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.
Later estimates suggested that every planetary system in the galaxy booted at least one planet into