Ask Dr. SETI ®

Dear Dr. SETI:
What are the current estimations of fraction of stars with planets and mean numbers of planets favourable for life, and how much uncertaintity is there in current estimations?
PT (not Barnum)

The Doctor Responds:
Roughly 10% of all stars are single F, G, or K class main sequence stars of about the right age for planets. We consider these the best candidate stars for habitable planets, and in fact the targeted searches (such as SETI Institute's Project Phoenix) concentrate on such stars in developing their target lists. Although we are just beginning to learn how to detect planets, about everywhere we look we find them. So I'd say f_p is pretty close to 10%.

Of about a dozen extra-solar planets detected so far, only one has been clearly in the habitable zone of its star. So for n_e, the number of Earthlike planets per solar system, I'd have to estimate only about 0.1. This does not consider the possibility of habitable moons.

Both of these estimates are highly uncertain, because our sample size is pitifully small. But we expect habitable planets to be rather commonplace throughout the cosmos (that's not a very specific answer, but an encouraging one).

See Solving the Drake Equation for a further perspective on this question.