Like many people, I’ve been watching the Kepler telescope’s exoplanet search with a great deal of interest. An exoplanet is a planet orbiting a star other than our Sun; the vast majority of known planets are now exoplanets, a big change over the last 10 years! One of the major goals of the Kepler mission is to identify terrestrial planets — planets that are roughly the same size and composition as Earth. Specifically, the ideal discovery would locate exoplanets in orbits that are within the host star’s habitable zone: the region where heat from the host star is sufficient for liquid water to exist. One potentially exciting discovery, Gliese 581g, was made last fall, which I wrote about here.

An even more interesting question is whether life exists on other worlds. As I’ve said before (and likely will again — professors are required by law to repeat themselves at least once per semester, and usually we are fully compliant with that law), we really don’t know what the odds are for life to develop in a given environment. In fact, the best candidate for life beyond Earth in our Solar System may be Enceladus, Saturn’s icy moon — which obviously doesn’t resemble Earth in very many ways! However, we know Earth can support life, so that means we have at least one template for comparison with exoplanets.

I’ve actually revised my opinion about the probability of life beyond Earth in recent months, and Kepler has played a major role in that. With so many exoplanets discovered and described (at least in a preliminary way), it seems to me that even if life is very rare, there are so many planets — more than many of us suspected — that even a rare event can occur more than once. With potentially tens of billions of planets in our galaxy alone, even if there’s a chance in a trillion of life arising on a given world … it will happen many places within the observable universe.

So now we have a set of steps: first we look for exoplanets, then we search for planets that are Earth-like in some important way (ability to harbor liquid water, to be specific), then we look for life on those worlds. Finally, we ask that question: how unique are we humans as conscious, self-aware, intelligent beings? Defining “consciousness” is beyond my pay scale, and there isn’t a scientific consensus on it anyway; “intelligent” is even worse: are dolphins intelligent? how about chimpanzees? and what about cephalopods? We have trouble recognizing self-awareness in any kind of objective sense, though if I understand the literature, cognitive scientists think all of these things lie on a continuum rather than a sharp line; dogs have limited self-awareness, dolphins are more, but it’s a hard thing to quantify. For the sake of searching for intelligent extraterrestrial life, we’ll have to assume we can figure something out heuristically, perhaps based on technology.

Despite my cautious optimism about discovering life elsewhere in the universe, I’m still agnostic about whether there’s intelligent life out there. However, I fully support the mission of the Search for Extraterrestrial Intelligence (SETI) Institute, and was very unhappy to hear that they are being forced by budget cuts to suspend operations at their Allen Telescope Array. This is one kind of project where (in my opinion) the small chance of success is more than balanced by the relatively small operating cost and totally overwhelmed by what a positive discovery would reveal.