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Entering orbit…
Habitability
0/ 100
Uninhabitable
GJ 15 A c rendered in TerraForge from its measured mass, orbit and star

GJ 15 A c

Also written GJ 15 Ac

Gas Giant12 light years awayFound 2018
Mass
36.00 × Earth
Radius
6.67 × Earth
Gravity
0.81 g
Surface (est.)
46 K
Year
20.8 years
From Earth
11.6 ly
Mass
36.00 × Earth
Radius
6.67 × Earth
Gravity
0.81 g
Surface (est.)
46 K
Year
20.8 years
From Earth
11.6 ly

GJ 15 A system

Type
M-type (Red Dwarf)
Spectral type
M1
Temperature
3,607 K
Radius
0.38 ☉
Mass
0.38 ☉
Planets
2
Luminosity
0.0221 ☉

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.

GJ 15 A b0.072 AU3.0 M⊕
GJ 15 A c5.40 AUYou are here36 M⊕
Habitable zone

Star and planets drawn to scale: the star is 6.2 times as wide as GJ 15 A c, 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

GJ 15 A c is a cold giant planet on a very long orbit. When it was reported in 2018, it was the longest-period planet lighter than Jupiter ever found by the radial velocity method, which detects planets from the slight back-and-forth motion their gravity gives a star. It circles GJ 15 A, a red dwarf about 11.6 light-years away, roughly once every 7,600 days, about 20 years, at around 5.4 astronomical units, a little farther than Jupiter is from the Sun. Its minimum mass is about 36 Earth masses, making it a super-Neptune.

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

Gas Giant
Magnetic FieldExtreme Activity
GJ 15 A c has 85.8% hydrogen and helium, combined with a mass of 36.0x 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: 85.8% (>60% required)✓ Mass: 36.0x Earth (>10x required)

Reading GJ 15 A c’s atmosphere

Dominant elements: H, He, O, C, NThis is what a telescope would measure while GJ 15 A c 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.GJ 15 A c sits about 12 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
Near absolute zero - all molecular activity ceases, impossible 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
36.00x Earth (2.15e+26 kg)
Radius
3.30x Earth (21,037 km)
This planet: 3.30× Earth radiusEarth: 1× (baseline)
Surface Gravity
3.30x Earth (32.4 m/s²)
Surface Temperature
46 K (-227 °C / -376 °F)
Rotation Period
24.0 hours/day
This planet: 24 h/dayEarth: 24 h/day

Elemental Composition

How TerraForge built this world

GJ 15 A c, from the NASA Exoplanet Archive (found 2018). Measured: 36 Earth masses, 5.4 AU, a 3607 K M-type star. Composition inferred from a estimated bulk density of 0.12x Earth's, modeled rather than measured since this isn't a transiting planet, which only hydrogen and helium can explain, with an assumed 24-hour day — rotation cannot be measured for exoplanets.

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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