In their recent book Rare Earth, Peter Ward and Donald Brownlee argue that although simple, durable one-celled life may be very common in the Universe, at most one nearby star out of several hundred may have any planet capable of evolving metazoan life, let alone intelligence. Given the vast number of stars within a few thousand light-years of us, though, it still seems very possible that our technology may allow us to identify some such stars. Even if no such worlds exist, we badly need to know much more about all the different types of planets around other stars in order to better understand just what the odds for complex life on them really are. Not surprisingly, there was considerable discussion of current and future techniques to detect extrasolar planets at the first Astrobiology Science Conference last month at NASA's Ames Research Center. Until this year, the only successful technique available to us was Doppler radial-velocity measurements.
Any planet orbiting a star naturally tends to drag the star around with it in a slight wobble.
An alternative means exists, though, to detect this wobble. The very sensitive spectrometers that we now have can detect the resulting very slight Doppler shifts in the frequency of the star's light spectrum as the star shifts towards and away from the Earth. For five years now this method has been sensitive enough to detect Jupiter-mass planets closely orbiting fairly bright stars, allowing astronomers to determine these planets' orbits and estimate their masses. This technique requires very sensitive Doppler measurements, capable of detecting stellar velocity changes of only a few dozen meters per second. The general feeling is that within a year, we'll be able to confidently detect Uranus-sized extrasolar planets if they are close to their suns.