Using nature's color palette from early Earth, Cornell University astronomers have created a cosmic "cheat sheet" in order to understand where discovered exoplanets may fall along their own evolutionary spectrum.

Jack O'Malley-James, a research associate at the Carl Sagan Institute, and Lisa Kaltenegger, professor of astronomy and director of the Carl Sagan Institute, co-authored "Expanding the Timeline for Earth's Photosynthetic Red Edge Biosignature" published in the Astrophysical Journal Letters [https://iopscience.iop.org/article/10.3847/2041-8213/ab2769].

"In our search to understand exoplanets, we're using the early Earth and its biological milestones in history as a Rosetta stone," O'Malley-James said. "Scientists can observe surface biosignatures beyond vegetation on Earth-like exoplanets by using our own planet as the key for what to look for."

For the last half-billion years - roughly 10 percent of our planet's lifetime - chlorophyll, present in many familiar forms of plant life such as leaves and lichen, has been the key component in Earth's biosignature.

But other flora, such as cyanobacteria and algae, are much older than land-based vegetation, but their chlorophyll-containing structures leave their own telltale signs on a planet's surface.

"Astronomers had concentrated only on vegetation before, but with a better color palette, researchers can now look beyond a half-billion years and up to 2.5 billion years back on Earth's history to match like periods on exoplanets," Kaltenegger said.

"If an alien had used color to observe if our Earth had life, that alien would see very different colors throughout our planet's history - going back billions of years - when different life forms dominated Earth's surface."

O'Malley-James and Kaltenegger modeled spectra of Earth-like exoplanets with different surface organisms that use chlorophyll.

Lichens (a symbiotic fungal and photosynthetic partnership) may have colonized Earth's land masses some 1.2 billion years ago and would have painted Earth in sage to mint green colors. This coverage would have generated a "nonvegetative" photosynthetic red-edge signature. A red-edge signature is the part of the spectrum that helps keep planets from getting burned by the Sun.

"When we discover an exoplanet, this research gives us a much wider range to look back in time," Kaltenegger said. "We extend the time that we can find surface biota from 500 million years (widespread land vegetation) to about 1 billion years ago with lichen and up to 2 or 3 billion years ago with cyanobacteria."

Research Report: "Expanding the Timeline for Earth's Photosynthetic Red Edge Biosignature"

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