Some Planets Play a Mean Game of Pool

Thank your lucky stars you live in a relatively peaceful corner of the galaxy. Astronomers have found six large planets whose off-kilter orbits suggest that they crashed through their solar systems, swallowing any smaller planets that got in their way. The findings indicate that solar system formation is often disorderly and unpredictable, and that some potential cousins of Earth may have been destroyed in the chaos.

Astronomers have long thought that the formation of a star and its solar system was straightforward. A large cloud of gas and dust begins to congeal gravitationally, starts rotating, and eventually flattens into an object called a protostellar disk. That rotation dictates the future spin of the sun and the orbit of its planets. This is essentially what happened in our own solar system. There can be irregularities, of course, such as the odd spin axis of Uranus, which for as-yet-unexplained reasons is nearly tilted on its side, and Pluto's odd orbit, which occasionally takes it inside the path of Neptune. But these oddballs are nothing compared with what astronomers are seeing with six newly discovered planets known as hot Jupiters.

Hot Jupiters resemble our largest planet in size and composition, yet they orbit much closer to their stars, sometimes well within what would be the orbit of Mercury. But the newly discovered worlds add an extra twist: all orbit in the opposite direction from all of the other objects in their solar system (a so-called retrograde orbit), and all orbit at severe angles.

The implications are twofold, a team of astronomers led by Andrew Collier Cameron of the University of St. Andrews in the United Kingdom reports today at a meeting of the Royal Astronomical Society in Glasgow in the U.K. First, objects at least as large as these planets, and probably larger, must have shoved them severely out of their original orbits, probably well above or below the plane of their solar system. The encounters hurtled the gas giants in toward their parent suns. Then, each star's gravity caught the errant planet and whipped it around in a highly irregular, tight, and retrograde orbit. During this period of irregularity, which may have lasted millions of years, any rocky inner world sitting in the path of the hot Jupiter and its new orbit would have been obliterated.

The finding "adds to the accumulating evidence that the pathways that bring these giant planets close to their parent stars are messy and chaotic," says planetary scientist Jonathan Fortney of the University of California, Santa Cruz. It also suggests that there's "an incredible diversity in the structure of planetary systems,” he says.

These new worlds imply that there's a "dramatic and exciting process of gravitational billiards" at work, agrees astrophysicist Alan Boss of the Carnegie Institution of Washington in Washington, D.C. The finding lends credence to the idea that some gas giants in nascent solar systems “have to fight it out among themselves to see what orbit they will win and occupy," says Boss, a process he likens to "battles for office space on university campuses."