Stars orbited by planets may burn up more of the lithium in their atmospheres than stars that are planet-free. Credit: ESO/L. Calcada

The amount of lithium on the surface of a Sun-like star could be a telltale sign that it is orbited by extrasolar planets — a finding that could speed astronomers' attempts to detect them.

Astronomer Garik Israelian of the Institute of Astrophysics of the Canary Islands in Tenerife, Spain, and his colleagues, found evidence for the link by comparing planet-hosting and planet-free stars. "It's a very clear signal," says Israelian.

Lithium burns via fusion in a hot star's core, but models show that it can survive nearer the surface. In a sun-like star, swirling convection currents shouldn't run deep enough to draw lithium into the core. "For a Sun-like star, the models predict a very high amount of lithium in the atmosphere," Israelian says.

However, that is not what astronomers observe: the Sun has 140 times less lithium than it is predicted to have had when it formed, far less than astronomers expect to see. And now it would seem that the Sun isn't alone in having this characteristic.

Israelian's team analysed data from surveys of 24 Sun-like stars (including the Sun) that had detectable orbiting planets and found that the vast majority — 22 stars in total — had unusually low levels of lithium in their atmospheres. If a star's low lithium levels are strongly correlated with the presence of extrasolar planets, it could provide an important new way to search for planets in other solar systems. The team's results are published in Nature1.

A question of age

Others, however, question whether the connection between lithium levels and extrasolar planets is real. The star's age might also be responsible for the low lithium readings, says Jorge Melendez, an astronomer at the University of Porto, Portugal.

We believe what they are seeing is just an age effect. Jorge Melendez , University of Porto

Stars grow hotter with time and burn more lithium as they age, so it would be unsurprising if the older stars in the sample had less lithium. Young stars are turbulent, and that would make it harder to see their planets, he adds. Thus the apparent link between the presence of planets and low lithium might be due to a systematic bias. Melendez wants Israelian and his team to provide more robust evidence for the age of the stars in their sample.

"We believe what they are seeing is just an age effect," he says.

Israelian counters that the group has already taken the stars' age into account: all the stars in their sample are more than a billion years old, and many without planets have relatively high levels of lithium in their atmospheres. The observation that so many systems with planets have low levels of lithium is unlikely to be due to chance, he believes. "It cannot be a simple coincidence," he says.

In a spin

As for why lithium is depleted in these stars, Israelian and his team propose that extrasolar planets are to blame. The group theorizes that orbiting planets sap stars of their angular momentum, causing them to spin more slowly. That, in turn, causes their atmospheres to mix more deeply, allowing lithium to be sucked down into the star where it burns and is destroyed.

Intriguingly, out of 60 planet-free stars the team examined, roughly half also had low levels of lithium. That could mean that these stars have extrasolar planets that have not yet been detected.