QUOTE (Jyril @ Jul 26 2007, 03:16 PM)
Well, the negative side of the COROT survey is that the search is limited to closely orbiting transiting planets so we get a very biased sample. Which is far better than nothing, of course.
Check my math in trying to characterize the bias.
Given two similar planets orbiting two similar stars, but with one planet N times farther from its star than the other, the ratio of likelihood of detection in a short time frame should be N^2.5. That is, the probability of appropriate geometry for a transit is decreased by N for the farther planet, whereas the probability of a transit taking place at the right time is a function of the orbital period, which introduces another factor of N^1.5.
For example, if Earth were orbiting at 5 AU, it would be precisely 1/5 as likely for its orbit to transit the Sun as seen from afar, and if it did, it would do so about 1/11th as often. So a factor of 5 in distance translates to a factor of 55 in transit observations. A factor of 10 in distance translates to a factor of 300 in transit observations.
The temporal factor is mitigated as the observations continue. Given a mission lasting Y years, we'd get one observation of every transiting planet with a period <=Y, two observations of every transiting planet with a period <=Y/2, and a probability Y/X of one observation of every transiting planet with a period X longer than Y.
The diameter of the planet is also a minor factor. Jupiter might graze the Sun's disk whereas a Pluto in the same location would just miss. As the planets get much smaller than the star, this factor almost vanishes.
COROT will survey a few different areas, none for more than 150 days or so, so repeat detections will be strictly limited to planets in close-in orbits. Single detections of planets farther out will (presumably!) take place, and could help us get an idea of the distribution of planets in different-sized orbits. But at some point out there, the data will be too sparse to make predictions significant.
So overall, I think it's going to be pretty sparing in telling us about the raw numbers of Venuses, Earths, and Neptunes. But a few data points would be a lot nicer than none.