Many of the planets discovered outside the Solar System are bigger than Jupiter. Some are larger than expected given the steady shrinking that occurs as gas-giant planets cool.
Using simulations, Laurent Ibgui and Adam Burrows of Princeton University in New Jersey have shown how the enormous size of some of these planets can be attributed to peculiar conditions at their birth. Just after formation, the planet could wind up in an eccentric orbit close to its star. Later, tides between the star and planet would pump heat into the planet's interior and inflate it.
The process, the authors say, might explain the lack of shrinkage for some older giants, giving them the look of a planet a billion years younger.