Nasa’s Kepler space telescope has discovered over 1200 extra-solar planet candidates, it was revealed at a press function yesterday.  The telescope only scans a small area, about 0.25% of the sky, which suggests that our galaxy is absolutely packed to the gills with planets.  Of these candidates, 54 are found in the so-called ‘Habitable Zone’, the distance from a star at which a planet could potentially have the liquid water which is so vital for life.

We must remember, of course, that these discoveries are only candidates – they await further study from larger ground-based telescopes to confirm that they are actually planets.  At the time of writing, 15 candidates have been confirmed so far.  What I find interesting is how many of these new planets orbit so close to their star that their surface temperatures exceed 1000°C.  This doesn’t mean that most planets are extremely close to their star, however, only that very close planets are easier to spot with current detection methods. 

Kepler relies on the ‘transit method’ to detect exoplanets.  If a stars ecliptic plane happens to lie close to our line of sight, then it’s planets will regularly pass in front of it from our point of view.  When this happens, the stars light will be dimmed by a tiny amount – far too little to be detected by traditional methods, but easy for Kepler’s ultra-sensitive CCD cameras.  Since a distantly orbiting object like Pluto could only transit its star once every 250 years, Kepler would need to keep an unblinking eye on that star for at least that long to have any chance of detecting the brief dip in brightness. 

A second consequence of the transit method is that a planet has to be reasonably large to cause a big enough dip in the stars light.  As the method continues to be refined, we’re able to spot smaller and smaller planets, but it is only very recently that we’ve been able to see anything as small as Earth, which implies that the true count of planets is far, far higher than what we’re seeing at the moment.