Why is HAT-P-1 so much larger than expected?

[Astronuc]

Puffy planet poses pretty puzzle
http://news.bbc.co.uk/2/hi/science/nature/5346998.stm

Astronomers have found a strange new world that has them pondering again the essential properties of a planet.

This new object, designated HAT-P-1, orbits one member of a pair of stars 450 light-years away in the constellation Lacerta.

Although HAT-P-1's radius is about 1.38 times that of our own Jupiter, it has a mass that is only half that of Jupiter.

This makes it much bigger and lower in density than planets are usually, raising questions about how it formed.

The mathematical equations describing planetary structure do not fit.

HAT-P-1 is the largest of the 200 or so planets that have been detected outside our Solar System.

Like many of these extrasolar bodies, it orbits close to its parent star, revolving around it in just once every 4.5 Earth days.

Scientists know of one other extrasolar planet, HD 209458b, which is also puffed up about 20% bigger than predicted by theory. HAT-P-1 is 24% larger than expected.

http://www.cfa.harvard.edu/press/pr0624.html

http://arxiv.org/abs/astro-ph/0609369

http://www.cfa.harvard.edu/~gbakos/

 

[GeorgeSol]

Puffy planet poses pretty puzzle

Hmmmm...probably a pretty peculiar planet.

Its semimajor axis is 0.055 AU. This proximity to the G0 star will produce ~450,000 watts/m^2 (comparing it to a 1,367 solar constant value).

I think the models limiting planet size to only slightly larger than Jupiter were not intended to handle hot atmospheres, but this is just an assumption. There may have been no exoplanets when they were developed.

 

[SueWhite82]

Puffy Planet - Idea

Maybe the planet started off like Jupitor, but it had some chemical process that produced a substance that formed/bubbled/etc at the surface, or condensed out of atmosphere into particles that floated and attached to other particles and eventually formed a shell surrounding the planet?
The when the planet cools, the planet shrinks and the shell remains the size it was. Leaving a planet that is basically hollow (except it has a core) and the density being what it is because of this "hollowness"...

 

[GeorgeSol]

That is imaginative and imagination is sure helpful.

The problem is it should never hove gotten that big in the first place. If you add lots of gas to Jupiter, it will increase the mass and, therefore, increase the gravity. This will cause the atmosphere to shrink, somewhat. This minimizes just how big a gas giant can become. They can be larger than Jupiter, but not 30% larger.

However, it wouldn't surprise me if this formulation of maximum size deals with cool and warm planets, not planets getting fried next to their star. A hot atmosphere will expand and I would not expect a 30% increase to cause a concern. I am not an astrophysicist by a long shot, however.