How could you possibly study the interior of a giant planet orbiting another star? Especially when that planet is so drowned in its star’s light that we can’t even see it? Various methods suggest themselves, including transits, those cases wherein the exoplanet happens to pass between us and the star it circles. A transit gives you the chance to measure both mass and size. Throw in inferences based on slowly evolving planetary models and you can draw some tentative conclusions. You also wind up with even more questions.

And as Tristan Guillot would probably point out, we now have twenty gas giants whose mass and size can be determined, including those within our own Solar System. Guillot, who works in one of the most celestially beautiful places on Earth (he’s at the Observatoire de la Cote d’Azur in Nice), makes it his business to compare and contrast what we see around Sol with the rising number of giant planets we’re finding around other stars using the transit method.

Several interesting things emerge. All gas giants seem to be made of hydrogen and helium wrapped around a dense core that is presumed to be made of compressed water and rocks. That’s from standard models of planet formation, and when you run the numbers, you expect to see a core of about ten Earth masses. And indeed, this accounts for Uranus and Neptune, but Jupiter and Saturn remain enigmatic: Jupiter’s core seems to be only a few times the mass of Earth, while Saturn’s may be as much as 25 times Earth mass.

So we have much to learn here in our own system. And exoplanets present an even greater discrepancy. The hydrogen/helium constituents are what we expect, but Guillot and colleagues are finding much larger cores — up to one hundred times that of Earth’s mass. Not surprisingly, the metallicity of the star (its relative richness in heavier elements) seems to correlate to the mass of the planetary core.

And Guillot, who has won the Harold C. Urey Prize from the Division for Planetary Sciences of the American Astronomical Society, points to future missions like Juno, a Jupiter-bound spacecraft that will measure the planet’s gravity with minute precision, not to mention the data expected from both COROT and Kepler, transit hunters that will soon take to the skies. COROT is scheduled for a December launch, with Kepler to follow in two years. A golden era of transit studies is upon us.

If you’re following gas giant research, Guillot’s Web site at the observatory offers papers, figures, images and other data. A useful starting point is his paper “The Interiors of Giant Planets: Models and Outstanding Questions,” in Annual Review of Earth and Planetary Sciences, Vol. 33, p.493-530, which is also available online.