The Kepler probe has announced the confirmation of its first five planets. Kepler (which was launched in March 2009) detects planets by measuring the dimming of their parent stars when the planet passes between them and the probe.
Four of the five are larger than Jupiter. All are very close to the parent star (of the order of a twentieth of the Earth-Sun distance, or about eight times closer than Mercury), so hellishly hot, with an orbital period of half a week.
This first clutch of planets might seem rather disappointing for a mission whose objective is to look for Earthlike planets in temperate orbits – but there are very good reasons why the easiest planets to find will be giants in close orbits.
Imagine I am standing some distance from you swinging a ball around my head by a string. Now, you can only see the ball when and if it passes in front of my face (this analogy works best if I am just a disembodied head). What properties of the ball/string/swing make it likely that you will see the ball?
Clearly if the swing doesn’t pass between us, you’ll never see it.
If the ball is large, there’s more chance you’ll notice it if it DOES pass between us.
If the string is short, the ball will pass between us frequently, giving you more chance to see it.
These three characters have a great deal to say about how many of the planets that are out there can be seen by Kepler. It also helps if the parent star is dim (if my face is face-sized, not like a hot-air balloon).
So the upshot is, Kepler is limited in the proportion of planets it can see and what kind of planets they are likely to be. The Kepler team worked all this out years ago, of course. For an Earthlike planet around a Sunlike star, the chance that Kepler will spot it is 1/200. As Kepler monitors 100,000 stars, if every star has an Earthlike planet, we’re looking at 500 planets. If only 1 in 100 systems has an Earthlike planet, that comes down to 5 detections. (Assuming for the moment that the monitored stars are all quite like the Sun.)
Kepler Mission Site