Meet the planet COROT-exo-3b. It orbits a star slightly larger, hotter, and brighter than the Sun. The star is not an unusual one in any way, but the planet is definitely weird: it orbits the star in just over 4 days, which is pretty close in, though not a record breaker in and of itself. What’s bizarre is that it has about the same diameter of Jupiter, but has 21.6 times Jupiter’s mass. That makes it denser than lead.

If I could stand on the surface of this planet, I’d weigh, over 9000 pounds!

This is by far the most massive planet found so close to its parent star. There is another extrasolar planet found with about that mass, but it orbits its star much farther out. The ones we’ve found that orbit their stars so close tend to have masses much smaller than this. For comparison, Jupiter takes 12 years to circle the Sun once. Mercury takes 88 days. So we’re talking big planets, really close to their stars.

This planet is challenging to models. How did it form? It most likely formed farther out from the star — gravitational influences make it hard for a large planet to form close to a star — and then gradually moved in. This can happen due to friction, of all things: when the star and planet are young, there is a disk of material leftover from the planetary formation. As the planet sweeps through this material it slows its orbit. It spirals in due to gravitational interaction with the disk, and eventually settles down when the disk material thins out a few million kilometers from the surface of the star itself.