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Entering orbit…
Habitability
19/ 100
Uninhabitable
HD 128311 b rendered in TerraForge from its measured mass, orbit and star

HD 128311 b

Also written HD 128311b

Gas Giant53 light years awayFound 2002
Mass
562.00 × Earth
Radius
13.50 × Earth
Gravity
3.08 g
Surface (est.)
200 K
Year
453 days
From Earth
53.2 ly
Mass
562.00 × Earth
Radius
13.50 × Earth
Gravity
3.08 g
Surface (est.)
200 K
Year
453 days
From Earth
53.2 ly

HD 128311 system

Type
K-type (Orange)
Spectral type
K0
Temperature
4,965 K
Radius
0.76 ☉
Mass
0.828 ☉
Planets
2
Luminosity
0.308 ☉

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 128311 b1.08 AUYou are here562 M⊕
HD 128311 c1.74 AU1,200 M⊕
Habitable zone

Star and planets drawn to scale: the star is 6.1 times as wide as HD 128311 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 128311 b, from the NASA Exoplanet Archive (found 2002). Measured: 562 Earth masses, 1.08 AU, a 4965 K K-type star. Composition inferred from a mass of 562 Earths, which is giant territory whatever its 0.23x density suggests, with an assumed 24-hour day — rotation cannot be measured for exoplanets.

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

Gas Giant
Magnetic FieldExtreme Activity
HD 128311 b has 86.0% hydrogen and helium, combined with a mass of 562.0x 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: 86.0% (>60% required)✓ Mass: 562.0x Earth (>10x required)

Reading HD 128311 b’s atmosphere

Dominant elements: H, He, O, C, NThis is what a telescope would measure while HD 128311 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 128311 b sits about 53 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
19
Contributing Factors
temperature20
Very cold - water frozen, challenging 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
rotation100
Optimal rotation period - balanced day/night cycle, moderate weather, and good heat distribution

Physical Properties

Mass
562.00x Earth (3.36e+27 kg)
Radius
8.25x Earth (52,576 km)
This planet: 8.25× Earth radiusEarth: 1× (baseline)
Surface Gravity
8.25x Earth (81.0 m/s²)
Surface Temperature
200 K (-73 °C / -100 °F)
Rotation Period
24.0 hours/day
This planet: 24 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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