Our Earth is a rare oasis in space and time, said Don Brownlee, University of Washington professor of astronomy and this year's Thomas Gold lecturer, in a March 31 public talk. If our planet were closer or farther from the sun, if it were covered by more or fewer oceans or had formed earlier or later in the history of the universe -- or if one of myriad other factors were different -- we may never have evolved.

Brownlee's lecture, "Is Earth a Rare Planet?" in Rockefeller Hall's Schwartz Auditorium, summarized current thinking by scientists about the prevalence of life in the universe. He has previously written about these ideas with paleontologist Peter Ward in the best-selling books "Rare Earth: Why Complex Life Is Uncommon in the Universe" and "The Life and Death of Planet Earth."

Brownlee described the two most widely held conceptions of the rarity of Earth: that Earth-like planets are common because life is the purpose of the universe, and that Earth is unique to the universe because it was made expressly for us. "I don't think either of these is right," he said.

The universe has 10²² planets, Brownlee estimated. That makes for a lot of possible Earths with chances for life to emerge, but animal life forms are probably exceedingly rare because they require special environments, and they are also likely too far away for us to contact.

"If there are no animals on planets around the closest thousand stars, we may never learn much about life," Brownlee said.

Over the past decades, humans have spent billions and billions of dollars studying Mars, our next-door neighbor, but we still can't say whether life does -- or ever did -- exist on the red planet. "If there is life on Mars, it is microbial and underground," Brownlee said.

Unlike animals, "microbial life is easy to form and hard to make extinct," he said. So think little green algae, not little green men.

If animal-like life is rare and distant, humans will probably never interact with an intelligent alien race. "That means there's no 'Star Wars' cantina," Brownlee said.

Not only is it rare for us to have evolved at all, but it will also be rare for us to survive another few million years -- 99 percent of all species that have lived on Earth have gone extinct during catastrophic events.

"Without interdiction, all intelligent life on planets will be done in by impacts," Brownlee said. Our survival will depend only on our technology, he argued. Specifically, it will depend on our capability to deflect any asteroid that comes hurtling toward Earth.

Technology, however, cannot save our planet from its inevitable fate in several billion years when the swelling, dying red giant sun will scorch and envelop the Earth.

"Some people get really depressed about that, but I don't. Just remember that we live in the best place in all the entire universe, and it's going to be this way for a really, really long period of time compared to anything you've ever read about in human history," Brownlee said.

The Thomas Gold Lecture Series is sponsored by the astronomy department and the College of Arts and Sciences in honor of Thomas Gold, the late Cornell professor of astronomy.

Graduate student Melissa Rice is a writer intern at the Cornell Chronicle.