A Planet Can Keep Liquid Water With NO Sun at All

A planet doesn't even need a star to keep liquid water. In theory, it could stay warm in total darkness.

The habitable zone assumes a planet is heated by its sun. But warmth can come from elsewhere, and some worlds may carry oceans through endless night, with no star above them at all.

Wandering between the stars are rogue planets, worlds flung out of their systems. With no sun, their surfaces should be frozen wastelands. Yet scientists suspect a few could still hide liquid water. A thick enough atmosphere, especially one rich in hydrogen, can trap a planet's own leftover heat like a heavy blanket. Under it, a deep ocean might survive for billions of years, kept liquid not by a star, but by the planet itself.

It flips the whole idea on its head. Life's water may not need a sun at all. The most common habitable spots in the universe could be drifting, starless worlds we can barely see.

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