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Entering orbit…
Habitability
8/ 100
Uninhabitable
Kepler-399 b rendered in TerraForge from its measured mass, orbit and star

Kepler-399 b

Also written Kepler-399b, Kepler 399b, Kepler 399 b

Rocky Terrestrial2416 light years awayFound 2014
Mass
0.84 × Earth
Radius
0.96 × Earth
Gravity
0.91 g
Surface (est.)
691 K
Year
14.4 days
From Earth
2,417 ly
Mass
0.84 × Earth
Radius
0.96 × Earth
Gravity
0.91 g
Surface (est.)
691 K
Year
14.4 days
From Earth
2,417 ly

Kepler-399 system

Type
G-type (Yellow (Sun-like))
Temperature
5,502 K
Radius
0.68 ☉
Mass
0.902 ☉
Planets
3
Luminosity
0.382 ☉

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-399 b0.10 AUYou are here0.8 M⊕
Kepler-399 c0.15 AU2.6 M⊕
Kepler-399 d0.26 AU4.2 M⊕
Habitable zone

Star and planets drawn to scale: the star is 77 times as wide as Kepler-399 b, so only its edge is in view. Orbits spaced as in the star system view.

Kepler-399 b, from the NASA Exoplanet Archive (found 2014). Measured: 0.84 Earth masses, 0.103 AU, a 5502 K G-type star. Composition inferred from a measured bulk density of 0.95x Earth's, which is rock and iron, with an assumed 24-hour day — rotation cannot be measured for exoplanets.

Surface. Rocky surface with impact craters, possible volcanic features

Rocky Terrestrial
Life unlikely — too hot for liquid waterMagnetic FieldModerate Activity
Kepler-399 b is dominated by silicate (24.7%) and iron (12.3%) 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: 24.7% + Iron: 12.3% + Magnesium: 3.7%✓ Mass: 0.8x Earth (terrestrial range)

Reading Kepler-399 b’s atmosphere

Dominant elements: O, Si, Fe, N, H, CThis is what a telescope would measure while Kepler-399 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-399 b sits about 2417 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.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 ScoreUninhabitable
8
Contributing Factors
temperature4
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
rotation100
Optimal rotation period - balanced day/night cycle, moderate weather, and good heat distribution

Physical Properties

Mass
0.84x Earth (5.02e+24 kg)
Radius
0.94x Earth (6,011 km)
This planet: 0.94× Earth radiusEarth: 1× (baseline)
Surface Gravity
0.94x Earth (9.3 m/s²)
Surface Temperature
691 K (417 °C / 783 °F)
Rotation Period
24.0 hours/day
This planet: 24 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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