A Shortcut for Detecting Alien Worlds

12 November 2009 (All day)

ESO/L. Calcada

Chemical clue. Stars that form solar systems tend to deplete their lithium content.

Finding new alien worlds may become a lot easier. Astronomers have discovered that stars containing low amounts of the element lithium tend to host solar systems, a result that could dramatically reduce the time it will take to detect another Earth-like planet.

Thus far, attempting to determine whether other stars play host to planets has required great patience and painstaking measurements. Astronomers scan the skies for two stellar phenomena. One is the barely perceptible but regular dimming of a star's brightness that occurs when an orbiting planet passes between the star and Earth. The other is the regular but minuscule variation in a star's radial velocity--its speed through the galaxy relative to Earth's speed--which indicates that the star is being tugged by an orbiting planet's gravity. Astronomers casually refer to the two phenomena as blink and wobble. Efforts to detect planets via these methods can take months--and they often come up empty.

Enter lithium. Some young stars form inside a small rotating disk of dust and gas, whereas others coalesce inside a much larger rotating disk, called a protoplanetary disk. Astronomers think that in the former case, the star rotates much faster because it isn't subject to the gravitational drag imposed by the larger disk. This faster rotation, the thinking goes, pushes the star's lighter elements, such as lithium, closer to the surface, where they can persist for billions of years. The presence of a protoplanetary disk, on the other hand, could hinder the star's rotation, thereby causing lithium and other lighter elements to sink deep inside the star's nuclear furnace, where they are consumed. If these scenarios are correct, stars with low levels of lithium should be more likely to host planets.

That's what astronomers have found. In today's issue of Nature, a team led by Garik Israelian of the Instituto de Astrofisica de Canarias in Spain's Canary Islands surveyed 46 stars in our galactic neighborhood that host planets, along with 116 stars where so far no planets have been detected. Analysis of the light from the 46 stars showed low lithium content, the team reports, whereas the 116 stars all had higher levels of lithium--with over half sporting about 10 times more than the planet-hosting stars.

Physicist Dejan Vinković, of the University of Split in Croatia calls the paper exciting, but he says it's still unclear exactly why stars that host planets have low lithium levels. It's possible, he says, that something other than the protoplanetary disk is contributing to the chemical's loss.

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