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Entering orbit…
Habitability
20/ 100
Extremely Harsh
YZ Cet b rendered in TerraForge from its measured mass, orbit and star

YZ Cet b

Also written YZ Cetb

Rocky Terrestrial12 light years awayFound 2017
Mass
0.70 × Earth
Radius
0.91 × Earth
Gravity
0.84 g
Surface (est.)
477 K
Year
2.0 days
From Earth
12.1 ly
Mass
0.70 × Earth
Radius
0.91 × Earth
Gravity
0.84 g
Surface (est.)
477 K
Year
2.0 days
From Earth
12.1 ly

YZ Cet system

Type
M-type (Red Dwarf)
Spectral type
M4.5 V
Temperature
3,151 K
Radius
0.157 ☉
Mass
0.142 ☉
Planets
3
Luminosity
2.19e-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.

YZ Cet b0.016 AUYou are here0.7 M⊕
YZ Cet c0.022 AU1.1 M⊕
YZ Cet d0.029 AU1.1 M⊕
Habitable zone

Star and planets drawn to scale: the star is 19 times as wide as YZ Cet 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

YZ Ceti b may be the first planet beyond the solar system with a magnetic field detected through radio waves. It is the innermost of three Earth-mass planets around YZ Ceti, a small red dwarf about 12 light-years away, and orbits the star in about two days.

Surface. Rocky surface with impact craters, possible volcanic features

Rocky Terrestrial
Life unlikely — too hot for liquid waterMagnetic FieldLow Activity
YZ Cet b is dominated by silicate (25.0%) and iron (12.5%) content, forming a rocky terrestrial body.
Rocky terrestrial planets are primarily composed of silicate rocks and metals. They're the most common type of small planet in the universe.
Classification Criteria:
✓ Silicon: 25.0% + Iron: 12.5% + Magnesium: 3.8%✓ Mass: 0.7x Earth (terrestrial range)

Reading YZ Cet b’s atmosphere

Dominant elements: O, Si, Fe, N, C, HThis is what a telescope would measure while YZ Cet 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.YZ Cet b sits about 12 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.
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Habitability ScoreExtremely Harsh
20
Contributing Factors
temperature10
Extremely hot - hostile to known life forms
atmosphere70

Thin CO₂ and nitrogen atmosphere

Nitrogen atmosphere provides protection but not breathable
water31
Limited water content - likely as steam (low gravity lets water escape to space)
magnetic Field100
Magnetic field provides radiation protection
geology60
Some geological activity provides slow nutrient cycling
Organic Chemistry80
Core organic elements (CHNO) present
rotation65
Slow rotation - significant day/night temperature variations, but life can adapt

Physical Properties

Mass
0.70x Earth (4.18e+24 kg)
Radius
0.89x Earth (5,657 km)
This planet: 0.89× Earth radiusEarth: 1× (baseline)
Surface Gravity
0.89x Earth (8.7 m/s²)
Surface Temperature
477 K (204 °C / 399 °F)
Rotation Period
48.5 hours/day (slower than Earth)
This planet: 48 h/dayEarth: 24 h/day

Time Dilation

Clocks run 8.6 seconds per year slower than deep space — about 11 minutes over a lifetime

Elemental Composition

How TerraForge built this world

YZ Cet b, from the NASA Exoplanet Archive (found 2017). Measured: 0.7 Earth masses, 0.0163 AU, a 3151 K M-type star. Composition inferred from an estimated bulk density of 0.92x 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 2.0-day year, as anything orbiting this close would be.

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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