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Entering orbit…
Habitability
0/ 100
Uninhabitable
HD 81817 b rendered in TerraForge from its measured mass, orbit and star

HD 81817 b

Also written HD 81817b

Brown Dwarf877 light years awayFound 2020
Mass
7670.00 × Earth
Radius
12.00 × Earth
Gravity
53.26 g
Surface (est.)
1,440 K
Year
2.8 years
From Earth
877 ly
Mass
7670.00 × Earth
Radius
12.00 × Earth
Gravity
53.26 g
Surface (est.)
1,440 K
Year
2.8 years
From Earth
877 ly

HD 81817 system

Type
K-type (Orange)
Spectral type
K3 III
Temperature
4,140 K
Radius
83.8 ☉
Mass
4.3 ☉
Planets
2
Luminosity
1.82e+3 ☉

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 81817 c2.33 AU7,190 M⊕
HD 81817 b3.23 AUYou are here7,670 M⊕
Habitable zone

Star and planets drawn to scale: the star is 763 times as wide as HD 81817 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 81817 b, from the NASA Exoplanet Archive (found 2020). Measured: 7670 Earth masses, 3.23 AU, a 4140 K K-type star. Composition inferred from a mass of 7670 Earths, which is giant territory whatever its 4.44x density suggests, with an assumed 24-hour day — rotation cannot be measured for exoplanets. Note: TerraForge comes out at 1440 K against the archive's 1016 K, because it models stars by spectral class, and HD 81817 is not an average K-type.

Surface. No surface: a deuterium-burning sphere the size of Jupiter and dozens of times its mass, glowing dull red from its own heat

Brown Dwarf
Magnetic FieldNone Activity
HD 81817 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: 7670.0x EarthTemperature: 1440K

Reading HD 81817 b’s atmosphere

Dominant elements: H, He, O, C, NThis is what a telescope would measure while HD 81817 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 81817 b sits about 877 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.
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
0
Contributing Factors
temperature0
Inferno world - temperatures exceed all possibility of life
atmosphere30

Hydrogen and helium with iron and silicate cloud decks - hot enough to rain molten metal

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
7670.00x Earth (4.58e+28 kg)
Radius
19.72x Earth (125,643 km)
This planet: 19.72× Earth radiusEarth: 1× (baseline)
Surface Gravity
19.72x Earth (193.5 m/s²)
Surface Temperature
1,440 K (1,167 °C / 2,132 °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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