When astronomers discovered the first exoplanet around a normal star 2 decades ago, there was joy—and bewilderment. The planet, 51 Pegasi b, was half as massive as Jupiter, but its 4-day orbit was impossibly close to the star, far smaller than the 88-day orbit of Mercury. Theorists who study planet formation could see no way for a planet that big to grow in such tight confines around a newborn star. It could have been a freak, but soon, more “hot Jupiters” turned up in planet searches, and they were joined by other oddities: planets in elongated and highly tilted orbits, even planets orbiting their stars “backward”—counter to the star’s rotation.
The planet hunt accelerated with the launch of NASA’s Kepler spacecraft in 2009, and the 2500 worlds it has discovered added statistical heft to the study of exoplanets—and yet more confusion. Kepler found that the most common type of planet in the galaxy is something between the size of Earth and Neptune—a “super-Earth,” which has no parallel in our solar system and was thought to be almost impossible to make. Now, ground-based telescopes are gathering light directly from exoplanets, rather than detecting their presence indirectly as Kepler does, and they, too, are turning up anomalies. They have found giant planets several times the mass of Jupiter, orbiting their star at more than twice the distance Neptune is from the sun—another region where theorists thought it was impossible to grow large planets. Other planetary systems looked nothing like our orderly solar system, challenging the well-worn theories that had been developed to explain it.
“It’s been really obvious things didn’t fit pretty much from day one,” says Bruce Macintosh, a physicist at Stanford University in Palo Alto, California. “There has never been a moment when theory has caught up with observations.”