Last fall an unexpected out-of-towner blazed a faint but memorable trail through the solar system. ‘Oumuamua, as it came to be called, had dive-bombed the sun from parts unknown, and was witnessed whizzing past Earth on an orbital path that would take it back out to interstellar space. That offered astronomers a brief, first-ever chance to study an object from another star.
As a singular event, ‘Oumuamua was gratifyingly weird: likely made of rock or metal, reddish in color, not gassy like a comet and stretched into an extremely elongated shape. On the off chance that the visitor was a probe with artificial origins, scientists with the Breakthrough Listen project and the SETI Institute even checked to see whether it was broadcasting radio waves. (It wasn’t.) Soon the speeding object had slipped beyond the reach of even our best telescopes.