How to Find Extraterrestrial Life

Unless you think that life is highly improbable -- that in a galaxy filled with planets and the basic molecules of biology, life only seldom appears -- then it's hard to believe that all three of these theories will fail.
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It's largely a technical achievement, but it augurs well for the chances of finding life trillions of miles from Earth.

A team of mostly Dutch astronomers have used the Very Large Telescope -- situated atop a barren ridge in the Chilean Andes -- to examine a planet hugging the star Tau Bootis, 51 light-years away. The planet is hellishly hot and impressively hefty; it packs the mass of a half-dozen Jupiters. But the truly interesting thing is that the VLT was able to detect this world's elusive infrared glow. Applying a bit of spectral analysis, the team uncovered carbon monoxide in its atmosphere.

Now carbon monoxide, familiar to many as a noxious tail pipe emission, is just another of the simple molecules that are as common as clover in the cosmos. No biggie. But could this same technique be used to find biologically relevant gases, such as ozone (a form of oxygen) or methane? If so, it would be a method for discovering life from afar.

Frankly, today's telescopes would have a hard time pulling this off. But new, outsized instruments sporting mirror diameters of 100 feet or more are in the offing. They're complex and they're costly, but they're also game changers, and could offer a life-detection scheme as promising as sending people or probes to Mars.

Which brings me back to the summer of 2006, when I was preparing a talk about the search for extraterrestrial life for a group near San Francisco. The newspapers were thick with stories relevant to biology beyond Earth: ice on Mars, discovery of a few more planets, etc. If you paid attention to it all, you'd be swamped. But it struck me that by stepping back from this daily dose of astronomical revelation, it was clear that all these stories neatly fell into three categories -- three approaches to determining if there's anything alive out there.

In other words, there's a three-way horse race to discover extraterrestrial life. For your consideration and contemplation, I list this trio of trotters below:

1. Discover life nearby. This is horse number one, and where you'll find the big effort and the big money. National space agencies back this nag by sending probes that might eventually ferret out microbes in the dark aquifers beneath Mars' sterile landscape, or in the hidden oceans of such moons as Europa, Callisto, Ganymede, Titan or Enceladus.

This is the search for life in our solar system, a holy grail ever since Galileo first swung a telescope towards the moon. But so far, no joy. Despite tantalizing suggestions of fossilized microbes in meteorites, puzzling and possibly biogenic methane gas in the martian atmosphere, and a long-standing controversy over the Viking lander experiments of nearly 40 years ago, there's still no Exhibit A that points unequivocally to biology in our own back yard.

Nonetheless, the search goes on, becoming ever more extensive and more sophisticated. It's like Prince Henry the Navigator, sending ship after ship into the South Atlantic, hoping to find a route to the East. He eventually succeeded, and maybe we will too.

2. Sniff it out. Horse number two is the strategy described at the start of this article. A decade ago, NASA had plans to build a space-based, infrared telescope called the Terrestrial Planet Finder. It could both image exoplanets as well as do the sort of spectral analysis that might detect atmospheric gases caused by biology. Alas, the TPF is presently in a state of "indefinite hold," which many researchers think is a euphemism for rigor mortis.

But the idea is good, and either something like the TPF will eventually fly, or the humongous telescopes currently on the drawing boards will use their cyclopean mirrors to search out life around other stars by sensing its exhaust gases.

3. Eavesdrop on ET. Otherwise known as SETI, this third horse is an effort to detect radio signals or laser flashes from technically savvy extraterrestrials. The down side? Life able to build transmitters or lasers is surely less widespread than the type of single-celled organisms that might be discovered by schemes 1 and 2. So SETI will have to search farther and wider to win this race. The up side? It could, in principle, succeed tomorrow.

Of course, some folks consider SETI a long shot. But it offers a big payoff if it noses across the finish line first: Namely, life (or at least intelligence) that could have something interesting to say.

So what are the chances that any of these horses will make it into the stretch?

That's a big guess of course, and not much more than a guess. But unless you think that life is highly improbable -- that in a galaxy filled with planets and the basic molecules of biology, life only seldom appears -- then it's hard to believe that all three of these horses will scratch.

As to who's favored to prance into the winner's circle, my opinion is that, a priori, each of the contenders boasts roughly equal odds. In the next two decades, we'll see yet more robotic explorers on Mars, and possibly even on the more seductive satellites of the outer solar system. That's good news for horse one. But within the same time frame, large telescopes and possibly a cousin to the Terrestrial Planet Finder will be analyzing the light from planets around nearby stars. And SETI experiments -- assuming that continued funding can be found -- will increase the number of stellar systems examined by a factor of a thousand.

So the next time you check out the myriad science stories here and elsewhere that pertain to the matter of life beyond Earth, consider this simple way to categorize their relevance. The field is small, the contenders well-matched, and the horses are out of the gate.

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