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Entering orbit…
Habitability
6/ 100
Uninhabitable
GJ 3998 b rendered in TerraForge from its measured mass, orbit and star

GJ 3998 b

Also written GJ 3998b

Rocky Terrestrial59 light years awayFound 2016
Mass
2.50 × Earth
Radius
1.39 × Earth
Gravity
1.29 g
Surface (est.)
745 K
Year
2.6 days
From Earth
59 ly
Mass
2.50 × Earth
Radius
1.39 × Earth
Gravity
1.29 g
Surface (est.)
745 K
Year
2.6 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 AUYou are here2.5 M⊕
GJ 3998 c0.090 AU6.8 M⊕
GJ 3998 d0.19 AU6.1 M⊕
Habitable zone

Star and planets drawn to scale: the star is 39 times as wide as GJ 3998 b, 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 b, from the NASA Exoplanet Archive (found 2016). Measured: 2.5 Earth masses, 0.03 AU, a 3726 K K-type star. Composition inferred from an estimated bulk density of 0.93x 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 a day locked to its 2.6-day year, as anything orbiting this close would be.

Surface. Rocky surface with impact craters, possible volcanic features

Rocky Terrestrial
Life unlikely — too hot for liquid waterMagnetic FieldHigh Activity
GJ 3998 b 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: 2.5x Earth (terrestrial range)

Reading GJ 3998 b’s atmosphere

Dominant elements: O, Si, Fe, N, C, HThis is what a telescope would measure while GJ 3998 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.GJ 3998 b 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.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 ScoreUninhabitable
6
Contributing Factors
temperature3
Scorching - hot enough to melt lead, though rock stays solid; no chance for life
atmosphere70

Thin CO₂ and nitrogen atmosphere

Nitrogen atmosphere provides protection but not breathable
water8
Limited water content - boiling away, little would be retained
magnetic Field100
Magnetic field provides radiation protection
geology90
Active geology recycles nutrients and drives carbon cycle
Organic Chemistry80
Core organic elements (CHNO) present
rotation65
Slow rotation - significant day/night temperature variations, but life can adapt

Physical Properties

Mass
2.50x Earth (1.49e+25 kg)
Radius
1.36x Earth (8,647 km)
This planet: 1.36× Earth radiusEarth: 1× (baseline)
Surface Gravity
1.36x Earth (13.3 m/s²)
Surface Temperature
745 K (472 °C / 882 °F)
Rotation Period
63.6 hours/day (slower than Earth)
This planet: 64 h/dayEarth: 24 h/day

Time Dilation

Clocks run 10.9 seconds per year slower than deep space — about 15 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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