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Entering orbit…
Habitability
0/ 100
Uninhabitable
HD 20781 b rendered in TerraForge from its measured mass, orbit and star

HD 20781 b

Also written HD 20781b

Lava World117 light years awayFound 2019
Mass
1.93 × Earth
Radius
1.21 × Earth
Gravity
1.32 g
Surface (est.)
1,026 K
Year
5.3 days
From Earth
117 ly
Mass
1.93 × Earth
Radius
1.21 × Earth
Gravity
1.32 g
Surface (est.)
1,026 K
Year
5.3 days
From Earth
117 ly

HD 20781 system

Type
G-type (Yellow (Sun-like))
Spectral type
K0 V
Temperature
5,256 K
Radius
0.834 ☉
Mass
0.7 ☉
Planets
4
Luminosity
0.490 ☉

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 20781 b0.053 AUYou are here1.9 M⊕
HD 20781 c0.10 AU5.3 M⊕
HD 20781 d0.17 AU11 M⊕
HD 20781 e0.34 AU14 M⊕
Habitable zone

Star and planets drawn to scale: the star is 75 times as wide as HD 20781 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 20781 b, from the NASA Exoplanet Archive (found 2019). Measured: 1.93 Earth masses, 0.0529 AU, a 5256 K G-type star. Composition inferred from an estimated bulk density of 1.09x Earth's, modeled rather than measured since this isn't a transiting planet — the radius is the archive's estimate, so it can't settle how much water there is; read as rock and iron, with a day locked to its 5.3-day year, as anything orbiting this close would be.

Surface. Molten surface with lava flows, volcanic activity everywhere

Lava World
Life unlikely — too hot for liquid waterExtreme Activity
HD 20781 b's proximity to its star (0.05 AU) results in extreme surface temperatures of 1026K.
Lava worlds orbit so close to their stars that rocky surfaces melt into oceans of molten rock. Recent planet formation can also create temporary lava worlds.
Classification Criteria:
✓ Temperature: 1026K (>1000K)

Reading HD 20781 b’s atmosphere

Dominant elements: O, Si, Fe, N, C, HThis is what a telescope would measure while HD 20781 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 20781 b sits about 117 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.Exoplanets: Diamond Worlds, Super Earths, Pulsar PlanetsMichael Summers & James TrefilThe strange end of the catalogue — carbon worlds, molten rock, planets orbiting dead stars.Imagined LifeJames Trefil & Michael SummersWhat could live on a world like this one: ice shells, supergravity, tidally locked twilight.
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Habitability ScoreUninhabitable
0
Contributing Factors
temperature0
Inferno world - temperatures exceed all possibility of life
atmosphere40

Vaporized rock and metals

Atmosphere present but composition not ideal for life
water8
Limited water content - boiling away, little would be retained
magnetic Field30
No magnetic field - vulnerable to solar radiation
geology40
Extreme volcanic activity may be hazardous
Organic Chemistry80
Core organic elements (CHNO) present
rotation30
Very slow rotation - days and nights each last several Earth days, bringing extreme temperature swings

Physical Properties

Mass
1.93x Earth (1.15e+25 kg)
Radius
1.25x Earth (7,932 km)
This planet: 1.25× Earth radiusEarth: 1× (baseline)
Surface Gravity
1.25x Earth (12.2 m/s²)
Surface Temperature
1,026 K (752 °C / 1,386 °F)
Rotation Period
127.4 hours/day (slower than Earth)
This planet: 127 h/dayEarth: 24 h/day

Time Dilation

Clocks run 8.8 seconds per year slower than deep space — about 12 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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