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Entering orbit…
Habitability
0/ 100
Uninhabitable
HD 209458 b rendered in TerraForge from its measured mass, orbit and star

HD 209458 b

Also written HD 209458b

Gas Giant157 light years awayFound 1999
Mass
232.00 × Earth
Radius
15.60 × Earth
Gravity
0.95 g
Surface (est.)
1,465 K
Year
3.5 days
From Earth
158 ly
Mass
232.00 × Earth
Radius
15.60 × Earth
Gravity
0.95 g
Surface (est.)
1,465 K
Year
3.5 days
From Earth
158 ly

HD 209458 system

Type
F-type (White)
Spectral type
G0 V
Temperature
6,091 K
Radius
1.19 ☉
Mass
1.23 ☉
Planets
1
Luminosity
1.70 ☉

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 209458 b0.047 AUYou are here232 M⊕
Habitable zone

Star and planets drawn to scale: the star is 8.3 times as wide as HD 209458 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

Much of what astronomers know how to do with exoplanet atmospheres was first tried on this world. In 1999 it was already known from the radial velocity method, which detects a star's slight wobble as a planet pulls on it. That year David Charbonneau and colleagues caught the planet passing in front of its star, dimming it slightly. It was the first time anyone had seen a planet transit, or cross the face of, a star like the Sun. The transit revealed the planet's size directly, about 1.27 times Jupiter's radius, and showed it was a gas giant.

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

Gas Giant
Magnetic FieldExtreme Activity
HD 209458 b has 86.0% hydrogen and helium, combined with a mass of 232.0x 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: 232.0x Earth (>10x required)

Reading HD 209458 b’s atmosphere

Dominant elements: H, He, O, C, NThis is what a telescope would measure while HD 209458 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 209458 b sits about 158 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.
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
0
Contributing Factors
temperature0
Inferno world - temperatures exceed all possibility of 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
rotation65
Slow rotation - significant day/night temperature variations, but life can adapt

Physical Properties

Mass
232.00x Earth (1.39e+27 kg)
Radius
6.14x Earth (39,147 km)
This planet: 6.14× Earth radiusEarth: 1× (baseline)
Surface Gravity
6.14x Earth (60.3 m/s²)
Surface Temperature
1,465 K (1,192 °C / 2,178 °F)
Rotation Period
84.5 hours/day (slower than Earth)
This planet: 84 h/dayEarth: 24 h/day

Time Dilation

Clocks run 12.9 seconds per year slower than deep space — about 17 minutes over a lifetime

Elemental Composition

How TerraForge built this world

HD 209458 b, from the NASA Exoplanet Archive (found 1999). Measured: 232 Earth masses, 0.0471 AU, a 6091 K F-type star. Composition inferred from a mass of 232 Earths, which is giant territory whatever its 0.06x density suggests, with a day locked to its 3.5-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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