Create your free account

Enter your email and we'll send a 6-digit code — no password needed.
We only keep your email and the username you choose. We never share or sell your email, and we don't send marketing — we use it only to email you a sign-in code. Read our privacy policy.
Entering orbit…
Habitability
6/ 100
Uninhabitable
K2-121 b rendered in TerraForge from its measured mass, orbit and star

K2-121 b

Also written K2-121b, K2 121b, K2 121 b

Gas Giant551 light years awayFound 2017
Mass
51.00 × Earth
Radius
7.16 × Earth
Gravity
0.99 g
Surface (est.)
792 K
Year
5.2 days
From Earth
551 ly
Mass
51.00 × Earth
Radius
7.16 × Earth
Gravity
0.99 g
Surface (est.)
792 K
Year
5.2 days
From Earth
551 ly

K2-121 system

Type
K-type (Orange)
Temperature
4,526 K
Radius
0.675 ☉
Mass
0.71 ☉
Planets
1
Luminosity
0.180 ☉

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.

K2-121 b0.052 AUYou are here51 M⊕
Habitable zone

Star and planets drawn to scale: the star is 10 times as wide as K2-121 b, so only its edge is in view. Orbits spaced as in the star system view.

K2-121 b, from the NASA Exoplanet Archive (found 2017). Measured: 51 Earth masses, 0.0525 AU, a 4526 K K-type star. Composition inferred from a mass of 51 Earths, which is giant territory whatever its 0.14x density suggests, with a day locked to its 5.2-day year, as anything orbiting this close would be.

Surface. No solid surface - thick gaseous envelope with possible rocky core deep within

Gas Giant
Magnetic FieldExtreme Activity
K2-121 b has 86.0% hydrogen and helium, combined with a mass of 51.0x Earth, which exceeds the gas giant threshold.
Gas giants form when massive cores capture enormous atmospheres of light gases. They typically form beyond the 'frost line' where volatiles can condense.
Classification Criteria:
✓ Hydrogen + Helium: 86.0% (>60% required)✓ Mass: 51.0x Earth (>10x required)

Reading K2-121 b’s atmosphere

Dominant elements: H, He, O, C, NThis is what a telescope would measure while K2-121 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.K2-121 b sits about 551 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
6
Contributing Factors
temperature3
Scorching - hot enough to melt lead, though rock stays solid; no chance for life
atmosphere30

Hydrogen and helium dominated with trace methane and ammonia

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 Field100
Magnetic field provides radiation protection
geology40
Extreme volcanic activity may be hazardous
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
51.00x Earth (3.05e+26 kg)
Radius
3.71x Earth (23,626 km)
This planet: 3.71× Earth radiusEarth: 1× (baseline)
Surface Gravity
3.71x Earth (36.4 m/s²)
Surface Temperature
792 K (519 °C / 966 °F)
Rotation Period
124.6 hours/day (slower than Earth)
This planet: 125 h/dayEarth: 24 h/day

Time Dilation

Clocks run 6.2 seconds per year slower than deep space — about 8 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.

Create your free account

Enter your email and we'll send a 6-digit code — no password needed.
We only keep your email and the username you choose. We never share or sell your email, and we don't send marketing — we use it only to email you a sign-in code. Read our privacy policy.

Send feedback

We're still building out the best experience we can — and we'd love your input. Tell us about features you enjoy, ideas you'd like to see, or pain points where something feels off, and we'll know where to look next.

Type
Idea
Bug
Pain point
Other