There are many methods used to find exo-planets (planets in other solar systems). The direct imaging of a planet (this is rather hard given the distances), the transit method, which is where you wait for the light levels to change as a planet passes in front and behind the star, and the radial velocitymethod which is probably the most successful method so far and a few more besides.

With 400+ found the suggestion is that there are plently of planets out there, but there is still quite a bit of effort in finding them. Recently however in an analysis of over 500 stars including 70 which have planets, a highly significant statistically result was identified correlating the presence of planets the amount of Lithium in the star. Systems with planets have as little as 1% of the lithium of stars without planets. What this means is that we can start to look at the stars which are most likely to have planets.   So why the low lithium? The current theory is that the angular momentum of the star is reduced based on the pull of the planets, allowing the lithium to fall deeper into the star and be destroyed.

But all this raises the question what if we start to limit our search to those stars which contain low lithium levels? Will we lose the ability to find planets where we least expect them? Sure we should take this data and use it, but perhaps the universe has more to say on this issue than we yet know. Already we have found Gas Giants larger than Jupiter inside the orbit or Mercury, which still challenges our understanding of how solor systems are formed.

While this is potentially excellent news for planet hunters lets hope it doesn’t narrow our focus too much. At least until we know more.  The paper which caused this stir is Israelian et al., “Enhanced lithium depletion in Sun-like stars with orbiting planets,” Nature 462 (12 November 2009), pp. 189-191 (abstract).