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Entering orbit…
Habitability
0/ 100
Uninhabitable
HD 18599 b rendered in TerraForge from its measured mass, orbit and star

HD 18599 b

Also written HD 18599b

Ice Giant126 light years awayFound 2022
Mass
24.10 × Earth
Radius
2.60 × Earth
Gravity
3.57 g
Surface (est.)
1,035 K
Year
4.1 days
From Earth
126 ly
Mass
24.10 × Earth
Radius
2.60 × Earth
Gravity
3.57 g
Surface (est.)
1,035 K
Year
4.1 days
From Earth
126 ly

HD 18599 system

Type
K-type (Orange)
Spectral type
K2 V
Temperature
5,145 K
Radius
0.767 ☉
Mass
0.863 ☉
Planets
1
Luminosity
0.372 ☉

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.

HD 18599 b0.048 AUYou are here24 M⊕
Habitable zone

Star and planets drawn to scale: the star is 32 times as wide as HD 18599 b, so only its edge is in view. Orbits spaced as in the star system view.

HD 18599 b, from the NASA Exoplanet Archive (found 2022). Measured: 24.1 Earth masses, 0.048 AU, a 5145 K K-type star. Composition inferred from a measured bulk density of 1.37x Earth's at this size, suggesting roughly 14% water/ice by mass, with a day locked to its 4.1-day year, as anything orbiting this close would be.

Surface. No solid surface - a deep volatile envelope over a supercritical water mantle and hot-ice interior

Ice Giant
Life unlikely — too hot for liquid waterMagnetic FieldExtreme Activity
HD 18599 b has moderate mass (24.1x Earth) with significant water, carbon, and nitrogen content, characteristic of ice giants.
Ice giants like Uranus and Neptune have icy interiors with thick atmospheres of hydrogen and helium, but not enough to be classified as gas giants.
Classification Criteria:
✓ Mass: 24.1x Earth (5-20x range)✓ Water/Carbon/Nitrogen rich composition

Reading HD 18599 b’s atmosphere

Dominant elements: O, Si, H, Fe, N, CThis is what a telescope would measure while HD 18599 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.HD 18599 b sits about 126 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.
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Habitability ScoreUninhabitable
0
Contributing Factors
temperature0
Inferno world - temperatures exceed all possibility of life
atmosphere30

Hydrogen and helium with water, methane, and ammonia ices

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
rotation65
Slow rotation - significant day/night temperature variations, but life can adapt

Physical Properties

Mass
24.10x Earth (1.44e+26 kg)
Radius
2.89x Earth (18,403 km)
This planet: 2.89× Earth radiusEarth: 1× (baseline)
Surface Gravity
2.89x Earth (28.3 m/s²)
Surface Temperature
1,035 K (762 °C / 1,403 °F)
Rotation Period
99.4 hours/day (slower than Earth)
This planet: 99 h/dayEarth: 24 h/day

Time Dilation

Clocks run 6.8 seconds per year slower than deep space — about 9 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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