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Entering orbit…
Habitability
9/ 100
Uninhabitable
GJ 3998 c rendered in TerraForge from its measured mass, orbit and star

GJ 3998 c

Also written GJ 3998c

Sub-Neptune59 light years awayFound 2016
Mass
6.82 × Earth
Radius
2.50 × Earth
Gravity
1.09 g
Surface (est.)
430 K
Year
13.7 days
From Earth
59 ly
Mass
6.82 × Earth
Radius
2.50 × Earth
Gravity
1.09 g
Surface (est.)
430 K
Year
13.7 days
From Earth
59 ly

GJ 3998 system

Type
K-type (Orange)
Spectral type
M1
Temperature
3,726 K
Radius
0.5 ☉
Mass
0.52 ☉
Planets
3
Luminosity
0.0439 ☉

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.

GJ 3998 b0.030 AU2.5 M⊕
GJ 3998 c0.090 AUYou are here6.8 M⊕
GJ 3998 d0.19 AU6.1 M⊕
Habitable zone

Star and planets drawn to scale: the star is 22 times as wide as GJ 3998 c, 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

GJ 3998 c, from the NASA Exoplanet Archive (found 2016). Measured: 6.82 Earth masses, 0.09 AU, a 3726 K K-type star. Composition inferred from a estimated bulk density of 0.44x Earth's, modeled rather than measured since this isn't a transiting planet, too low for bare rock at this size, suggesting a rocky core under roughly a 4% hydrogen/helium envelope by mass, with a day locked to its 13.7-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
GJ 3998 c is a sub-Neptune: a 6.8x 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: 6.8x Earth (under 20x, below the giants)✓ A hydrogen-helium envelope over the core

Reading GJ 3998 c’s atmosphere

Dominant elements: O, Si, Fe, N, C, HThis is what a telescope would measure while GJ 3998 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.GJ 3998 c sits about 59 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.
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
9
Contributing Factors
temperature12
Extremely hot - hostile to known life forms
atmosphere18

Deep hydrogen-helium envelope with photochemical haze

Hydrogen-rich atmosphere typical of gas giants (partially stripped by slow rotation)
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
6.82x Earth (4.07e+25 kg)
Radius
1.90x Earth (12,082 km)
This planet: 1.90× Earth radiusEarth: 1× (baseline)
Surface Gravity
1.90x Earth (18.6 m/s²)
Surface Temperature
430 K (157 °C / 315 °F)
Rotation Period
328.8 hours/day (slower than Earth)
This planet: 329 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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