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Entering orbit…
Habitability
10/ 100
Uninhabitable

Reading TOI-521 b’s atmosphere

Dominant elements: H, O, Si, Fe, N, CThis is what a telescope would measure while TOI-521 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.TOI-521 b sits about 199 light-years away, found by the transit 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.

TOI-521 b

BarrenUninhabitable199 light years awayFound 2025

TOI-521 b, from the NASA Exoplanet Archive (found 2025). Measured: 5.3 Earth masses, 0.0195 AU, a 3544 K M-type star. Composition inferred from a measured bulk density of 0.68x Earth's at this size, suggesting roughly 51% water/ice by mass, with a day locked to its 1.5-day year, as anything orbiting this close would be.

Host star

TOI-521

Type
M-type (Red Dwarf)
Spectral type
M3
Temperature
3,544 K
Radius
0.421 ☉
Mass
0.42 ☉
Known planets
1 (this one)
Luminosity
0.0252 ☉

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.

Barren

No life — too hot for liquid waterMagnetic FieldExtreme Activity
Why is this a Barren?Your planet 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: 5.3x EarthTemperature: 889K
Further reading
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.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 ScoreUninhabitable
10
Contributing Factors
temperature5
Scorching - hot enough to melt lead, though rock stays solid; no chance for life
atmosphere70
Nitrogen atmosphere provides protection but not breathable
water15
Abundant water - boiling away, little would be retained
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
5.30x Earth (3.17e+25 kg)
Radius
1.74x Earth (11,108 km)
This planet: 1.74× Earth radiusEarth: 1× (baseline)
Surface Gravity
1.74x Earth (17.1 m/s²)
Surface Temperature
889 K (616 °C / 1,141 °F)
Rotation Period
37.0 hours/day (slower than Earth)
This planet: 37 h/dayEarth: 24 h/day

Orbital Environment

Star Type
M-type (Red Dwarf)
Distance from Star
0.02 AU (2.92 million km)
Orbital Period
2 days
Time Dilation
Clocks run 7.2 seconds per year slower than deep space — about 10 minutes over a lifetime

Atmosphere

Thin CO₂ and nitrogen atmosphere

Elemental Composition

H - Hydrogen
33.8%
O - Oxygen
30.6%
Si - Silicon
14.2%

Surface Characteristics

Barren rocky surface with unusual elemental composition
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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