The Search For Alien Life: How Do Scientists Know What To Look For?

At the 2014 World Science Festival, WNYC radio journalist John Hockenberry compared searching for alien life to searching for and settling into a new home: You need a real estate agent (an astronomer to find the right planet), an interior decorator (a planetary scientist to assess whether the environment is hospitable to life), a chef to check if there are the right ingredients in the kitchen (a biochemist), and an accountant to juggle all the numbers and make sure you’re looking in the right neighborhood (a physicist or computer scientist).

Thankfully, the Festival panel Alien Life: Will We Know It When We See It? had all those essential ingredients in place: Harvard astronomer Dimitar Sasselov, MIT planetary scientist Sara Seager, Howard Hughes Medical Institute geneticist and biochemist Jack Szostak, and Arizona State University physicist Paul Davies.

Hockenberry, who moderated the panel, asked the team to rate just how close we might be to discovering extraterrestrials.

“If we’re talking about intelligent aliens, I suspect we’re a very long way away from that, but if we’re talking about microbes on Mars, it could be any time,” Davies said.

Seager estimated that with the larger and more complicated space telescopes on the way (such as the James Webb), we could find evidence of alien life within 10 to 40 years.

The first logical step to seeking out extraterrestrials, of course, is to find a planet they’re living on. “We know there are billions of planets in our galaxy, and now we’re after the best ones, just like in real estate—location, location, location,” Sasselov said.

Scientists are especially interested in planets lying within the habitable zones of stars, where the temperatures is not too hot, nor too cold, but at that right range that allows liquid water to exist. Because stars are variable in size and heat, this habitable zone can vary as well in range and distance from the home sun. To find planets, Sasselov and other scientists make use of the Kepler telescope, which looks for dips in the brightness of stars as planets “transit,” or pass between them and our view. If those dips occur on regular periods, that’s a good sign that one or more planets are circling the star.

But one drawback to the transit method of looking for exoplanets is that the conditions of the planetary system have to be just right—the orbits of the planets have to be lined up so that we can see the transit from our vantage point. And this happens for less than 1 in every 100 stars, Sasselov said.

We also can’t really study the stars closest to us with this method. Kepler is designed to look at a lot of stars, so the odds are better of finding ones with planets around them; to do this, it has to look farther away from Earth. Seager compared the situation to being directed to stand in Times Square and look at 200,000 people at once. If you just look right down 42nd Street, you might see a few hundred, but if you used binoculars to look across the Hudson to New Jersey, your field of vision might encompass hundreds of thousands of people. Employing a similar strategy, Kepler mostly finds planets lying 500 to 2,000 light years away.

Even with these limitations, the Kepler scientists have found some exciting worlds. Very recently, the telescope picked up the first Earth-sized planet orbiting in the habitable zone around Earth’s star: Kepler 186-f, which completes an orbit around a star in the constellation Cygnus every 130 days. Even more recently, researchers found a surprising “mega-Earth,” which is 17 times more massive than our planet but still rocky, and seemingly not the massive gas giant that scientists would have expected.

But even finding a planet that looks to be the right size and in the right zone to support life can’t tell you if the conditions are right for life to flourish. For that, scientists like Seager use a technique called spectroscopy to examine the atmospheres of these far-off worlds to find spikes of interesting chemicals, like oxygen, which on Earth is much more abundant than a naïve alien observer would expect, thanks to the actions of Terran life—plants and photosynthesizing bacteria.

How can you find gases just by using starlight reflected off of these exoplanets? It turns out that if you separate the light into its component colors, there will be areas of dark bands where certain minute ranges of colors are missing. This is because the different molecules in a planet’s atmosphere (if it has one) will absorb certain wavelengths of light.

Basically, different ingredients in the atmosphere are “taking out bites from the rainbow,” Seager said.

While Seager and other planetary scientists are scrutinizing atmospheres, biochemists interested in the search for alien life are trying to see if they can recreate life from basic building blocks in the lab. “If we can show that there’s a simple pathway and all the steps are easy, maybe that makes it look like it’s easy for life to get started,” Szostak said. “Conversely, if we showed that there are some difficult steps, it could show that life is maybe rare.”

Could alien creatures originate in some unusual and extreme environment? On Earth, life manages to find a way in all sorts of unusual places: near hydrothermal vents, or inside salt structures in Chile’s Atacama Desert, which are normally bone dry but manage to pull just enough water vapor from the air to support tiny microorganisms. However, “the environments where life could get started are probably much more restricted,” cautioned Szostak. “There are so many extreme environments that are populated, but that doesn’t mean life could get started there.”

Szostak’s work does indicate that some of the steps for life are pretty simple. Fatty acids (like the ones in our cellular membranes) will arrange themselves into little soap bubble forms and filaments automatically, and can divide into daughter structures. But evolution as we know it requires something to spread information; something like DNA.

“You can make membranes out of lots of different molecules, as long as you have one part that’s oil-loving and another part that likes to be in water,” Szostak said. But could an alien analogue of DNA arise with different ingredients?  “We know there’s a whole large family of related molecules that could work perfectly well as genetic molecules.”

But it’s not obvious how most of these alternatives could arise from early-Earth-type conditions.

And there’s the larger question of how you distinguish what life actually is. “If you ask a physicist or chemist, ‘what is life?’, you’ll get an answer about molecules and thermodynamics and activation energy,” Davies said. “You ask a biologist, ‘what is life?’ and you’ll get an explanation about coded instructions, signals, translation.”Life is complex, and its origin requires both hardware (the exact ingredients it’s made of) and software (the way that it manipulates processes and manages and organizes information).

Are we not thinking weirdly enough? Could scientists be overlooking life on strange gaseous planets, or creatures that subsist on methane, or use silicon or nitrogen as a basic building block? Our participants suggested those sci-fi scenarios aren’t quite as practical. “The chemistry of carbon is so much richer than anything else,” Szostak said.

If E.T. does exist, and swings by for a visit, Earth might well call in Davies for that first encounter; he chairs SETI’s grandly named Post-Detection Task Group (check out our extensive interview with Davies about what we might do when the aliens come knocking). But the evidence for life may not come in such a convenient manner. “It may be that we detect some weird thing out there that has no natural explanation,” Davies said.

And so, on many fronts, the search continues. Astronomers are trying out new methods of exoplanet hunting that don’t rely on the transit method or the Kepler telescope. Seager heads up a group working on a new concept called Starshade, a flower-shaped device that will block out most of the bright glare of a target star so scientists can get a better look at exoplanets directly. Seager compared it to holding up your hand to block out a spotlight, so that you can see some object nearby.

“The beauty of astrobiology is that it brings together chemists and physicists and astronomers and mathematicians and computer scientists and mixes them all together,” said Davies. “We need something of everything.”

Want to learn even more about science’s search for alien life? Watch the full program right here.

Image credit: Ruletkka/Dreamstime.com