For any potential ETs looking for their own alien life forms, Earth would have attracted more attention during the time of the dinosaurs than it does today.
Cornell University astrobiologist Rebecca Payne and astronomer Lisa Kaltenegger modeled the last 540 million years of Earth’s history—the Phanerozoic Eon—to see how different biomarkers used to detect signs of life at great distances might have changed. They found that two key pairs of biomarkers would have been stronger about 100 to 300 million years ago: oxygen and methane, and ozone and methane. This is largely due to the proliferation of vegetation that provided significantly higher levels of oxygen to the atmosphere at that time. This means that any hypothetical alien telescopes looking for light fingerprints (or transmission spectra) could have spotted our planet much more easily during the Jurassic era than they do today. “The light fingerprint of modern Earth has long been our model for identifying potentially habitable planets, but there was a time when this fingerprint was even more pronounced—the better for showing signs of life,” Kaltenegger says.
“This gives us hope that it might be a little easier to find signs of life – even large, complex life – elsewhere in the cosmos.” All sorts of factors influence the amount of oxygen in the atmosphere, from the level of forest cover on land to the different types of marine species in the ocean to the prevailing weather patterns. For much of the past 400 million years, atmospheric oxygen levels are thought to have been in the range of 16 to 35 percent. This is known as the “fire window”: if it were to drop, fires wouldn’t start, and they would be very difficult to put out. “The Phanerozoic is only the most recent 12 percent of Earth’s history, but it encompasses almost the entire time when life was more complex than microbes and sponges,” Payne says.
Astronomers are also scouring the rest of the universe for the same glowing fingerprints, looking for signs of an atmosphere that could support a form of life we might be familiar with. Now that we know what a Phanerozoic planet might look like, that search can be more precise. With advanced technology aboard the James Webb Space Telescope (JWST), scientists now have the ability to analyze exoplanet atmospheres, though we still can’t be sure that life would evolve in the same way it did on Earth. “We hope to find some planets that have more oxygen than Earth right now, because that will make the search for life a little bit easier,” Kaltenegger says. “And who knows, maybe there are other dinosaurs out there waiting to be found.”
