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Entering orbit…
Habitability
0/ 100
Uninhabitable
Kepler-21 b rendered in TerraForge from its measured mass, orbit and star

Kepler-21 b

Also written Kepler-21b, Kepler 21b, Kepler 21 b

Lava World355 light years awayFound 2012
Mass
7.50 × Earth
Radius
1.64 × Earth
Gravity
2.79 g
Surface (est.)
2,014 K
Year
2.8 days
From Earth
356 ly
Mass
7.50 × Earth
Radius
1.64 × Earth
Gravity
2.79 g
Surface (est.)
2,014 K
Year
2.8 days
From Earth
356 ly

Kepler-21 system

Type
F-type (White)
Spectral type
F6 IV
Temperature
6,305 K
Radius
1.9 ☉
Mass
1.41 ☉
Planets
1
Luminosity
5.19 ☉

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.

Kepler-21 b0.043 AUYou are here7.5 M⊕
Habitable zone

Star and planets drawn to scale: the star is 127 times as wide as Kepler-21 b, so only its edge is in view. Orbits spaced as in the star system view.

Kepler-21 b, from the NASA Exoplanet Archive (found 2012). Measured: 7.5 Earth masses, 0.0434 AU, a 6305 K F-type star. Composition inferred from a measured bulk density of 1.70x Earth's, far too high for silicates alone — an iron-rich world, with a day locked to its 2.8-day year, as anything orbiting this close would be.

Surface. Molten surface with lava flows, volcanic activity everywhere

Lava World
Magnetic FieldExtreme Activity
Kepler-21 b's proximity to its star (0.04 AU) results in extreme surface temperatures of 2014K.
Lava worlds orbit so close to their stars that rocky surfaces melt into oceans of molten rock. Recent planet formation can also create temporary lava worlds.
Classification Criteria:
✓ Temperature: 2014K (>1000K)

Reading Kepler-21 b’s atmosphere

Dominant elements: Si, Fe, Mg, Ni, S, AlThis is what a telescope would measure while Kepler-21 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.Kepler-21 b sits about 356 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.Imagined LifeJames Trefil & Michael SummersWhat could live on a world like this one: ice shells, supergravity, tidally locked twilight.
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Habitability ScoreUninhabitable
0
Contributing Factors
temperature0
Inferno world - temperatures exceed all possibility of life
atmosphere10

Minimal - too hot to retain

No atmosphere - exposed to radiation and temperature extremes
water0
No water - the recipe has no hydrogen and oxygen to form it
magnetic Field100
Magnetic field provides radiation protection
geology40
Extreme volcanic activity may be hazardous
Organic Chemistry20
Limited organic chemistry potential
rotation65
Slow rotation - significant day/night temperature variations, but life can adapt

Physical Properties

Mass
7.50x Earth (4.48e+25 kg)
Radius
1.96x Earth (12,471 km)
This planet: 1.96× Earth radiusEarth: 1× (baseline)
Surface Gravity
1.96x Earth (19.2 m/s²)
Surface Temperature
2,014 K (1,741 °C / 3,166 °F)
Rotation Period
67.0 hours/day (slower than Earth)
This planet: 67 h/dayEarth: 24 h/day

Time Dilation

Clocks run 14.0 seconds per year slower than deep space — about 19 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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