A New Clue in the Search for Forests on Distant Planets

To find signs of plant life on other worlds, it helps to understand the history of our own.

An aerial view of a river and a forest
An aerial view of vegetation on Earth (Goddard Space Flight Center / NASA)

Astronomers remotely detected signs of life on a planet for the first time in history in December 1990. “The Galileo spacecraft found evidence of abundant gaseous oxygen, a widely distributed surface pigment with a sharp absorption edge in the red part of the visible spectrum, and atmospheric methane in extreme thermodynamic disequilibrium,” astronomers wrote in a paper in Nature. “Together, these are strongly suggestive of life.”

It wasn’t exactly a major discovery. The planet in question was Earth; at the urging of Carl Sagan, the Galileo spacecraft, on its way to study Jupiter, had pointed its instruments home to determine whether it was possible to observe life on our planet from space.

The results were significant nonetheless because they presented a tantalizing possibility. Galileo’s detection of a “sharp absorption edge in the red part of the visible spectrum,” translated into layman’s terms, means that the spacecraft had picked up the distinct signature of vegetation. When photosynthetic plants are exposed to sunlight, they absorb the frequencies they need for energy, which happen to be the ones we see as red, and reflect others. That’s why vegetation appears green to us. Astronomers can use this phenomenon as a signal to determine the presence of plant life.

Since a spacecraft has successfully identified this signature on Earth, perhaps, with more advanced technology, it could do the same for planets beyond our solar system. Perhaps someday, astronomers could report the discovery of alien flora on another world.

In the years since the Galileo experiment, astronomers have continued to point to the signature of vegetation as a worthwhile clue in the search for extraterrestrial life. And the possibility has only become more pressing. At the time of Galileo’s flyby, only the planets in our own solar system were known to us, and we were sure none of them had forests or grasslands or swamps. The first exoplanet was discovered five years later, and today, there are more than 2,300 confirmed exoplanets. Thirty of them are about the size of Earth and orbit in their star systems’ habitable zones. The thought that some of them may have plant life doesn’t seem as unfathomable as it once did.

Existing telescopes aren’t powerful enough to study the light coming from these worlds in detail, and new ones are under construction and several years away from operation. In the meantime, astronomers continue to hone their strategies for finding alien plant life. For some of them, the best clues can be found right here on Earth.

“We really should look at our planet like a Rosetta Stone,” says Lisa Kaltenegger, the director of the Carl Sagan Institute at Cornell. “We are on the verge of figuring this out, and we need archival data on our planet to be able to spot it somewhere else.”

Kaltenegger and Jack O’Malley-James, a fellow astronomer at the institute, recently used computer simulations to study the spread of vegetation across Earth over the past 500 million years. Over this time period, the landscape shifted from simple mosses and liverworts, to leafy ferns, and, finally, to mature plant forms, including trees.

The astronomers found that as time went on, and as plant life became more widespread and diverse, the signal for vegetation strengthened. “Of course, the whole signal gets modulated by how much snow and ice you have,” Kaltenegger adds. But what this means, she says, is that some of the best candidates for the search for life may be older, rather than younger, Earthlike exoplanets.

It also means that if there are alien astronomers out there with interests that overlap with humanity’s and equipped with very powerful instruments, they could have found signs of flora on Earth as early as 500 million years ago. The task would be easier now than all those years ago; the signal given off by early mosses is weaker than that of present-day trees, Kaltenegger says. But it would still be a challenge. The Galileo spacecraft collected data from only 600 miles away, not millions of light-years. And Earth’s overall vegetation signal is actually quite weak, Kaltenegger says, thanks to all our oceans.

There’s one important caveat in all this. Kaltenegger and O’Malley-James’s research assumes, as have previous studies on using vegetation signatures to find life, that other worlds look like our own. Plant life on Earth evolved in response to our sun’s particular properties. Plants on other planets—if they even exist—would have arisen in response to their own stars, which could be much brighter or dimmer than ours.

These scenarios beckon imagination over empirical evidence.“Will the light they absorb be red, or a different color? Will the light they reflect be green or something completely bizarre, like magenta or bright blue?” Nicholos Wethington, a science writer at Universe Today, wondered more than a decade ago. We can still only wonder about the looks of alien plants, but the technology that could help us find out more is on its way. With the new research from Kaltenegger and O’Malley-James, astronomers may better know where to look for the answers.