Create your free account

Enter your email and we'll send a 6-digit code — no password needed.
We only keep your email and the username you choose. We never share or sell your email, and we don't send marketing — we use it only to email you a sign-in code. Read our privacy policy.
Entering orbit…
Habitability
0/ 100
Uninhabitable
HATS-21 b rendered in TerraForge from its measured mass, orbit and star

HATS-21 b

Also written HATS-21b, HATS 21b, HATS 21 b

Gas GiantUninhabitable932 light years awayFound 2016
exoplanet.euListed by the Extrasolar Planets Encyclopaedia; not yet in NASA's confirmed catalogue.View on exoplanet.eu

HATS-21 b, from the Extrasolar Planets Encyclopaedia (found 2016). Measured: 106 Earth masses, 0.0468 AU, a 5695 K G-type star. Composition inferred from a mass of 106 Earths, which is giant territory whatever its 0.05x density suggests, with a day locked to its 3.5-day year, as anything orbiting this close would be.

Host star

HATS-21

Type
G-type (Yellow (Sun-like))
Spectral type
G4V
Temperature
5,695 K
Radius
1.02 ☉
Mass
1.08 ☉
Known planets
1 (this one)
Luminosity
0.988 ☉

Published by the Extrasolar Planets Encyclopaedia. This world’s temperature and habitability score are computed from its star’s published radius and temperature.

Gas Giant

Magnetic FieldExtreme Activity
Why is this a Gas Giant?
HATS-21 b has 86.0% hydrogen and helium, combined with a mass of 106.0x 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: 106.0x Earth (>10x required)

Reading HATS-21 b’s atmosphere

Dominant elements: H, He, O, C, NThis is what a telescope would measure while HATS-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.HATS-21 b sits about 933 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.
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-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
106.00x Earth (6.33e+26 kg)
Radius
4.73x Earth (30,152 km)
This planet: 4.73× Earth radiusEarth: 1× (baseline)
Surface Gravity
4.73x Earth (46.4 m/s²)
Surface Temperature
1,284 K (1,010 °C / 1,851 °F)
Rotation Period
85.2 hours/day (slower than Earth)
This planet: 85 h/dayEarth: 24 h/day

Orbital Environment

Star Type
G-type (Yellow (Sun-like))
Distance from Star
0.05 AU (7 million km)
Orbital Period
4 days
Time Dilation
Clocks run 10.0 seconds per year slower than deep space — about 13 minutes over a lifetime

Atmosphere

Hydrogen and helium dominated with trace methane and ammonia

Elemental Composition

H - Hydrogen
53.8%
He - Helium
32.3%
O - Oxygen
5.4%

Surface Characteristics

No solid surface - thick gaseous envelope with possible rocky core deep within
Love TerraForge? Please help support us.Keeps it ad-free, and funds new features and hosting.
The surface of TOI-700 d as TerraForge draws it: red-leaved trees over green ground under a lavender sky

New: Stand on a Planet in VR 🥽

Open any planet's surface view on your phone and look around just by moving it: hold it up, turn on the spot, and the view turns with you. In Chrome on Android you also get a VR button, in the row along the bottom:

The surface view's bottom row of buttons on a phone, with an arrow pointing to the VR button beside full screen

Tap it, drop your phone into a headset viewer, and you're standing on the ground under a star that isn't ours. It only appears where your browser can do VR.

Create your free account

Enter your email and we'll send a 6-digit code — no password needed.
We only keep your email and the username you choose. We never share or sell your email, and we don't send marketing — we use it only to email you a sign-in code. Read our privacy policy.

Send feedback

We're still building out the best experience we can — and we'd love your input. Tell us about features you enjoy, ideas you'd like to see, or pain points where something feels off, and we'll know where to look next.

Type
Idea
Bug
Pain point
Other