The Planet Without Dawn: An Ocean May Exist Beneath a World with No Star

In 1999, scientist David Stevenson published a model that reverses the direction of the search for life. Instead of asking which planet orbits at the right distance from a star, the model asks which planet still holds water. An Earth-sized planet could be thrown out of its star system after a gravitational encounter and still keep its water and a thick layer of hydrogen gas. The surface freezes in darkness. Beneath the ice, heat from the interior and heat held down by the atmosphere can keep water liquid.

A rogue planet does not orbit a star. It may be ejected by a giant planet or form alone inside a cloud of gas and drift between star systems. Without sunlight its surface is nothing like Earth. A large planet, though, still carries heat left from formation and heat released by radioactive decay. Pressure keeps gas and water from escaping into space.