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Entering orbit…
Habitability
34/ 100
Extremely Harsh

Reading HD 216520 c’s atmosphere

Dominant elements: H, O, Si, Fe, N, CThis is what a telescope would measure while HD 216520 c 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 216520 c sits about 64 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.

HD 216520 c

Ice GiantExtremely Harsh64 light years awayFound 2020

HD 216520 c, from the NASA Exoplanet Archive (found 2020). Measured: 9.44 Earth masses, 0.528 AU, a 5103 K K-type star. Composition inferred from a estimated bulk density of 0.34x Earth's, modeled rather than measured since this isn't a transiting planet, larger than even a pure water/ice world of this mass, suggesting a water-rich interior under roughly a 4% hydrogen/helium envelope by mass, with an assumed 24-hour day — rotation cannot be measured for exoplanets.

Host star

HD 216520

Type
K-type (Orange)
Spectral type
K2
Temperature
5,103 K
Radius
0.76 ☉
Mass
0.82 ☉
Known planets
2, including this one
Luminosity
0.353 ☉

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.

Ice Giant

No life — no phosphorus — DNA and ATP are built on itMagnetic FieldExtreme Activity
Why is this a Ice Giant?Your planet has moderate mass (9.4x 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: 9.4x Earth (5-20x range)✓ Water/Carbon/Nitrogen rich composition
Further reading
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.Envisioning ExoplanetsMichael CarrollWhat these worlds might look like from the ground, painted from the same measurements this page renders.Imagined LifeJames Trefil & Michael SummersWhat could live on a world like this one: ice shells, supergravity, tidally locked twilight.
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 ScoreExtremely Harsh
34
Contributing Factors
temperature100
Optimal temperature range for liquid water and life
atmosphere30
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
rotation100
Optimal rotation period - balanced day/night cycle, moderate weather, and good heat distribution

Physical Properties

Mass
9.44x Earth (5.64e+25 kg)
Radius
2.11x Earth (13,465 km)
This planet: 2.11× Earth radiusEarth: 1× (baseline)
Surface Gravity
2.11x Earth (20.7 m/s²)
Surface Temperature
309 K (36 °C / 97 °F)
Rotation Period
24.0 hours/day
This planet: 24 h/dayEarth: 24 h/day

Orbital Environment

Star Type
K-type (Orange)
Distance from Star
0.53 AU (79 million km)
Orbital Period
167 days

Atmosphere

Hydrogen and helium with water, methane, and ammonia ices

Elemental Composition

H - Hydrogen
54.9%
O - Oxygen
29.2%
Si - Silicon
5.8%

Surface Characteristics

No solid surface - a deep volatile envelope over a supercritical water mantle and hot-ice interior
A binary star system's orrery: two suns close together, a red dashed dead-zone boundary, and a debris trail left by a destroyed world

New: Binary Star Systems 🌀

A system can now orbit two suns. Add a companion star when you build one — close, medium, or wide — and the physics decides what survives: a shared center of mass carves out a dead zone where gravity from both stars tears a world apart, leaving it as debris, and every surviving planet's orbit goes elliptical, moving fastest at closest approach and slowest out at aphelion. Push the separation to wide and the dead zone opens tens of AU across, wide enough to gut a system that would have been stable around one star alone. Stand on a world that made it through and you'll watch two suns cross the sky together.

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