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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 binary star system's orrery: two suns close together, a red dashed dead-zone boundary, and a debris trail left by a destroyed world

New: Binary Star Systems 🌀

A system can now orbit two suns. Add a companion star when you build one — close, medium, or wide — and the physics decides what survives: a shared center of mass carves out a dead zone where gravity from both stars tears a world apart, leaving it as debris, and every surviving planet's orbit goes elliptical, moving fastest at closest approach and slowest out at aphelion. Push the separation to wide and the dead zone opens tens of AU across, wide enough to gut a system that would have been stable around one star alone. Stand on a world that made it through and you'll watch two suns cross the sky together.

Create your free account

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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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