What’s the most likely way we’ll find life on other planets?

Credit: ESO/L. Calçada ESO

Despite science fiction's fondness for alien invasions, the first signs of life on another planet most likely won't come from radio beacons, let alone ships decimating the White House with giant energy beam. They may not come from Earth robots hunting for life, either. Instead, the first evidence for life may be in the form of passive signals, telltale signs of life and its processes, that astronomers will find in a planet's atmosphere. And those could very well come from the most passive life forms we know: plants.

Planets that have atmospheres at all are a good place to start looking for life — as far as we know, life (of any sort) needs a layer of gas covering a rocky world, which provides energy sources and protection from the unfriendly environment of space. This is one reason why the Mars rover Curiosity has been trying so hard to characterize the atmosphere of Mars, and why the forthcoming MAVEN mission (Mars Atmosphere and Volatile Evolution) will try to understand how it grew so thin. For scientists studying exoplanets, atmospheric science poses a greater challenge. But two different approaches could help astronomers study alien atmospheres in unprecedented detail, perhaps someday finding the signature of life.

Atmospheric Chemistry

The first step in the search for exoplanet life is determining whether a planet has an atmosphere worth studying, said Heather Knutson, a planetary scientist at Caltech. For instance, plenty of scorched rocky worlds spotted by the Kepler space telescope may not have an atmosphere at all. "For one, you have to ask, is the planet we found potentially suitable for life? Does it have an atmosphere at all, and is it hot, is it cold, does it have a solid surface? You have to do the basic characterization," Knutson told DVICE.

Given an atmosphere of a nice, Earth-like thickness, astronomers would next want to study its constituents. Aliens might not be little green men, "but you could have very, very little green men, like microbes," Nicolas Cowan, a postdoctoral fellow at Northwestern University's Center for Interdisciplinary Exploration and Research in Astrophysics, told us. "You can do a little bit of chemistry and measure the molecules in the atmosphere of a planet, and you can infer something is going on.”

Astronomers could make discoveries just by following some basic rules of chemistry. Earth has an abundance of oxygen, which is a highly reactive gas, as a direct result of all the life here. A different planet with an atmosphere full of oxygen and methane could mean there's something down below replenishing those gases — that's because oxygen would normally react with the with methane and there wouldn't be anything left, Cowan explained. "If you look at Earth's spectrum, we have both oxygen and methane, because life is here and metabolizing," Cowan said.

To find oxygen, methane, and other chemical species that could be life's calling cards, astronomers would either watch the planet transit its star, or directly image the planet and its star (the latter technique is much more tricky). Transiting exoplanets appear to move across the faces of their stars as seen from Earth, which causes a small decrease in the stars' brightness. If the planet has a thick atmosphere, it will be transparent in some wavelengths of light but opaque in others, altering the spectrum of light that we see, Knutson said. This can help scientists tell which chemicals are present in the atmosphere.

Then, astronomers could watch as the planet moves around the back side of its star as seen from our perspective. This is called a secondary eclipse, and it helps astronomers see which wavelengths of light the planet is emitting. Like the absorption spectrum, this emission spectrum helps scientists pinpoint the chemicals that exist in a planet's atmosphere.

So far, astronomers have done this for about 50 planets, Knutson said. Most of them are short-period 'hot Jupiters,' meaning large gas giants that orbit very close to their stars — not likely homes for life. And there's the rub: it’s difficult to spot these gases in the atmospheres of very distant worlds, especially smaller, potentially life-friendly ones. Oxygen is difficult to see in a spectrum, and while methane may be easier, it would still require an extremely powerful telescope, Cowan said. "It's not clear that we would be able to detect methane with any of the planned missions people are talking about," he said.

The Red Edge

Along with atmospheric components, astronomers may be able to spot life itself — or at least its reflection. All life forms get rid of waste energy they can't use; in plants' case, it’s in the form of reflected ultraviolet photons, which plants cannot use to produce food. When photosynthetic life is abundant, like in a rain forest or a mat of algae, this reflection can be picked up as a slight infrared glow, known as a "red edge." A similar reflection could conceivably show up on an exoplanet, too, Cowan said.

Again, this would be hard to detect at a great distance. That's especially true if an exoplanet has lots of plant life, since whole planet could glow in infrared, and it would be impossible to tell whether plants are responsible. It would be better to find a planet with a vast, dark ocean in one hemisphere, and a massive rain forest in another, Cowan said.

"It's a pretty subtle feature. If you look at Earth from far away, you can detect a red edge if you understand what you’re looking for," he said. "If you happen to be looking at the Amazon rain forest, you're doing great if it's not a cloudy day. But it usually is a cloudy day in the Amazon — that’s why it's a rain forest. So it's basically a crapshoot."

Some researchers theorize that plants also polarize light when they reflect it, like water does — that’s why polarized sunglasses are helpful at the beach. Spectral polarimetry might be another way to detect life, essentially by the glint of light bouncing off the leaves, Cowan said.

Seeing Planets In Living Color

Astronomers recently made the first direct image of an exoplanet, and found it was a lovely shade of blue. Could they see a green planet, teeming with photosynthetic life? Or better yet, a red one? It's possible, depending on whether photosynthetic organisms evolved around sun-like or red dwarf stars, according to Nancy Kiang, a scientist at NASA’s Goddard Institute for Space Studies.

On Earth, plants use a pigment called chlorophyll A to harvest light from the sun and produce food. It's tuned to the wavelengths of light the sun emits, and it's why plants (for the most part) are green. Kiang is working with a species of algae, discovered on the underside of a sea squirt, that uses chlorophyll D instead. It absorbs light more toward the infrared edge of the spectrum, she said. "It's stretching the wavelength limit for doing oxygenic photosynthesis," she told DVICE. "It's not too unlike what you might find on the surface of planets that you might find orbiting red dwarf stars."

In 2007, Kiang and colleagues wrote that alien plants on alien planets could have varying color depending on the brightness of their star, adapting their photosynthetic pigments to whatever light is available. For this reason, future telescopes should be designed to capture the whole range of light waves a plant could conceivably use for photosynthesis, Kiang said. Meanwhile, she is trying to characterize the limits of photosynthesis for the life forms we have here on Earth.

If red plants absorb infrared light, their host planet may have plants, but no red edge — so how would astronomers know what they’re seeing? "That's the golden question," Kiang said. "If we see a pigment signature, how do we know it is a pigment signature and not rocks fooling us?"

Far from listening for intelligent extraterrestrials trying to communicate with us directly, detecting alien life on other planets will involve heavy amounts of data processing, Kiang said. "For me, it would be nice if we could say if any surface signature has signs of photosynthesis. That's still an area of research."

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