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Entering orbit…
Habitability
0/ 100
Uninhabitable
HD 18438 b rendered in TerraForge from its measured mass, orbit and star

HD 18438 b

Also written HD 18438b

Brown Dwarf730 light years awayFound 2023
Mass
6670.00 × Earth
Radius
12.10 × Earth
Gravity
45.56 g
Surface (est.)
1,070 K
Year
803 days
From Earth
731 ly
Mass
6670.00 × Earth
Radius
12.10 × Earth
Gravity
45.56 g
Surface (est.)
1,070 K
Year
803 days
From Earth
731 ly

HD 18438 system

Type
K-type (Orange)
Spectral type
M2.5 III
Temperature
3,860 K
Radius
88.5 ☉
Mass
1.84 ☉
Planets
1
Luminosity
929 ☉

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 18438 b2.10 AUYou are here6,670 M⊕
Habitable zone

Star and planets drawn to scale: the star is 799 times as wide as HD 18438 b, so only its edge is in view. Orbits spaced as in the star system view.

Estimated: this planet doesn't transit its star, so its size is inferred, not measured

HD 18438 b, from the NASA Exoplanet Archive (found 2023). Measured: 6670 Earth masses, 2.1 AU, a 3860 K K-type star. Composition inferred from a mass of 6670 Earths, which is giant territory whatever its 3.77x density suggests, with an assumed 24-hour day — rotation cannot be measured for exoplanets.

Surface. No surface: a cooling failed star, its methane atmosphere absorbing so much red light it would look magenta to the eye

Brown Dwarf
Magnetic FieldNone Activity
HD 18438 b has an unusual composition that doesn't fit standard classifications, or lacks the elements needed for complex geological processes.
Barren worlds can form from uncommon element combinations or may be remnants of larger bodies that lost their atmospheres and volatile materials.
Classification Criteria:
Composition doesn't match standard planet typesMass: 6670.0x EarthTemperature: 1070K

Reading HD 18438 b’s atmosphere

Dominant elements: H, He, O, C, NThis is what a telescope would measure while HD 18438 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 18438 b sits about 731 light-years away, found by the radial velocity 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
0
Contributing Factors
temperature0
Inferno world - temperatures exceed all possibility of life
atmosphere30

Hydrogen and helium dominated by methane absorption, with water clouds below

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 Field20
A strong field, but no surface beneath it — there is nothing here for it to shelter
geology30
Geologically dead - no nutrient recycling
Organic Chemistry80
Core organic elements (CHNO) present
rotation100
Optimal rotation period - balanced day/night cycle, moderate weather, and good heat distribution

Physical Properties

Mass
6670.00x Earth (3.98e+28 kg)
Radius
18.82x Earth (119,927 km)
This planet: 18.82× Earth radiusEarth: 1× (baseline)
Surface Gravity
18.82x Earth (184.7 m/s²)
Surface Temperature
1,070 K (797 °C / 1,466 °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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