Wandering worlds — with no parent star — raise questions about what it means to be a planet
A. M. Quetz/MPIA
Out among the stars, toward the constellation Capricornus, a red sphere floats freely through space. It doesn’t have enough mass to fuse atoms for fuel, as stars do, and it’s too small to be a failed star. In nearly every way, this drifter, known as PSO J318.5-22, is like a planet. Except it fails one key test for planethood: It does not orbit a star.
PSO J318.5-22 is homeless. With no parent star to give it heat or light, it drifts in eternal darkness, a rogue of the Milky Way.
Computer simulations in the 1970s gave planetary scientists their first hints that rogue planets might exist. As planets formed around a star, some planetary material would have been scattered into far-flung orbits. A few miniplanets may have been tossed far enough to be ejected completely from the star’s gravitational grasp.