Over the past two decades, thousands of exoplanets—worlds that orbit other stars—have been discovered. Some of these alien places could potentially host life, spurring scientists to develop sophisticated strategies to detect “biosignatures,” or signs of life, over the vast cosmic distances between our Sun and other stars.
Astronomer Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell University, is helping to lead this effort to streamline the search for extraterrestrial creatures on known exoplanets. “The idea is basically, among these plentiful options, how can we identify the ones that are the best to look at for biosignatures, in the air as well as on the surface?” she told me in a phone call. “Our work is trying to figure out, with theoretical studies, the couple of cases that will give us the best shot of finding signs of life out there.”
Kaltenegger, along with fellow Cornell astronomer Jack T. O'Malley-James, explore this question through the lens of extraterrestrial vegetation in a new paper published in Astrobiology. The research focuses on the vegetation red edge (VRE), which is the high infrared reflectivity observed in Earth’s plants—a biosignature that could be discerned from space if a planet is covered with enough photosynthetic life.
The team uses Earth’s history of vegetation as a “Rosetta Stone” or an “archive,” in Kaltenegger’s words. “The only thing we can do is use what we know—the Earth—as a template,” she said. “So far people have used the Earth as it is right now, but our planet has gone through an amazing history.”
By modeling the evolution of plants over the past half-billion years on Earth, the team found that VRE signals grew stronger over time, as complexifying plant species colonized the world. If an alien civilization had observed Earth during the Cambrian period 500 million years ago, when mosses were the primary vegetation on the planet, it would have been much more difficult to detect a VRE signature than it is today.
These VRE signals are likely to become even more pronounced as Earth ages and warms, developing into either an arid planet of cacti (which have very strong VRE signatures) or perhaps a lush tropical biome. For this reason, Kaltenegger and O'Malley-James suggest that the best exoplanets to study for signs of life are “older Earth analogues” that may be either “hot jungle worlds” or “hot arid desert worlds,” according to the paper.
While it would be ideal to find such an exoplanet around a Sunlike star, most known Earth-scale worlds orbit low-mass dwarf stars. These stars, which are typically five to ten times less massive than the Sun, are plentiful, and theoretically could nourish photosynthetic life. But whether vegetation would evolve at the same clip in a dimmer star system, or if it would have an Earth-analogous VRE signature, is still an open question.
The VRE study is the latest of several projects Kaltenegger has organized to cumulatively build a “database of spectral fingerprints for habitable planets,” she said. She co-authored another Astrobiology paper, published in July, that showed how albedos, spectra, and colors of solar system bodies could act as blueprints for classifying exoplanets.
The overarching idea is to create an exhaustive catalog of known planetary features in time for the refined data expected from the next generation of planet-hunting observatories, like the James Webb Space Telescope or the European Extremely Large Telescope. These observatories will be exponentially more capable of pinpointing biosignatures, so scientists will need to be primed to read the subtle exoplanetary clues that could revolutionize our place in the cosmos.
“Once we figure out, or I hope we figure out, that we are not alone in the universe, the connection to the cosmos is just going to be even stronger,” Kaltenegger said. “We are wishing that there is something out there, but that we live in the time when we have the technology to figure it out—I think that’s amazing.”
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