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Entering orbit…
Habitability
21/ 100
Extremely Harsh

Reading KMT-2021-BLG-0171L b’s atmosphere

Dominant elements: H, O, Si, Fe, N, CThis is what a telescope would measure while KMT-2021-BLG-0171L 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.KMT-2021-BLG-0171L b sits about 15003 light-years away, found by the microlensing 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.

KMT-2021-BLG-0171L b

Ice GiantExtremely Harsh14996 light years awayFound 2022

KMT-2021-BLG-0171L b, from the NASA Exoplanet Archive (found 2022). Measured: 11.2 Earth masses, 3.5 AU, a 5200 K G-type star. Composition inferred from a estimated bulk density of 0.30x 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 6% hydrogen/helium envelope by mass, with an assumed 24-hour day — rotation cannot be measured for exoplanets.

Host star

KMT-2021-BLG-0171L

Type
G-type (Yellow (Sun-like))
Temperature
5,200 K
Mass
0.78 ☉
Known planets
1 (this one)

Published by the NASA Exoplanet Archive. This star’s luminosity has not been published, so this world’s temperature and habitability score fall back to a typical G-type star.

Ice Giant

Magnetic FieldModerate Activity
Why is this a Ice Giant?Your planet has moderate mass (11.2x 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: 11.2x 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
21
Contributing Factors
temperature20
Very cold - water frozen, challenging 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
11.20x Earth (6.69e+25 kg)
Radius
2.24x Earth (14,254 km)
This planet: 2.24× Earth radiusEarth: 1× (baseline)
Surface Gravity
2.24x Earth (21.9 m/s²)
Surface Temperature
156 K (-118 °C / -180 °F)
Rotation Period
24.0 hours/day
This planet: 24 h/dayEarth: 24 h/day

Orbital Environment

Star Type
G-type (Yellow (Sun-like))
Distance from Star
3.50 AU (524 million km)
Orbital Period
6.55 years

Atmosphere

Hydrogen and helium with water, methane, and ammonia ices

Elemental Composition

H - Hydrogen
54.9%
O - Oxygen
28.5%
Si - Silicon
5.6%

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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