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Entering orbit…
Habitability
0/ 100
Uninhabitable
HAT-P-21 b rendered in TerraForge from its measured mass, orbit and star

HAT-P-21 b

Also written HAT-P-21b, HAT P 21b, HAT P 21 b

Gas Giant903 light years awayFound 2010
Mass
1550.00 × Earth
Radius
12.40 × Earth
Gravity
10.08 g
Surface (est.)
1,377 K
Year
4.1 days
From Earth
903 ly
Mass
1550.00 × Earth
Radius
12.40 × Earth
Gravity
10.08 g
Surface (est.)
1,377 K
Year
4.1 days
From Earth
903 ly

HAT-P-21 system

Type
G-type (Yellow (Sun-like))
Spectral type
G3
Temperature
5,588 K
Radius
1.21 ☉
Mass
1.24 ☉
Planets
1
Luminosity
1.52 ☉

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.

HAT-P-21 b0.050 AUYou are here1,550 M⊕
Habitable zone

Star and planets drawn to scale: the star is 11 times as wide as HAT-P-21 b, so only its edge is in view. Orbits spaced as in the star system view.

HAT-P-21 b, from the NASA Exoplanet Archive (found 2010). Measured: 1550 Earth masses, 0.0504 AU, a 5588 K G-type star. Composition inferred from a mass of 1550 Earths, which is giant territory whatever its 0.81x density suggests, with a day locked to its 4.1-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
HAT-P-21 b has 86.0% hydrogen and helium, combined with a mass of 1550.0x 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: 1550.0x Earth (>10x required)

Reading HAT-P-21 b’s atmosphere

Dominant elements: H, He, O, C, NThis is what a telescope would measure while HAT-P-21 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.HAT-P-21 b sits about 903 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.
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
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
1550.00x Earth (9.26e+27 kg)
Radius
11.57x Earth (73,731 km)
This planet: 11.57× Earth radiusEarth: 1× (baseline)
Surface Gravity
11.57x Earth (113.5 m/s²)
Surface Temperature
1,377 K (1,104 °C / 2,020 °F)
Rotation Period
98.9 hours/day (slower than Earth)
This planet: 99 h/dayEarth: 24 h/day

Time Dilation

Clocks run 9.3 seconds per year slower than deep space — about 12 minutes over a lifetime

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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