Under extremely high pressures, matter stops behaving normally. The atoms get squashed in weird ways and turn into a state called degenerate matter. Planets about as massive as Jupiter and larger are big enough to contain degenerate matter. This leads to the interesting property that planets lighter than Jupiter are smaller than Jupiter, but planets heavier than Jupiter are actually about the same size as Jupiter…or so we thought.
Then HAT-P-1b was discovered.
The first planet discovered by the HATNet Project in 2006, HAT-P-1b is only about half the mass of Jupiter, but its diameter is 22% larger. We know that because it passes in front its sun and blocks some of the starlight. Since we have a good estimate of how big the star is, we can also estimate how big the planet is.
HAT-P-1b is only about a quarter as dense as water. For comparison, the least dense planet in our Solar System, Saturn, is two-thirds as dense as water. There’s no known reason for a planet to have that low a density. Even though it’s a hot Jupiter orbiting very close to it’s sun, normal models of planetary atmospheres say it should not be that fluffy.
And HAT-P-1b isn’t even the least dense planet. That honor goes to WASP-17b (compared with Jupiter in the artist’s rendition above), which is only one seventh the density of water, or about the density of balsa wood. In all, we’ve discovered as many as a few dozen of these “puffy planets”, but how they got so puffed up is not clear. Our best guess is that it’s caused by a combination of tidal heating and electromagnetic heating effects. As we discover more planets and put together a more continuous landscape of their properties, we may be able to learn more.