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Entering orbit…
Habitability
0/ 100
Uninhabitable
TOI-3998 Ab rendered in TerraForge from its measured mass, orbit and star

TOI-3998 Ab

Also written TOI 3998 Ab

Gas GiantUninhabitable1324 light years awayFound 2026
exoplanet.euListed by the Extrasolar Planets Encyclopaedia; not yet in NASA's confirmed catalogue.View on exoplanet.eu

TOI-3998 Ab, from the Extrasolar Planets Encyclopaedia (found 2026). Measured: 334 Earth masses, 0.0305 AU, a 6240 K F-type star. Composition inferred from a mass of 334 Earths, which is giant territory whatever its 0.11x density suggests, with a day locked to its 1.8-day year, as anything orbiting this close would be.

Host star

TOI-3998 A

Type
F-type (White)
Spectral type
F8V
Temperature
6,240 K
Radius
1.2 ☉
Mass
1.15 ☉
Known planets
1 (this one)
Luminosity
1.96 ☉

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?
TOI-3998 Ab has 86.0% hydrogen and helium, combined with a mass of 334.0x 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: 334.0x Earth (>10x required)

Reading TOI-3998 Ab’s atmosphere

Dominant elements: H, He, O, C, NThis is what a telescope would measure while TOI-3998 Ab 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.TOI-3998 Ab sits about 1324 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-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
334.00x Earth (1.99e+27 kg)
Radius
6.94x Earth (44,203 km)
This planet: 6.94× Earth radiusEarth: 1× (baseline)
Surface Gravity
6.94x Earth (68.1 m/s²)
Surface Temperature
1,883 K (1,610 °C / 2,931 °F)
Rotation Period
43.4 hours/day (slower than Earth)
This planet: 43 h/dayEarth: 24 h/day

Orbital Environment

Star Type
F-type (White)
Distance from Star
0.03 AU (4.56 million km)
Orbital Period
2 days
Time Dilation
Clocks run 19.9 seconds per year slower than deep space — about 27 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
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The surface of TOI-700 d as TerraForge draws it: red-leaved trees over green ground under a lavender sky

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