• When searching for exoplanets with potentially habitable atmospheres, astronomers really only have Earth as an example of what to look for. But a new study says that a past era of Earth may be a better indicator of complex life.
• Because of high concentrations of particular biosignatures, finding worlds similar to Earth during the Mesozoic Era—when dinosaurs roamed the Earth—would be much easier.
• This data will come in handy as missions like the James Webb Space Telescope attempt to piece together which atmospheres signal potentially habitable exoplanets.

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When astronomers search for worlds that could possibly host life, they’re operating off of just one example: Earth. So far as we can tell, our specific cocktail of nitrogen, oxygen, and other trace gasses is the only atmosphere capable of supporting life. So, when we go looking for that ever-elusive “second Earth,” we tend to search for the only gaseous combo that we know for sure can produce life-creating results.

But according to new research from Cornell University, the Earth of today may not be the best template for finding habitable planets. That accolade belongs to the high-oxygen Earth that existed during the Mesozoic Era and gave rise to the age of dinosaurs.

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Using estimates from two established climate models (GEOCARB and COPSE), scientists determined that telescopes could more easily detect the light fingerprint of “Jurassic Worlds.” This is due to the presence of higher concentrations of two pairs of biosignatures—ozone/methane and oxygen/methane—in Earth’s atmosphere around 100-300 million years ago. The results of the study were recently published in the journal Monthly Notices of the Royal Astronomical Society: Letters.

“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,” Lisa Kaltenegger, director of the Carl Sagan Institute and associate professor of astronomy at Cornell University, said in a press statement. “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.”

To determine when exactly Earth would have been most detectable by telescope, scientists analyzed atmospheric composition—as well as its resulting transmission spectra—over the entire Phanerozoic eon. This is the 500 million year timeframe (that we’re still in) that represents the only period of Earth’s history to ever support complex life.

The key ingredient is oxygen, which has existed within a range that scientists call a “fire window” (from 16 percent to 35 percent concentration) for the past 400 million years. If oxygen concentration dipped below that window, it’d never ignite, and if concentrations exceeded that window—well, fires would never go out (which, believe it or not, is bad news for life).

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Other theories suggest that the rise in dinosaurs coincided with the peak of oxygenation in the atmosphere some 300 million years ago. As a result, surveys of distant planets with similar fingerprints have a higher chance of supporting some sort of complex life.

“The Phanerozoic is just the most recent 12% or so of Earth’s history, but it encompasses nearly all of the time in which life was more complex than microbes and sponges,” research associate and lead author Rebecca Payne said in a press statement. “These light fingerprints are what you’d search for elsewhere, if you were looking for something more advanced than a single-celled organism.”

Understanding these “loud” atmospheric fingerprints will come in handy as missions like the James Webb Space Telescope use their capabilities to piece together the gaseous compositions of potentially habitable planets. While dreams of a space T. rex remain frustratingly out of reach at the moment, scientists at least have a better idea of how to sniff out complex life in its prime.