The huge gas-swaddled planet 51 Pegasi b orbits just 7.5 million kilometers from its parent star (a distance about one-eighth the size of Mercury’s orbit)—so close, in fact, that its “year” lasts just 4 Earth days or so. Other such “hot Jupiters” orbit their suns even closer than that. Ever since this odd class of planets was first discovered about 2 decades ago, scientists have wondered how gas giants, a type of planet found only in our outer solar system, end up in orbits so exceedingly close to their parent stars? Previous studies have suggested that gravitational interactions with other large planets in their solar systems created orbital chaos that drove them inward from a more distant orbit. A new analysis supports that notion. Researchers used software to simulate the planetary motions within a two-planet system: one being a Jupiter-sized planet (depicted in the artist’s concept above) orbiting its parent star at about the same distance Earth orbits the sun, and the other a large planet in a highly tilted orbit that was slightly larger. Over a simulated interval of 200 million years, the inner planet slowly migrated even farther inward to become a “warm Jupiter” orbiting its parent star at about the same distance Mercury does in our solar system, the researchers report online today in Science. (Previous studies have identified six distant solar systems that include both a warm Jupiter and a large companion planet in a highly tilted orbit, the researchers note.) Over the life of a star, which can last billions of years, continued gravitational interactions with the other large planet in the system could drive a warm Jupiter even closer to its parent star, converting it into a hot Jupiter.