Want to Find Alien Life? Look at Older, Hotter Earths.

If alien astronomers are out there searching for signs of life on Earth, they might just find it in the telltale pattern of light reflected by our plants, from redwood forests to desert cacti to grass-covered plains. That reflected fingerprint has been visible since vegetation first began carpeting our rocky terrestrial landscape about half a billion years ago. And as Earth aged and evolution marched onward, the reflected signal strengthened.

Now, two astronomers are suggesting that plants could leave similar fingerprint-like patterns on distant exoplanets, and perhaps the first signs of life beyond our solar system could come from light reflected by forests covering an alien moon like Endor or cacti living in Tatooine’s deserts.

“We’re trying to figure out—with all the planets we’re finding—what are the signatures that could indicate habitability?” says Cornell University’s Lisa Kaltenegger, who recently described Earth’s leafy signature in a study published in the journal Astrobiology.

“We really want to identify the handful, or two or three, that give us the best chance to pick up signs of life.” (How would people react to finding alien life? Science says, it’s complicated.)

While this isn’t the first time scientists have suggested looking for life in a far-off planet’s light, Kaltenegger’s team adds a twist: Such reflections can also offer a good estimate for an alien planet’s evolutionary advancement, based on our knowledge of how things work on Earth.

“This idea that you could find vegetation on another planet has been around. But nobody ever used Earth’s own geological history as an archive,” Kaltenegger says. “We don’t have a second planet with habitability, but we do have our Earth through time, and it would be really smart to study it.”

Finding Signs of Life … on Earth

Several decades ago, the Galileo spacecraft, which was headed for Jupiter, swiveled to stare at Earth’s reflected light. It spied the signs of biology at work in the presence of atmospheric gases such as ozone and methane. More recently, astronomers have teased apart Earthshine, or the bit of Earth-light that sometimes dimly illuminates the darker part of a crescent moon’s face. They found life’s fingerprints there, too.

Now, scientists searching for life beyond Earth are debating how biology might leave molecular marks in alien atmospheres, either by producing particular compounds or by shifting the mix of gases swaddling a planet.

“In recent years, the possible remote detection of biosignatures from exoplanets has become one of the most exigent—but also most challenging—problems in modern astrophysics,” says Michael Sterzik of the European Southern Observatory.

The signature Kaltenegger is working with is a bit different, though. For reasons that aren’t exactly clear, photosynthetic plants reflect specific wavelengths of infrared light, with some plants being more reflective than others. Called the vegetation red edge, this pattern is visible in near-infrared wavelengths, which are a bit longer than the colors our eyes perceive yet are easily detected by the right kind of telescope.

Detecting the infrared signature depends on a number of factors, including how much of a planet’s surface is covered by vegetation, whether the planet is warmer or cooler, what the cloud cover is like, and the sensitivity of the telescope being used. (On some alien worlds, trees might even bloom black.)

Sterzik, who looked for this very signature in Earthshine in 2012, notes that it’s quite difficult to spot. Even the Galileo spacecraft could barely see it, and it was much closer to Earth than the nearest alien telescope is likely to be. Today’s telescopes aren’t up to the task of finding such a signature on exoplanets. But space- and ground-based observatories that are currently in development, such as the Extremely Large Telescope, could do the trick.

“It’s super doable. It’s hard, and it’s a small signature, but it’s there,” Kaltenegger says.

Searching the Ancient Past

As part of their work, Kaltenegger and Cornell’s Jack O’Malley-James wanted to find out how long the fingerprints of vegetation on Earth have been visible.

Based on the fossil record, scientists know that the first mosses began creeping across Earth’s barren surface around 500 million years ago, capturing sunlight and turning it into energy. Eventually, those primordial plants diversified and erupted into a riotous mix of ferns, trees, and flowers that, at times, have covered nearly 90 percent of Earth’s land surface.

By simulating the light reflected by plants and placing it within a simulated planetary atmosphere, Kaltenegger and O’Malley-James found that as soon as plants appeared and colored the Earthly landscape, they produced a detectable signature. It was weak at first, but that fingerprint grew stronger over time as plants spread across more of Earth’s surface, although the planet’s total reflectivity during ice ages would have made that burgeoning vegetation signature harder to tease from the mix.

What’s more, the team thinks the signal could get even brighter in the future—depending, of course, on how we take care of our planet. If we end up with a parched, desert landscape dominated by highly reflective cacti, life-forms on Earth will be easy for alien astronomers to spot. But developing a more opaque greenhouse gas-trapping atmosphere means that, among many other consequences, leafy Earth-life will no longer be detectable by curious extraterrestrials.

Narrowing the Candidates

The pair suggests that scanning the light reflected by distant exoworlds for these leafy signals should be one of several tools used to scrutinize the thousands of potentially habitable planets being discovered across the cosmos by missions such as Kepler and TESS.

“Even with big telescopes, it’s going to be incredibly time-consuming to look at these planets in detail, so we’ll have to pick out the best ones,” Kaltenegger says.

Based on this work, she suggests that older, hotter exo-Earths might be the best targets for life-hunting telescopes, because leafy reflected light increases as a planet ages. But she’s quick to point out that this signature alone would not be definitive proof that an alien biosphere exists. Various minerals could mimic the infrared signal, Kaltenegger says, which is why she’d also like to see, for instance, a tantalizing combination of atmospheric gases as well.

If such a signature was found on an alien world, it might then be possible to determine how quickly evolution is at work on that planet. We’d know the planet’s age, based on the age of its star, and we’d be able to compare how much vegetation is covering its surface relative to the age at which a comparable amount blanketed Earth.

“There’s a lot of discussion about what sets the evolutionary speed on a planet,” Kaltenegger says, “and we only have one example: us.”