In our Solar System, four planets stand out for their sheer mass and size. Jupiter, Saturn, Uranus and Neptune indeed qualify as ``giant planets'' because they are larger than any terrestrial planet and much more massive than all other objects in the Solar System, except the Sun, put together.

According to Dr. Guillot, "the giant planets, because of their gravitational might, they have played a key role in the formation of the Solar System, tossing around many objects in the system, preventing the formation of a planet in what is now the asteroid belt, and directly leading to the formation of the Kuiper belt and Oort cloud. They also retain some of the gas (in particular hydrogen and helium) that was present when the Sun and its planets formed and are thus key witnesses in the search for our origins.

In the past few years many giant planets have also be discovered around other stars. Those that transit, by chance, between us and their star can be directly characterized: we can measure both their mass and size, which allows us to infer, with the help of models, their compositions. Including our own giant planets and the recent discoveries by ground based surveys and Hubble Space Telescope, we now know the masses and sizes of twenty giant planets!

By carefully studying the interior structure of these planets, T. Guillot and his colleagues try to discover what they are made of and how they have evolved. This is a delicate task, involving precise comparison between detailed measurements of our giant planets and less precise data obtained for extra-solar planets. But new data are flowing in, and the future looks even brighter: Measurements by the Cassini spacecraft are leading to an extremely precise measurement of Saturn's gravity and better constraints on its interior composition. The future Juno mission, to be launched in 2011 will yield a measurement of Jupiter's gravity field with a precision that will be only second to that of the Earth. The Juno spacecraft, skimming over Jupiter's cloud tops will also measure the abundance of a key ingredient for planet formation: water in Jupiter's deep atmosphere. Last but not least, looking out of our Solar System, an extrasolar planet bonanza is expected from the missions COROT, to be launched in December this year, and Kepler, two years later.

What can we say so far? All giant planets seem to be made of a gaseous envelope of hydrogen and helium -the same elements that our Sun is made of- surrounding a central dense core made probably of compressed water and rocks. This core is at the heart of the formation of these planets. Standard planet formation theories would predict it to be of the order of ten Earth masses. This works for Uranus and Neptune, but not so well for Jupiter and Saturn: Jupiter appears to have a rather small core of a few times the mass of the Earth, while Saturn seems to have a larger core, around 10 to 25 times the mass of the Earth [Guillot 2005].

What about extra-solar planets? While more difficult to model, astronomers have confirmed that they are indeed made of mostly hydrogen and helium. But according to recent work by T. Guillot and his colleagues, some of them possess surprisingly large cores, up to one hundred times the mass of the Earth. Furthermore, there seem to be a correlation between how rich the star is in elements such as iron, silicate and oxygen and the mass of the planetary cores. This indicates that indeed planetary cores grew by the sticking of grain particles, but that the process was more effective than previously thought.

Obviously, the road to a clear understanding of how planets form and how our Solar System originated is still a long one. But by comparing detailed observations of the giant planets in our Solar System to the less precise observations of distant extra- solar planets, astronomers are hoping to directly test planet formation models and to learn more about planets. But billions of new worlds are awaiting us in our Galaxy. And for sure, as many surprises!

The Harold C. Urey Prize prize is awarded annually (or less frequently) by the Division for Planetary Sciences (DPS) of the American Astronomical Society to recognize and encourage outstanding achievements in planetary science by a young scientist.

Observatoire de La Cote D' Azur
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