When Lisa Kaltenegger first saw the Grand Prismatic Spring in Yellowstone National Park, its azure valentine-shaped pool surrounded by lurid, fiery yellow-orange tendrils, stunned her. As an astronomer who studies other worlds, Kaltenegger understood better than most people here on Earth that the kinds of bacteria that thrive in hot springs like the ones at Yellowstone tend to have striking coloration. "I knew it before, logically," Kaltenegger says. "But seeing it was different."
So like any good scientist, Kaltenegger did a little reading. It turned out that different colored bacteria preferred different temperatures. And that gave her an idea. If an entire planet were covered in microbes…whatever conditions they lived under would affect their color. If life exists on other planets, it's probably not as toothy xenomorphs or doe-eyed wanderers. It's probably germs---bacteria like those that dominated Earth for the majority of its history. With new space and ground-based telescopes set to begin exploring alien worlds in the next few years, scientists are poised—for the first time ever—to identify signs of alien life. Kaltenegger realized that they could do it by looking for pretty colors.
When astronomers characterize stars, they often use a system called a color-color diagram. Basically it's a chart that maps the brightness of the star as seen in two different wavelengths, through a telescope. Measured this way, stars tend to cluster into groups based on things like how old they are. Kaltenegger, director of the Institute for Pale Blue dots at Cornell University, teamed up with her student at the time, an astronomer named Siddarth Hegde who's now also at Cornell, to try the same thing with bacteria. They took five species, each of them tough enough or weird enough to potentially exist on another planet (hypothetically) and looked at them with visible and near-infrared light. Then they plotted a color-color dagram.
It turned out that the measurements formed a diagonal line—suggesting that other bacteria might fall on the same graph. Now, in a study published today, they expand those results to 137 microbial species, a catalog of colors that, if you see them glinting off a planet outside our solar system, might suggest bacteria on the surface. "So far we haven't found anything that falls outside the line," Kaltnegger says. And since actually looking directly at an exoplanet isn't easy right now, these kind of signatures might be the first sign anyone sees of ET.
The pattern is far from a definitive signature of life, but it's a way to prioritize which planets to focus on. “It’s not that this planet has life,” Kaltenegger explains. “It’s that this planet is more interesting than that other one.” That kind of prioritization is going to be crucial, because astronomers expect that they're about to discover lots more planets. The Transiting Exoplanet Survey Satellite, set for launch in 2017, aims to find dozens---maybe hundreds---of Earth-sized planets, all orbiting stars at a distance that would allow for liquid water. That "habitable zone" makes it at least plausible that life could exist on their surfaces.
Meanwhile, the next generation of planet-bound telescopes will actually be able to see the kinds of colors that Kaltenegger and Hegde's catalog covers. The Giant Magellan Telescope will start observing in 2021. The Thirty Meter Telescope in Hawaii will also be finished in a few years. And if one of these telescopes finds a planet promising for life, then astronomers can zoom in with the James Webb Space Telescope, which is scheduled for launch in 2018. JWST will be able to detect gases in the atmosphere—including the byproducts of processes unique to living things.
Obviously, you'd want to find those molecules as relatively sure signs of life. But some Earthly organisms don't produce any detectable gases at all. One type of such microbes, called anoxygenic phototrophs, flourished on Earth for a billion years. They could be similar to what astronomers might see on other planets. “You start with looking for what you know,” says Niki Parenteau, a researcher at the SETI Institute and NASA Ames Research Center. “Because these are very simple phototrophs that can inhabit a variety of niches, it seems reasonable to search for them as a starting point.” Conveniently, one of the phototrophs is purple.
In fact, Esther Sanromá, an astronomer at the Astrophysical Institute of the Canary Islands, used a computer model to see if they were purple enough. She built a computer model to calculate whether an Earth-like planet covered in the bugs would look purple to a telescope. Parenteau is also measuring the colors of other anoxygenic phototrophs, which span a spectrum of greens, yellows, and reds. “Instead of a purple exoplanet,” she says, “you have a rainbow sherbet exoplanet.”
Of course, whether or not astronomers can see such colors will depend on how much land or water covers the planet, and how thick its clouds are. Sanromá found that if purple microbes only lived in oceans, a telescope wouldn't be able to detect them. But the galaxy is filled with all kinds of planets with different landscapes and seascapes. "I think there's a big diversity of worlds out there," Kaltenegger says. And each could be home to a rainbow of lifeforms.
