Newly discovered planet is nearly 25 percent the size of its star

Enlarge (credit: University of Warwick/Mark Garlick)

What does it take to build a gas giant? Building models of planet formation and studying exosolar systems have both provided us with some hints. But there's a small but growing list of cases where the two of these approaches disagree about what's possible. A new paper adds to that list by describing a gas giant planet that orbits a dwarf star, creating a situation where the planet is 25 percent the size of its host-the smallest difference between planet and star yet observed.

Gas giants, as their name implies, are mostly hydrogen and helium. But models of planet formation have suggested that they can only form in systems with a lot of heavier elements around. The idea is that a large core of rocky material has to form quickly, before the star fully ignites and drives off any nearby gas. If the rocky body gets big enough early enough, it can grab enough gas to start a runaway atmospheric accumulation, turning itself into a gas giant.

Studying exosolar systems provides some support for this idea. We can get a sense of how many heavier elements-generically termed metals-were around during planet formation by looking at their presence in the host star. If the star has a high metal content, then the planets probably had access to lots of heavier elements, too. For small, rocky planets, it doesn't seem to matter how many heavier elements were around, as they're found at stars with various degrees of metal content. The same is true for super-Earths and Neptune-sized planets.

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