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What if an alien civilization was a billion or more years older than ours and detected Earth in its early history? “For billions of years of Earth history, an alien astronomer may have even been sufficiently misled to conclude that Earth was sterile—despite the fact that life was flourishing in our ocean at the time,” says Stephanie Olson, an astrobiologist at the University of California, Riverside with NASA’s Astrobiology Institite.

In a 2018 paper in Science Advances, Olson and her colleagues simulated how Earth’s atmosphere has changed over time. Even three billion years ago, aliens may have been able to infer life by sniffing out methane and carbon dioxide in the early atmosphere. But our modern atmosphere—a literal beacon for life—arrived only about 500 million years ago.

“The phrase Earth-like does not refer to a planet that necessarily resembles modern-day Earth at all,” explains Olson, an astrobiologist at the University of California, Riverside. “It’s actually a very broad term that encompasses a broad variety of worlds. It includes hazy worlds like the Archean; it includes icy worlds like the ‘snowball Earth’ intervals; it includes anoxic worlds with exclusively microbial ecosystems; it includes worlds with complex and intelligent life; and it includes worlds that we haven’t even seen yet.”

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