Hot Jupiters are planetary curiosities. These giant balls of gas are found very close to the stars they orbit, often at less than half the distance from the Earth to the Sun. They are strikingly hot, and take just hours or days to circle their stars. These mysterious objects were discovered about twenty years ago.
Ever since, scientists have been figuring out how these planets formed. Originally, it was thought that hot Jupiters started their lives at much greater distances from their stars, and gradually migrated inwards. In this theory, as the planet core gets closer to the central star, it picks up a coat of gas, transforming from a ball of rock to a gas giant.
Recently, the Kepler telescope has allowed astronomers to track rocky planets in other solar systems. For the first time, rocky planets have been spotted at very small distances from their stars, much closer than Mercury is to our sun. These planets were exactly the type that were expected to turn into hot Jupiters. This implied a problem with the old theory.
However, a new model developed by Aaron Bole and two others suggests another route to hot Jupiters. One type of arrangement scientists have discovered involves several rocky planets orbiting very close to the star. This class of system is unstable: the planets eventually crash into each other and are destroyed.
The new research suggests that if the collapse of the system is slow enough, these planets can combine when they crash into each other, picking up debris and gas to become a hot Jupiter. “This is a totally different way of thinking about [hot Jupiter] formation,” said Kathryn Volk, from the University of Arizona. While the old theory still applies in some cases, these planets are now better understood than ever.