In the search for life in the Universe, Earth provides a template of evolution for the one habitable planet we know. Earth’s atmospheric composition has changed significantly throughout its history. The last 500 million years — the Phanerozoic Eon, which includes the origins of animals, dinosaurs, and land plants — saw oxygen rise from around 10% to 35%. But the resulting transmission spectra are a crucial missing piece in our search for signs of life in exoplanet atmospheres. In a new study, astronomers modeled the atmospheric composition and transmission spectra of five stages in Earth’s Phanerozoic Eon. Two key biosignature pairs — oxygen and methane, and ozone and methane — appeared stronger in models of Earth roughly 100 million to 300 million years ago, when oxygen levels were significantly higher.
An artist’s impression of the Gliese 3470 planetary system. Image credit: Sci.News.
“Modern Earth’s light fingerprint has been our template for identifying potentially habitable planets, but there was a time when this fingerprint was even more pronounced — better at showing signs of life,” said Dr. Lisa Kaltenegger, director of the Carl Sagan Institute.
“This gives us hope that it might be just a little bit easier to find signs of life — even large, complex life — elsewhere in the cosmos.”
Using estimates from two established climate models (GEOCARB and COPSE), Dr. Kaltenegger and Cornell University astronomer Rebecca Payne simulated Earth’s atmospheric composition and resulting transmission spectra over five stages (approximating 500, 400, 300, 200, and 100 million years) of the Phanerozoic Eon.
Each features significant changes as a complex ocean biosphere diversified, forests proliferated and terrestrial biospheres flourished, influencing the mix of oxygen and other gasses in the atmosphere.
“It’s only the most recent 12% or so of Earth’s history, but it encompasses pretty much all of the time in which life was more complex than sponges,” Dr. Payne said.
“These light fingerprints are what you’d search for elsewhere, if you were looking for something more advanced than a single-celled organism.”
R.C. Payne & L. Kaltenegger found that telescopes could more easily detect an exoplanet with higher levels of atmospheric oxygen than modern Earth, as existed during the dinosaur age. Image credit: Rebecca Payne / Carl Sagan Institute.
While similar evolutionary processes may or may not unfold on exoplanets, the team’s models fill in a missing puzzle piece of what a Phanerozoic would look like to a telescope, creating new templates for habitable planets with varying atmospheric oxygen levels.
The authors pioneered modeling of what Earth would look like to faraway observers based on changes over time in its geology, climate and atmosphere — our ground truth for identifying potential evidence of life on other worlds.
“To date, about 35 rocky exoplanets have been discovered in habitable zones where liquid water could exist,” Dr. Kaltenegger said.
“Analyzing an exoplanet’s atmosphere — if it has one — is at the edge of technical capability for the NASA/ESA/CSA James Webb Space Telescope but is now a possibility. But scientists need to know what to look for.”
If an industrial civilization had existed on Earth many millions of years prior to human era, what traces would it have left and would they be detectable today? Image credit: Michael Osadciw, University of Rochester.
The models identify planets like Phanerozoic Earth as the most promising targets for finding life in the cosmos.
They also allow scientists to entertain the possibility — purely theoretical — that if a habitable exoplanet is discovered to have an atmosphere with 30% oxygen, life there might not be limited to microbes, but could include creatures as large and varied as the megalosauruses or microraptors that once roamed Earth.
“If they’re out there, this sort of analysis lets us figure out where they could be living,” Dr. Payne said.
“Dinosaurs or not, the models confirm that from a great distance, such a planet’s light fingerprint would stand out more than a modern Earth’s.”
The paper was published in the Monthly Notices of the Royal Astronomical Society: Letters.
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R.C. Payne & L. Kaltenegger. 2024. Oxygen bounty for Earth-like exoplanets: spectra of Earth through the Phanerozoic. MNRASL 527 (1): 151-155; doi: 10.1093/mnrasl/slad147
