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Entering orbit…
Habitability
6/ 100
Uninhabitable
HD 153557 b rendered in TerraForge from its measured mass, orbit and star

HD 153557 b

Also written HD 153557b

Gas Giant58 light years awayFound 2022
Mass
20.30 × Earth
Radius
4.77 × Earth
Gravity
0.89 g
Surface (est.)
797 K
Year
7.3 days
From Earth
58.4 ly
Mass
20.30 × Earth
Radius
4.77 × Earth
Gravity
0.89 g
Surface (est.)
797 K
Year
7.3 days
From Earth
58.4 ly

HD 153557 system

Type
K-type (Orange)
Spectral type
K0
Temperature
4,837 K
Radius
0.745 ☉
Mass
0.79 ☉
Planets
3
Luminosity
0.274 ☉

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.

HD 153557 b0.068 AUYou are here20 M⊕
HD 153557 c0.11 AU18 M⊕
HD 153557 d21.40 AU8,730 M⊕
Habitable zone

Star and planets drawn to scale: the star is 17 times as wide as HD 153557 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

HD 153557 b, from the NASA Exoplanet Archive (found 2022). Measured: 20.3 Earth masses, 0.068 AU, a 4837 K K-type star. Composition inferred from a estimated bulk density of 0.19x 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 29% hydrogen/helium envelope by mass, with a day locked to its 7.3-day year, as anything orbiting this close would be.

Surface. No solid surface - thick gaseous envelope with possible rocky core deep within

Gas Giant
Magnetic FieldExtreme Activity
HD 153557 b has 63.5% hydrogen and helium, combined with a mass of 20.3x Earth, which exceeds the gas giant threshold.
Gas giants form when massive cores capture enormous atmospheres of light gases. They typically form beyond the 'frost line' where volatiles can condense.
Classification Criteria:
✓ Hydrogen + Helium: 63.5% (>60% required)✓ Mass: 20.3x Earth (>10x required)

Reading HD 153557 b’s atmosphere

Dominant elements: H, O, He, Si, N, CThis is what a telescope would measure while HD 153557 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.HD 153557 b sits about 58 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.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.
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Habitability ScoreUninhabitable
6
Contributing Factors
temperature3
Scorching - hot enough to melt lead, though rock stays solid; no chance for life
atmosphere30

Hydrogen and helium dominated with trace methane and ammonia

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
geology40
Extreme volcanic activity may be hazardous
Organic Chemistry80
Core organic elements (CHNO) present
rotation30
Very slow rotation - days and nights each last several Earth days, bringing extreme temperature swings

Physical Properties

Mass
20.30x Earth (1.21e+26 kg)
Radius
2.73x Earth (17,380 km)
This planet: 2.73× Earth radiusEarth: 1× (baseline)
Surface Gravity
2.73x Earth (26.8 m/s²)
Surface Temperature
797 K (524 °C / 975 °F)
Rotation Period
175.2 hours/day (slower than Earth)
This planet: 175 h/dayEarth: 24 h/day

Elemental Composition

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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