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 moon is open for business.

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.