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Entering orbit…
Habitability
84/ 100
Highly Habitable

GJ 3998 d

Water WorldHighly Habitable59 light years awayFound 2025

GJ 3998 d, from the NASA Exoplanet Archive (found 2025). Measured: 6.07 Earth masses, 0.189 AU, a 3726 K K-type star. Composition inferred from a measured bulk density of 0.47x Earth's — too light for bare rock, so a large fraction of water and ice, with an assumed 24-hour day — rotation cannot be measured for exoplanets.

Host star

GJ 3998

Type
K-type (Orange)
Spectral type
M1
Temperature
3,726 K
Radius
0.5 ☉
Mass
0.52 ☉
Known planets
3, including this one
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.

Water World

Life DetectedMagnetic FieldExtreme Activity
Why is this a Water World?Your planet has abundant oxygen (41.2%) and hydrogen (17.6%), 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: 41.2% + Hydrogen: 17.6% (water-forming elements)✓ Temperature: 315K (habitable range)✓ Distance: 0.19 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.
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Habitability ScoreHighly Habitable
84
Contributing Factors
temperature70
Challenging but potentially habitable temperature
atmosphere100
Nitrogen-oxygen atmosphere - breathable and protective
water100
Abundant water
magnetic Field100
Magnetic field provides radiation protection
geology40
Extreme volcanic activity may be hazardous
Organic Chemistry80
Core organic elements (CHNO) present
rotation100
Optimal rotation period - balanced day/night cycle, moderate weather, and good heat distribution

Physical Properties

Mass
6.07x Earth (3.63e+25 kg)
Radius
1.82x Earth (11,622 km)
This planet: 1.82× Earth radiusEarth: 1× (baseline)
Surface Gravity
1.82x Earth (17.9 m/s²)
Surface Temperature
315 K (42 °C / 107 °F)
Rotation Period
24.0 hours/day
This planet: 24 h/dayEarth: 24 h/day

Orbital Environment

Star Type
K-type (Orange)
Distance from Star
0.19 AU (28.3 million km)
Orbital Period
36 days

Atmosphere

Nitrogen and oxygen with water vapor, mild greenhouse effect

Elemental Composition

O - Oxygen
41.2%
H - Hydrogen
17.6%
Si - Silicon
11.8%

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.
The TerraForge exoplanet catalog: 5,572 real exoplanets run through the simulation, with a search field and rendered worlds grouped by type

New: The Exoplanet Catalog 🔭

Every confirmed planet NASA has measured well enough to build — 5,572 of them — is now in here, and none of them arrived with a label. Their real mass, radius, orbit and star go through the same simulation that runs everything you make, and it decides what each one is: gas giant, lava world, ocean, ice. Sixteen come out Earth-like. Find TRAPPIST-1e, stand on its surface, and see what a red dwarf's light actually looks like from the ground.

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