbit and star

Kepler-138 e

Also written Kepler-138e, Kepler 138e, Kepler 138 e

Rocky Terrestrial218 light years awayFound 2022
Mass
0.43 × Earth
Radius
0.80 × Earth
Gravity
0.68 g
Surface (est.)
322 K
Year
38.2 days
From Earth
218 ly
Mass
0.43 × Earth
Radius
0.80 × Earth
Gravity
0.68 g
Surface (est.)
322 K
Year
38.2 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 AU0.1 M⊕
Kepler-138 c0.091 AU2.3 M⊕
Kepler-138 d0.13 AU2.1 M⊕
Kepler-138 e0.18 AUYou are here0.4 M⊕
Habitable zone

Star and planets drawn to scale: the star is 73 times as wide as Kepler-138 e, 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

Kepler-138 e, from the NASA Exoplanet Archive (found 2022). Measured: 0.43 Earth masses, 0.18 AU, a 3841 K K-type star. Composition inferred from an estimated bulk density of 0.85x Earth's, modeled rather than measured since this isn't a transiting planet — the radius is the archive's estimate, so it can't settle how much water there is; read as rock and iron, with an assumed 24-hour day — rotation cannot be measured for exoplanets.

Surface. Rocky surface with impact craters, possible volcanic features

✓ Conditions could support life. This planet’s modelled conditions could support biological processes: stable temperatures, liquid water and the chemistry life as we know it needs. That makes life possible, not present.

Rocky Terrestrial
Life possibleLow Activity
Kepler-138 e is dominated by silicate (25.0%) and iron (12.5%) content, forming a rocky terrestrial body.
Rocky terrestrial planets are primarily composed of silicate rocks and metals. They're the most common type of small planet in the universe.
Classification Criteria:
✓ Silicon: 25.0% + Iron: 12.5% + Magnesium: 3.8%✓ Mass: 0.4x Earth (terrestrial range)

Reading Kepler-138 e’s atmosphere

Dominant elements: O, Si, Fe, N, C, HThis is what a telescope would measure while Kepler-138 e 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 e sits about 218 light-years away, found by the transit timing variations 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.
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
50
Contributing Factors
temperature76
Warm - livable, but heat stress grows with every degree
atmosphere70

Thin CO₂ and nitrogen atmosphere

Nitrogen atmosphere provides protection but not breathable
water35
Limited water content (low gravity lets water escape to space)
magnetic Field30
No magnetic field - vulnerable to solar radiation
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
0.43x Earth (2.57e+24 kg)
Radius
0.75x Earth (4,809 km)
This planet: 0.75× Earth radiusEarth: 1× (baseline)
Surface Gravity
0.75x Earth (7.4 m/s²)
Surface Temperature
322 K (49 °C / 120 °F)
Rotation Period
24.0 hours/day
This planet: 24 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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