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Entering orbit…
Habitability
30/ 100
Extremely Harsh

Reading HN Lib b’s atmosphere

Dominant elements: H, O, Si, Fe, N, CThis is what a telescope would measure while HN Lib 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.HN Lib b sits about 20 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.

HN Lib b

Ice GiantExtremely Harsh20 light years awayFound 2023

HN Lib b, from the NASA Exoplanet Archive (found 2023). Measured: 5.46 Earth masses, 0.142 AU, a 3347 K M-type star. Composition inferred from a estimated bulk density of 0.51x Earth's at this size, modeled rather than measured since this isn't a transiting planet, suggesting roughly 85% water/ice by mass, with an assumed 24-hour day — rotation cannot be measured for exoplanets.

Host star

HN Lib

Type
M-type (Red Dwarf)
Spectral type
M4.0 V
Temperature
3,347 K
Radius
0.299 ☉
Mass
0.291 ☉
Known planets
1 (this one)
Luminosity
0.0101 ☉

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

Magnetic FieldModerate Activity
Why is this a Ice Giant?Your planet has moderate mass (5.5x 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: 5.5x 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.
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Habitability ScoreExtremely Harsh
30
Contributing Factors
temperature40
Extreme temperature - very difficult for 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
geology90
Active geology recycles nutrients and drives carbon cycle
Organic Chemistry80
Core organic elements (CHNO) present
rotation100
Optimal rotation period - balanced day/night cycle, moderate weather, and good heat distribution

Physical Properties

Mass
5.46x Earth (3.26e+25 kg)
Radius
1.76x Earth (11,219 km)
This planet: 1.76× Earth radiusEarth: 1× (baseline)
Surface Gravity
1.76x Earth (17.3 m/s²)
Surface Temperature
250 K (-23 °C / -10 °F)
Rotation Period
24.0 hours/day
This planet: 24 h/dayEarth: 24 h/day

Orbital Environment

Star Type
M-type (Red Dwarf)
Distance from Star
0.14 AU (21.2 million km)
Orbital Period
36 days

Atmosphere

Hydrogen and helium with water, methane, and ammonia ices

Elemental Composition

H - Hydrogen
49.1%
O - Oxygen
30.2%
Si - Silicon
8.4%

Surface Characteristics

Icy mantle surrounding rocky core, with hydrogen-helium atmosphere
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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