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Entering orbit…
Habitability
0/ 100
Uninhabitable
K2-138 d rendered in TerraForge from its measured mass, orbit and star

K2-138 d

Also written K2-138d, K2 138d, K2 138 d

Sub-Neptune662 light years awayFound 2017
Mass
7.92 × Earth
Radius
2.39 × Earth
Gravity
1.39 g
Surface (est.)
986 K
Year
5.4 days
From Earth
662 ly
Mass
7.92 × Earth
Radius
2.39 × Earth
Gravity
1.39 g
Surface (est.)
986 K
Year
5.4 days
From Earth
662 ly

K2-138 system

Type
G-type (Yellow (Sun-like))
Spectral type
G8 V
Temperature
5,356 K
Radius
0.863 ☉
Mass
0.935 ☉
Planets
6
Luminosity
0.517 ☉

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.

K2-138 b0.034 AU3.1 M⊕
K2-138 c0.045 AU6.3 M⊕
K2-138 d0.059 AUYou are here7.9 M⊕
K2-138 e0.078 AU13 M⊕
K2-138 f0.10 AU1.6 M⊕
K2-138 g0.23 AU4.3 M⊕
Habitable zone

Star and planets drawn to scale: the star is 39 times as wide as K2-138 d, so only its edge is in view. Orbits spaced as in the star system view.

Sub-Neptune, likely · its density could also fit a water world

K2-138 d, from the NASA Exoplanet Archive (found 2017). Measured: 7.92 Earth masses, 0.0589 AU, a 5356 K G-type star. Composition inferred from a measured bulk density of 0.58x Earth's, too low for bare rock at this size, suggesting a rocky core under roughly a 3% hydrogen/helium envelope by mass, with a day locked to its 5.4-day year, as anything orbiting this close would be.

Surface. No solid surface - a hydrogen-helium envelope wrapped around a rocky core (a sub-Neptune)

Sub-Neptune
Life unlikely — too hot for liquid waterLow Activity
K2-138 d is a sub-Neptune: a 7.9x Earth core wrapped in a deep hydrogen-helium envelope, with no solid surface to stand on.
Sub-Neptunes are the most common planets Kepler found, though our own solar system has none. Too light to be a giant, they still hold on to a thick envelope of light gas over a rocky or icy core.
Classification Criteria:
✓ Mass: 7.9x Earth (under 20x, below the giants)✓ A hydrogen-helium envelope over the core

Reading K2-138 d’s atmosphere

Dominant elements: O, Si, Fe, N, C, HThis is what a telescope would measure while K2-138 d 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.K2-138 d sits about 662 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.Exoplanets: Diamond Worlds, Super Earths, Pulsar PlanetsMichael Summers & James TrefilThe strange end of the catalogue — carbon worlds, molten rock, planets orbiting dead stars.The Little Book of ExoplanetsJoshua WinnHow these worlds are found, and what a measurement can honestly tell us.
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 ScoreUninhabitable
0
Contributing Factors
temperature0
Scorching - hot enough to melt lead, though rock stays solid; no chance for life
atmosphere30

Deep hydrogen-helium envelope with photochemical haze

Hydrogen-rich atmosphere typical of gas giants
water15
No solid surface — any water is locked in a supercritical mantle and hot ice, not oceans
magnetic Field30
No magnetic field - vulnerable to solar radiation
geology60
Some geological activity provides slow nutrient cycling
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
7.92x Earth (4.73e+25 kg)
Radius
1.99x Earth (12,699 km)
This planet: 1.99× Earth radiusEarth: 1× (baseline)
Surface Gravity
1.99x Earth (19.6 m/s²)
Surface Temperature
986 K (712 °C / 1,314 °F)
Rotation Period
129.6 hours/day (slower than Earth)
This planet: 130 h/dayEarth: 24 h/day

Time Dilation

Clocks run 7.9 seconds per year slower than deep space — about 11 minutes over a lifetime

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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