Here at the Astrobiology Science Conference 2008 in Santa Clara, Charley Lineweaver, a Senior Fellow at the Planetary Science Institute is trying to figure out where in a galaxy aliens might live - the so-called "galactic habitable zone." Planetary habitable zones are well-known - for our sun Sol, we think it's roughly between Venus' orbit and the asteroid belt outside Mars' orbit. Galactic habitable zones are little tougher to pin down - as usual you've got to worry about having liquid water (or liquid something), but things get interesting when you consider the risk of getting blown up by a supernova.
Lineweaver's four main preconditions for a piece of galactic real estate being hospitable to intelligent life are:
- Distance from galactic center. Our sun is about 8.5 kiloparsecs from the center of the Milky Way which is about right. The further you go out from the center of a galaxy, the fewer stars there are. The further you go in, the more likely a nearby star will go supernova, and wipe out life in your start system. LIneweaver figures between 7 and 9 kpc is about right.
- Age. Life takes time to evolve into something resembling intelligent. This takes a few billion years.
- Metallicity. This is Lineweaver's way of measuring how much of the supernova leftovers are accumulated in a given region of space. If there's less than 1% of the metals found in our solar system, there's probably not enough to build a rocky planet (in astronomers' parlance, 'metals' includes everything that's not hydrogen and helium, so stuff, like water, too).
- The likelihood of forming a gas giant. Like supernovas, Jupiters, Saturns and other giant planets make bad neighbors for harboring life. During the early stages of star system formation, they have a tendency to come crashing through planetary habitable zones, annihilating rocky planets that may one day harbor life.
In short, it's a galactic jungle out there, and in 2004 Lineweaver's beginning to get his head around narrowing down the best places we may find our interstellar neighbors, whether in this galaxy or the next. All that said, though, he's careful to point out that we don't even really know what a good definition of life is, so instead of "habitable zone" maybe the name should be changed to the more sensible (and way less-exciting) "pre-habitable zone."