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Brown Dwarfs

Failed stars: too heavy to be planets, too light to burn hydrogen.

Past about 13 Jupiter masses a ball of hydrogen gets hot enough in its core to fuse deuterium, and it stops being a planet at all. It is not a star either: that needs roughly 75 Jupiters, the mass at which ordinary hydrogen ignites. In between sits the brown dwarf, glowing on deuterium and the heat of its own contraction, cooling slowly over billions of years from dull red through magenta to barely visible. Strangely, piling on mass barely changes its size, because degeneracy pressure holds it near Jupiter's radius no matter how heavy it gets. Build one by pushing the mass slider past 13 Jupiters on a hydrogen-helium recipe.

See all brown dwarfs on Explore

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.

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.

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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.

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