A new study has found that the Earth’s early atmosphere was very, very different from what it is today.

Scientists studying bubbles in ancient Australian lava have found that the atmosphere would have been half as thick eons ago, contradicting long-held beliefs that the Earth’s atmosphere was always this thick, according to a University of Washington statement.

It’s a big discovery because it greatly expands the list of planets that could at some point support life, as it shows that a planet doesn’t have to start out with a thick atmosphere to allow life to grow.

The era would have been 2.7 billion years ago when the Earth was spinning at a much faster rate and the moon was raising higher tides than it does today. The atmosphere was much different, and yet it was quite alive, scientists said — this depsite the fact that it would have been exposed to much more ultraviolet light as it lacked an ozone layer.

Scientists were looking into the issue due to the “faint young sun” paradox, which astronomers Carl Sagan and George Mullen first raised decades ago noting that the host star should have been much fainter billions of years ago resulting in an Earth covered with glaciers. But the Earth was not that cold, so scientists were trying to figure out what caused the Earth to stay so warm. Many theories had been proposed, but this study puts forward a new hypothesis: that the Earth had a thin atmosphere of mostly nitrogen and the pressure was probably half the pressure today.

Normally, this results in a colder planet because it can’t trap heat as well, but the lower pressure would have meant a higher concentration of greenhouse gases, leading to a balmy Earth.

“For the longest time, people have been thinking the atmospheric pressure might have been higher back then, because the sun was fainter,” lead author Sanjoy Som, who did the work as part of his UW doctorate in Earth and space sciences, said in the statement. “Our result is the opposite of what we were expecting.”