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Entering orbit…
Habitability
8/ 100
Uninhabitable

Reading K2-128 b’s atmosphere

Dominant elements: O, Si, H, Fe, N, CThis is what a telescope would measure while K2-128 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.K2-128 b sits about 375 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.

K2-128 b

Rocky TerrestrialUninhabitable375 light years awayFound 2017

K2-128 b, from the NASA Exoplanet Archive (found 2017). Measured: 2.6 Earth masses, 0.056 AU, a 4470 K K-type star. Composition inferred from a measured bulk density of 0.91x Earth's at this size, suggesting roughly 18% water/ice by mass, with a day locked to its 5.7-day year, as anything orbiting this close would be.

Host star

K2-128

Type
K-type (Orange)
Temperature
4,470 K
Radius
0.778 ☉
Mass
0.711 ☉
Known planets
1 (this one)
Luminosity
0.151 ☉

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.

Rocky Terrestrial

No life — too hot for liquid waterHigh Activity
Why is this a Rocky Terrestrial?Your planet is dominated by silicate (21.0%) and iron (10.7%) 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: 21.0% + Iron: 10.7% + Magnesium: 3.0%✓ Mass: 2.6x Earth (terrestrial range)
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.
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 ScoreUninhabitable
8
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 Field30
No magnetic field - vulnerable to solar radiation
geology90
Active geology recycles nutrients and drives carbon cycle
Organic Chemistry80
Core organic elements (CHNO) present
rotation30
Very slow rotation - extreme temperature swings between eternal day and night sides

Physical Properties

Mass
2.60x Earth (1.55e+25 kg)
Radius
1.38x Earth (8,761 km)
This planet: 1.38× Earth radiusEarth: 1× (baseline)
Surface Gravity
1.38x Earth (13.5 m/s²)
Surface Temperature
771 K (498 °C / 928 °F)
Rotation Period
136.3 hours/day (slower than Earth)
This planet: 136 h/dayEarth: 24 h/day

Orbital Environment

Star Type
K-type (Orange)
Distance from Star
0.06 AU (8.38 million km)
Orbital Period
6 days
Time Dilation
Clocks run 5.8 seconds per year slower than deep space — about 8 minutes over a lifetime

Atmosphere

Thin CO₂ and nitrogen atmosphere

Elemental Composition

O - Oxygen
31.0%
Si - Silicon
21.0%
H - Hydrogen
16.5%

Surface Characteristics

Rocky surface with impact craters, possible volcanic features
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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