So far, the only place in the universe where we know there’s life is right here on Earth. But scientists are pondering the possibility of life elsewhere, from the moons of the solar system to planets in other star systems. They’ve even thought about life on “rogue” planets — worlds that roam through the galaxy in the darkness, far from any star. A rogue planet probably started out orbiting a star. It was kicked away from the star by the gravity of other planets in the system. And astronomers have discovered several dozen possible rogues. Without a star to warm it, such a world would quickly cool off. But the planet or an orbiting moon might provide enough heat for life to evolve and survive — perhaps for billions of years. For one thing, a rocky world similar to Earth should have radioactive elements in its core. Heat from the decay of these elements could keep water below an icy crust from freezing — a global ocean that could be miles deep. If the planet were born with a hydrogen-rich
