GIANT planets orbit up to 8 per cent of all Sun-like stars, say astronomers in the US. The estimate comes not from counting planets directly, but from counting stars that appear to be swallowing planets.
When stars such as the Sun exhaust the hydrogen fuel in their centres and become bloated "red giants", they grow big enough to swallow any closely orbiting planets. This won't happen to the Sun for another 5 billion years or so. But Mario Livio and Lionel Siess of the Space Telescope Science Institute in Baltimore, Maryland, reason that since the red giant phase lasts a few hundred million years, other Sun-like stars should be gobbling up their planets at this very moment-yielding clues about how many planets are out there.
They simulated what would happen if giant planets with masses comparable to that of Jupiter were gobbled by their stars, and found three strong markers. First, planet eaters glow with an excess of infrared light, as gravitational energy dumped into the star by the planet heats up the star, causing it to expand and puff off its cool outer layers as expanding shells of dust. Second, angular momentum transferred from the planet speeds up the spin of normally sluggish red giants to about 10 per cent of the speed at which they will eventually fly apart. Finally, the planet contaminates the star with lithium, an element that does not normally survive for long in the heat of a star.
Remarkably, Livio and Siess say that many red giants show all of these telltale signs: they are spinning very rapidly, emitting anomalous amounts of infrared and their light shows the spectral "fingerprint" of lithium. "We've found that about 4 to 8 per cent are showing the signs of swallowing planets," says Livio. "This percentage agrees very well with more direct estimates of the prevalence of planets." Livio and Siess's work will be published in Monthly Notices of the Royal Astronomical Society.
Other astronomers are impressed. "The planet-swallowing hypothesis is the best explanation I've seen for the origin of these lithium-rich giants," says Caty Pilachowski of the National Optical Astronomy Observatories at Kitt Peak in Arizona. The work also highlights the fact that planets can have a profound effect on the evolution of stars, she says.
Author: Marcus Chown
New Scientist issue 5 June 1999
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