Lone Rovers

How do stars and planets break the bonds of gravity, and how do we know they do? Most stars form with hundreds, if not thousands, of nearby neighbors, and yet the Sun is all alone; we learn about its crowded birthplace from decayed radioactive products and by the examples of stellar clusters all around in which supernova explosions can either trigger starbirth or terminate the growth of planetary systems. Planets form as the entourage of stars, and yet many have been found floating freely in interstellar space; such dark bodies, thrown free from their original planetary systems by migrating sibling planets and now drifting far from stars, are found by their bending of starlight, working as gravitational lenses—as predicted by Albert Einstein—when they pass in front of distant stars.