The ancient Greeks looked into the night and saw that some of the brightest stars were moving on a regular basis. They called them “wanderers”—planets. A few thousand years later, humanity itself has become the planetary wanderer, by sending robots to explore them. Rovers crawl on the surface of Mars; a lander clings precariously to a comet; a spacecraft swooshes past Pluto.
The questions posed by these probes are not all that different from those of our forebears: How did the solar system come to be? Is Earth the only place where the ingredients for life were catalyzed into the real thing? Robotic planetary exploration began a half century ago with sobering reality checks: our nearest neighbors—the moon, Mars and Venus—were fairly horrible places for life. More recently, scientists have been intrigued by not-so-inhospitable conditions in the icy worlds of the outer solar system—moons such as Europa, Titan, and Enceladus.
Now, the race is on to look past the edge of the solar system and into the planetary gardens of our galactic neighbors. New astronomical tools have brought thousands of exoplanets into view; scientists are on the cusp of discovering a true Earth twin. Strange new planetary species abound, such as “hot Jupiters”, giant planets that hug their parent stars in an all-too-warm orbital embrace—evidence that, at least in other star systems, planets really do wander. The menagerie of exoplanets has provided new natural laboratories for scientists to test out models of planetary formation and evolution. As always, however, the questions remain roughly the same: Is the solar system a freakish accident, and Earth even more special? Or could there be fellow travelers on our fellow wanderers?