Some planets float alone through space, with no sun and skies that are always dark. These rogue worlds are incredibly difficult to spot, but two new ones have been discovered. One of them is among the smallest we’ve ever seen, and there may be more small planets like it in the Milky Way than stars.

When astronomers normally spot planets beyond our solar system, they do so by observing a planet passing in front of its star , blocking out some of the …