Seth Shostak: Thanks very much to all of our panelists for describing four different kinds of alien worlds. I'd like to hear their prognoses about when we might, if ever, find such worlds showing evidence for life. Are we talking about one year, 10 years, 100 years?

Tori Hoehler: The planet we're most likely to find is a function of a couple of things. First of all, how frequently do each of those worlds occur? We have no idea how frequently life arises, but certainly Slime World is likely to be a lot more frequently occurring than Complex Life World. Probably every Slime World also will be a Subsurface Life World.

But this raises the next point, which is, how easy is it to see this life? The technology that you use to detect each one of these worlds will be quite different. As T.C. suggested, to find a Subsurface Life World you need to be able to dig beneath the ground, so you would need to send some sort of a probe. In the case of a Slime World, you would need very powerful telescopes flying in space to do the job correctly.

I think it's conceivable that when we fly the right sort of technology to find a Slime World, which may be 20 or 30 years from now, all of a sudden we'll get a flood of data. When we can expect to find planets with life depends on when we can deploy the right technology.

Seth Shostak: Sounds like a bid for money. If we find Slime World in 20 or 30 years, that means many of us will still be around to see that happen. We'll be slimed. T.C., you talked about cryptic life. What are the opportunities there?

T.C. Onstott: I totally agree with Tori on this. We have an opportunity in the next 20 to 30 years, if we have the programmatic drive to do it, to encounter and touch and feel subsurface life on Mars or Europa, and also the chance of detecting slime or even vegetative planets in other solar systems. It's another proof of life out there.

The only difference is that in one case you're picking up an optical signal and in the other case you're actually looking at the molecular structure. Combine that with the fact that scientists are getting very close to engineering life forms in the lab, it means that our lifetime is very special. We may not only encounter the signs of extraterrestrial life, but also understand the origins of our own life.

Seth Shostak: We have a horse race here, and there are several different horses that seem capable of crossing the finish line on the same time scale. Peter? Odds of finding complex life?

Peter Ward: Could I defer to Victoria Meadows? She knows this sort of thing far better than me.

Victoria Meadows: I think Tori has already given the number. You're looking at the capability for missions like the Terrestrial Planet Finder, which is a proposed NASA mission. It has a sister mission concept in the Darwin mission from ESA.

We're hoping we can get those up in the next 20 years or so. And if we can do that, then we can definitely go after things like Complex Life World and Slime World. In the best of all possible worlds, within the next 20 years we'll have the answer to whether or not those things are out there.

Seth Shostak: Jill, the other panelists say stupid life within 20 or 30 years.

Jill Tarter: I'm not so sure I believe that. For Slime World, or Subsurface World, or complex life without technology, the technologies we're talking about to find them will have a reach into our galaxy of only hundreds of light years within the next 10, 20 or 30 years. It may not be enough, even if life is quite prevalent.

There aren't that many stars within the reach of these instruments. So our "first generation net" may not be successful in grabbing it. With respect to technology, the answer might well be "never." Technology World might not be out there. It also might be that we've already detected it but we don't yet recognize it.

There are new things that are getting my colleagues really excited, new kinds of signals that are detected from the sky that we didn't anticipate, and the theorists are being very fleet of foot to provide more prosaic explanations for them.

But in fact it might turn out that they are evidence of another technology that we don't yet recognize as such. And then with respect to the things that we can imagine, 21st century Earth on steroids, that kind of technology, you may be in the 20 to 30 year time scale if a thousand light years is a large enough reach. If you need to go farther because they are rarer, then the search will take longer. But the time it'll take isn't infinitely long, unless the answer is never.

Seth Shostak: I assume you don't think that's the case.

Jill Tarter: I don't know what the answer is. I think it's worth looking.

Seth Shostak: Now it's time to hear from the audience. Everyone is welcome to grill our panelists like a cheese sandwich.

Q: Do you think the human race will survive long enough to find intelligent life and make contact with it? And actually communicate, rather than just find it?

Jill Tarter: Just detecting the intelligent life out there might help us survive longer. Because if you can detect a technological signal, it means that it's possible for technologies to last a long time. If the average length of a technological civilization is short in cosmic time scales, we'll never detect it. I don't expect cosmic salvation; I don't think they're going to come solve our problems.

But it might put us into the realm of the long-lived technologies if we know that somebody else has done it. I think we ought to be working on our survival for many reasons, including learning about someone else out there.

Peter Ward: The average mammal species lasts about 5 million years. Modern humans have been around for 200,000 years. If we are average, we've got 4.8 million years left. But my favorite species, the Nautilus, has been around for 500 million years, so there's no reason that we couldn't be here until the end of the sun, if we play our cards right.

Jill Tarter: The Nautilus didn't invent nuclear weapons. (laughs)