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Entering orbit…
Habitability
15/ 100
Uninhabitable
Kepler-399 d rendered in TerraForge from its measured mass, orbit and star

Kepler-399 d

Also written Kepler-399d, Kepler 399d, Kepler 399 d

Sub-Neptune2416 light years awayFound 2014
Mass
4.23 × Earth
Radius
1.89 × Earth
Gravity
1.18 g
Surface (est.)
434 K
Year
58.0 days
From Earth
2,417 ly
Mass
4.23 × Earth
Radius
1.89 × Earth
Gravity
1.18 g
Surface (est.)
434 K
Year
58.0 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 AU0.8 M⊕
Kepler-399 c0.15 AU2.6 M⊕
Kepler-399 d0.26 AUYou are here4.2 M⊕
Habitable zone

Star and planets drawn to scale: the star is 39 times as wide as Kepler-399 d, so only its edge is in view. Orbits spaced as in the star system view.

Sub-Neptune, likely · its density could also fit a water world

Kepler-399 d, from the NASA Exoplanet Archive (found 2014). Measured: 4.23 Earth masses, 0.261 AU, a 5502 K G-type star. Composition inferred from a measured bulk density of 0.63x Earth's, too low for bare rock at this size, suggesting a rocky core under roughly a 1% hydrogen/helium envelope by mass, with an assumed 24-hour day — rotation cannot be measured for exoplanets.

Surface. No solid surface - a hydrogen-helium envelope wrapped around a rocky core (a sub-Neptune)

Sub-Neptune
Life unlikely — too hot for liquid waterMagnetic FieldLow Activity
Kepler-399 d is a sub-Neptune: a 4.2x Earth core wrapped in a deep hydrogen-helium envelope, with no solid surface to stand on.
Sub-Neptunes are the most common planets Kepler found, though our own solar system has none. Too light to be a giant, they still hold on to a thick envelope of light gas over a rocky or icy core.
Classification Criteria:
✓ Mass: 4.2x Earth (under 20x, below the giants)✓ A hydrogen-helium envelope over the core

Reading Kepler-399 d’s atmosphere

Dominant elements: O, Si, Fe, N, C, HThis is what a telescope would measure while Kepler-399 d 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 d 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.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
15
Contributing Factors
temperature12
Extremely hot - hostile to known life forms
atmosphere30

Deep hydrogen-helium envelope with photochemical haze

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
geology60
Some geological activity provides slow nutrient cycling
Organic Chemistry80
Core organic elements (CHNO) present
rotation100
Optimal rotation period - balanced day/night cycle, moderate weather, and good heat distribution

Physical Properties

Mass
4.23x Earth (2.53e+25 kg)
Radius
1.62x Earth (10,304 km)
This planet: 1.62× Earth radiusEarth: 1× (baseline)
Surface Gravity
1.62x Earth (15.9 m/s²)
Surface Temperature
434 K (161 °C / 322 °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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