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."
Image: NASA
Chart: Science













Comments
Don't we have two gas giants in our solar system?
Though I seem to remember computer modeling that showed that gas giants of exactly Jupiter and Saturn's size and position were optimal for not screwing up Earth.
Ding ding ding! Looks like we over grown psychotic hairless apes won the cosmic lottery. Unfortunately, we're just in the process of pissing it away, like most lottery winners.
@ManchuCandidate: Actually, I think the roaches and opossums won, and they're doing just fine, thankyouverymuch.
@braak: Actually we have four Gas Giants (Jupiter, Saturn Uranus and Neptune).
I heard that gas giants were actually helpful in the long run as they attract small objects like asteroids that tend to smack into small rocky planets like Earth every once in a while if left unchecked.
@Pete: Yeah, the article I read said that the models they did showed that if the gas giants were any smaller, they probably wouldn't hoover up the meteors and protect us--but if they were any bigger, they'd start dragging in extra ones that would become dangerous.
So, just the right size.
@braak: Pete just called Uranus a gas giant. You gonna take that sitting down?
@braak: Why would gas giants be any more beneficial than non-gas giants of similar mass?
Our gas giants are in unusual positions. Most are closer to their star, precluding solar systems like ours. See all the stories about the discovery of another "hot jupiter".
I seem to recall reading some years ago that we're in an oxygen rich area - part of the metallicity mentioned in this story, I guess.
Whoa, logical holes big enough to drive a gas giant through, added to incredible terra-centricism that defines life as "us" and/or "us-ish" and then draw a wobbly color-coded chart that shows how very very danged special we are, not just as a species (thank you verra mush), but as a class of planet.
How's about we look at it like this: we are what we are because of where we are. if we were not here, we wouldn't be us. But that does not necessarily mean that we aren't somebody else, if'n you kin wrap yo tiny noodle around that.
@beercheck: Actually, upon further consideration, I can see how gas giants might be better at catching near-miss meteors, rather than just slingshotting them on by.
The aliens in my part of the galaxy hang out in the Home Depot parking lot.
@braak:
Ah, that makes sens. And as Mathmos mentionned, ours are also in the right spot. Guess we lucked out. ;)
@beercheck:
Ugh, I can't wait until 2620 when scientists change its name from Uranus to Urectum, to stop that stupid joke forever!
@Pete: Urectum? You damned near killed 'em!
So do aliens live in a galaxy far, far away or not?
@Pete: I seem to remember something about them stabilizing orbits as well, although if the gas giant is anywhere near our comfort zone, that would not be good.
I can't begin to count the number of things wrong with this. I'm just going to voice the most glaring criticism, which is that it is unbelievably anthropocentric.
@bonniegrrl: You know, after the, oh, twentieth time I watched Star Wars, I theorized that all the aliens (and alien life forms) in Star Wars are actually evolved from space-faring humans (and the flora and fauna they transported) that got stranded long long long long ago, and eventually evolved to be different from their original forms. Of course, this would require the Star Wars galaxy to be much older than previously thought. This also meant that aliens didn't originally exist. As cool as I found that scenario, I always felt that it was kind of sad. It's like the penultimate group of imaginary friends.
@Bob_of_Mars:
That is kinda of sad, like those Garfield comics where the cat never existed at all.
The sample size for this sort of thing is just a bit low.
I think this isn't anthropocentric enough. It's not that I believe that non-human or non-carbon based life forms are a scientific impossibility, but since we have no idea what they might be based on we cannot speculate much on them and we might as well speculate on carbon-based life forms and mammals. In other words, speculations about life in other systems should take into account things like Carbon, Hydrogen, Oxygen Nitrogen traces and planets with the correct gravity, as much as considerations of the "havitable zone".
I'm of the opinion that we are the only "intelligent" species in the Universe. I think it's a nice theory that their may be others out there, but I require evidence. We seem isolated enough so that it won't be an issue any time soon. As far as I'm concerned this is all just illusion and we just think it's "real."
- Age. Life takes time to evolve into something resembling intelligent. This takes a few billion years.
That's a sample size bias right there, especially when you consider how long we suffered the tyranny of single-celled life here on Earth-- let alone all the extinction events that followed.
There's no real reason why life couldn't evolve faster. In fact, it's still a big mystery why it took so long here. It might take less time elsewhere-- or more time.
As for *intelligent* life-- every species on Earth adapated just fine to their environment without human-level intelligence. I personally feel that the universe is probably teeming with life, but human-level (or beyond) intelligence is probably extraordinarily rare (just ask Fermi).
Damn sample size.
@Jeff-Minor: Talk to Hugo Garis-- we're all just a simulation in some alien race's Artilect.
@Huxleyhobbes: "I'm just going to voice the most glaring criticism, which is that it is unbelievably anthropocentric."
It's hard for us to be any other way. I've never heard a believable "life basis" other than the CHON version. You can talk about ammonia or methane breathers, but even they need (theoretically at least) a moderate temperature range and gravity.
@Daveinva:"but human-level (or beyond) intelligence is probably extraordinarily rare"
You're probably right. It certainly is on earth.
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