Life out there - Is it slime or ET?

This image, released on November 12, 1996, by the Jet Propulsion Laboratory in Pasadena shows Jupiter's ice-covered moon, Europa. Every new discovery makes it seem more likely that we are not alone

THE search for extraterrestrial life heats up as scientists discover a microbe that lives on arsenic, expanding our knowledge of life. Some say the probability of alien life is greater than ever. Seth Boorstein calls home.

Lately, a handful of new discoveries make it seem more likely that we are not alone - that there is life somewhere else in the universe.

In the past several days, scientists have reported there are three times as many stars as they previously thought. Another group of researchers discovered a microbe can live on arsenic, expanding our understanding of how life can thrive under the harshest environments. And earlier this year, astronomers for the first time said they'd found a potentially habitable planet.

"The evidence is just getting stronger and stronger," says Carl Pilcher, director of NASA's Astrobiology Institute, which studies the origins, evolution and possibilities of life in the universe. "I think anybody looking at this evidence is going to say, 'There's got to be life out there'."

A caveat: Since much of this research is new, scientists are still debating how solid the conclusions are.

Another reason to not get too excited is that the search for life starts small - microscopically small - and then looks to evolution for more. The first signs of life elsewhere are more likely to be closer to slime mold than to ET. It can evolve from there.

Scientists have an equation that calculates the odds of civilized life on another planet. But much of it includes factors that are pure guesswork on less-than-astronomical factors, such as the likelihood of the evolution of intelligence and how long civilizations last. Stripped to its simplistic core - with the requirement for intelligence and civilization removed - the calculations hinge on two basic factors: How many places out there can support life? And how hard is it for life to take root?

What earlier this month's findings did was both to increase the number of potential homes for life and broaden the definition of what life is. That means the probability for alien life is higher than ever before, agree 10 scientists interviewed by The Associated Press.

Seth Shostak, senior astronomer at the SETI Institute in California, ticks off the astronomical findings about planet abundance and earthbound discoveries about life's hardiness. "All of these have gone in the direction of encouraging life out there and they didn't have to."

Scientists who looked for life were once dismissed as working on the fringes of science. Now, Shostak says, it's the other way around. He says that given the mounting evidence, to believe now that Earth is the only place harboring life is essentially like believing in miracles. "And astronomers tend not to believe in miracles."

Astronomers, however, do believe in proof. They don't have proof of life yet. There's no green alien or even a bacterium that scientists can point to and say it's alive and alien. Even that arsenic-munching microbe discovered in Mono Lake in California isn't truly alien. It was manipulated in the lab.

But, says NASA astrobiologist Chris McKay, who has worked on searches for life on Mars and extreme places on Earth, "There are real things we can point to and show that being optimistic about life elsewhere is not silly."

First, there's the basic question of where such life might exist. Until a few years ago, astronomers thought life was only likely to be found on or around planets circling stars like our sun. So that's where the search of life focused - on stars like ours.

That left out the universe's most common stars: red dwarfs, which are smaller than our sun and dimmer. Up to 90 percent of the stars in the universe are red dwarf stars. And astronomers assumed planets circling them would be devoid of life.

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