© OCA Correlation of metallicity between Pegasids and their parent stars.
Correlation of metallicity between Pegasids and their parent stars.
In little more than a decade, enormous progress has been made in planetary science with the discovery of more than 200 planets beyond our solar system. But how exactly these planets form is still up for debate. New work in the field has exposed some of the mysteries behind a group of giant planets located in Earth's vicinity, at least by galactic standards (up to 650 light years away). Pegasids, also called “hot Jupiters,” make up a class of very large extrasolar planets (outside our solar system) that orbit in close proximity to their star, making them very hot. A team of six European astronomers led by CNRS researcher Tristan Guillot, from the Cassiopée Laboratory in Nice,1 has found a correlation between content levels of heavy elements2 in Pegasids,3 and in their respective parent stars. The findings support, for the most part, the “core accretion” model of planet formation, which posits that heavy elements play a capital role in planet formation. These elements condense at low temperatures–like those in space–and create solids that agglomerate to make up planet cores. “It is thus expected that planet formation would be easier around stars that are metal-rich and that these planets would have a larger core,” explains Guillot. The team's results confirmed this hypothesis:4 They found that Pegasids born around metal-rich stars–that is, two or three times our Sun's metal content–have large cores, while those born around stars with a low metal content have smaller ones.
Recent advances in planet observation methods allowed the astronomers to “look inside” the planets. Transit detection involves measuring how much the light emitted from a star dims when a planet's orbit takes it between the star and Earth. “What we see is a very slight dimming of the order of 1%,” says Guillot. “It lasts for a few hours, then the star returns to normal intensity, and then it dims again after each orbital revolution of the planet, typically every few days. This lets us determine the ratio between the size of the planet and that of the star and hence the size of the planet itself,” continues the researcher. The astronomers also used the radial velocity method to measure the “tug” each planet exerts on its star–which results in perceptible motion–and thus its mass. Combining the data obtained through these two techniques, they were then able to calculate planet density and, ultimately, the proportion of heavy and light elements.
One unexpected observation was that some of these large planets contain a considerable mass of heavy elements (up to 100 times the mass of the Earth). “Previously, the theory was that once the core of a planet grows to 10 times the mass of Earth, it begins to capture hydrogen and helium very efficiently. This should prevent large planets from having high metallicity. But that is not what we are finding,” says the researcher. The team is now eager to test its method on a wider spectrum of planet sizes, pinning their hopes on the French-led Corot mission,5 set to launch in November 2006, a part of which is dedicated to the detection of exoplanets. “Right now our results are only tentative because we only have complete data on twelve planets. With Corot, we will look at a large number of stars to see these transiting planets. We are hoping to detect many more large planets, but also smaller ones that are more like Earth. That will give us much more data to work with.”
1. CNRS / Observatoire de la Côte d'Azur joint lab
2. Astronomers, unlike other scientists, refer to elements that are heavier than hydrogen and helium as “heavy elements” or “metals.”
3. The planets in this study are called transiting planets because their orbit takes them between their star and Earth, making them visible to astronomers. Their masses are between 110 and 430 times that of Earth.
4. T. Guillot et al., “A correlation between the heavy element content of transiting extrasolar planets and the metallicity of their parent stars,” Astronomy and Astrophysics. 453 (2): L21-L24. 2006. and N. C. Santos et al., “High resolution spectroscopy of stars with transiting planets,” Astronomy and Astrophysics. 450,(2): 825-831. 2006.
5. The Corot (Convection, Rotation & planetary Transits) space telescope will be put on a mini-satellite platform and launched in November 2006 for a mission of two and a half years or more.
Observatoire de la Côte d'azur, Nice.