STARS rich in metals are more likely to host planets than your average star. This discovery will help astronomers refine how they search for planets, and is already providing fresh clues to how planets form.
In a survey of 754 nearby stars, those with roughly the same amount of iron and other metals as the sun have an 8 per cent chance of hosting planets. 鈥淥nce we see stars with three times the metal content of our sun, the planet detection rate goes up to 20 per cent,鈥� says Debra Fischer, an astronomer at the University of California, Berkeley, who revealed the finding. But none of the 29 stars with less than one-third the sun鈥檚 level of metal had planets (see Graphic). Fischer is a member of the team that has tracked down most of the 100 or so extrasolar planets that have been discovered to date.
Fischer and Jeff Valenti of the Space Telescope Science Institute in Baltimore, Maryland, worked out the stars鈥� metal composition by analysing the spectra of their light. Unlike previous studies that have correlated metallicity with planet formation, their survey includes 693 stars that, available evidence suggests, lack planets.
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The idea that metal-rich stars are more likely to host planets supports the most popular hypothesis about how planets form. Its proponents say that as an interstellar cloud collapses to form a star, solid grains in the rotating mass of matter around the new star aggregate to form planets. 鈥淢ore metal would provide more grains to stick together,鈥� says Fischer. A rival explanation says the gaseous outer reaches of the rotating mass break apart to form planets, a process which would not depend on the content of solid particles such as metallic grains.
The finding also suggests that younger stars are more likely to host planets, since they tend to be richer in metals. But many questions remain. The limitations of current planet-detection methods mean that the stars that have been deemed planetless could be hiding planets that are too small or distant to spot.
Then there is the question of whether metal-rich stars are more likely to spawn planets, or whether planet-spawning stars become metal-rich because fledgling planets crash into their surfaces and add to their metal content. Valenti and Fischer favour the former idea. That鈥檚 because the metal content of a star is measured in the outer convective layer, a gaseous region that bubbles like a boiling pot. If metallicity was due to planets crashing into the surface of the star, stars with a thinner convective layer should contain more metal because it would be less diluted, and this is not what the researchers found.
鈥淧lanet formation is hard to observe. This [new result] is really an interesting clue that ties what we see now to the initial conditions that might have been when planets form,鈥� says Michael Liu, an astronomer at the University of Hawaii in Honolulu.
For now, our best technology can detect nothing smaller than Saturn-sized planets, but it is expected to improve. 鈥淏y improving our understanding of how planets form, we will be better able to predict where Earth-like planets might be,鈥� says Valenti.
