PRINCETON — If you want to imagine that there is extraterrestrial life in the universe, go ahead. Just don't use the same baseless assumptions that science has relied on, so far.

That is the simplest conclusion from a study recently published by two Princeton astrophysicists — one at the Institute for Advanced Study and one at Princeton University.

Scientists, they claim, have been too apt to make assumptions about the likelihood of life elsewhere based on observations about how life developed here on Earth.

Those assumptions are proof of nothing beyond our own globe, the study said, and not a very convincing argument for life elsewhere.

For that, you might need one or two more examples.

“Certainly it seems like a more interesting reality, a universe rich with all sorts of extraterrestrial life,” said Edwin Turner, a professor of astrophysical sciences at Princeton. “There’s nothing in our paper that shows that isn’t true — it’s a subtle point.

“Our paper really just calls into question one of the few pieces of evidence we use for life elsewhere. The fact that life developed on Earth quickly has encouraged people to think that the probability of it happening elsewhere is quite high.

“It’s consistent, but it’s not really strong evidence. In fact there is no evidence.”

David Spiegel, a member of the Institute’s School of Natural Sciences, co-authored the paper with Turner. The study began as a simple conversation between colleagues with a shared love of statistical analysis.

The study was published earlier this spring in the Proceedings of the National Academy of Sciences and was supported by grants from NASA, the National Science Foundation and the Keck Fellowship, as well as a grant from the Japanese Ministry of Education, Science and Technology.

Spiegel and Turner investigated the possibility of alien life by using a Bayesian analysis, which uses a mathematical formula to contrast actual data with assumptions about that data.

Once they took away those assumptions, the authors found very little evidence to go on that extraterrestrial life exists.

“Life showed up fairly early on Earth, so it must be common through the galaxy — that is sort of a general way of thinking and the way scientists have proceeded in some segments of the astrobiology community. But it’s just not that conclusive,” says Spiegel.

It would be better, he says, if scientists had a clearer concept of abiogenesis — or, the emergence of life — here on Earth. Or if chemistry and biology were advanced enough to detect a “shadow life” here, a species with a completely separate origin about which larger conclusions could be drawn.

“Our analysis suggests that abiogenesis could be a rather rapid and probable process for other worlds, but it also cannot rule out that abiogenesis is a rare, improbable event,” Spiegel said. “We really have no idea, even to within orders of magnitude, how probable abiogenesis is.”

Turner, 62, has a Ph.D. in astronomy from the California Institute of Technology. He joined Princeton’s faculty in 1978. He has carried out extensive astronomical observations throughout the world. He currently serves as chair of the Board of Governors of the Astrophysical Research Consortium.

Spiegel, 34, met Turner as a post-doctoral researcher at Princeton before moving over to the Institute. His work focuses primarily on exoplanets — planets outside our own solar system — and questions about life elsewhere in the universe.

In recent decades, scientists have discovered in excess of 2,000 planets orbiting stars outside our solar system. In fact in December NASA announced the first observation of a planet just 600 light years from Earth that is orbiting the habitable zone of a Sun-like star.

These discoveries fuel optimism that scientists will detect life on other planets in short order.

Even Turner and Spiegel remain hopeful.

“I turn 63 next week,” says Dr. Turner. “I wouldn’t confidently forecast that we’ll know in my lifetime, but it doesn’t seem like it’s hundreds of years away. It seems more like it might be tens of years away.”

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