Tim Slater, associate professor of astronomy at the University of Arizona in Tucson, said it begins by understanding the life right here on Earth.

Slater spoke to about 200 students in the Love Library auditorium Thursday night.

In the lecture, titled "The Search for Life," he explained that knowing the conditions under which life can exist on our own planet will take scientists a long way toward deciding which planets outside our solar system may contain life.

The single largest indicator of life on a planet is the presence of water, Slater said.

"It's the magic connection. If you don't have water, you can't have life," he said. "It doesn't even have to be a large body of water, but there must be some."

Scientists have discovered if water is present, life can exist even in very extreme conditions, he said.

At the North and South poles, for example, bacteria can thrive on the snow's surface, as well as deep in frozen ice, surviving on ice that melts every few years, he said.

And ocean hot spots, which spew volcanic material from vents on the ocean floor, are inhabited by healthy colonies of heat-loving bacteria, proof that life can emerge without the presence of light, he said.

Bacteria, known to be the first life on Earth, are important starting points for other forms of life because they produce nitrogen and oxygen, and have the potential to completely change the nature of the atmosphere, Slater said.

"It's (humans) who have a low tolerance for extreme settings," he said.

Mars has been the focus of many scientists interested in whether life exists on other planets, he said, particularly because of convincing evidence water once flowed there.

But now, the atmospheric pressure on the planet is too low to prevent water from evaporating, he said, though water may still exist below the surface.

While many planets are too close to the sun to support water and thus life, he said, many have moons that have the right ingredients to create life.

Europa, one of Jupiter's moons, is covered with what some scientists think is a mile-thick layer of ice, under which lie oceans of water, he said.

The Galileo space probe has taken many pictures of the moon, but the probe's mission will end Sunday when it crashes into Jupiter.

Scientists decided to crash the probe into Jupiter instead of Europa to avoid contaminating the moon with bacteria from Earth, he said.

"When we visit Europa, we want to be sure that any life we find originated there and not on a space probe that wasn't cleaned properly," he said.

People have been thinking about whether life exists on other planets for more than 100 years, Slater said, but the field of astrobiology is relatively young with only a few programs throughout the country.

The field is interdisciplinary, combining areas such as chemistry, physics, astronomy, biology, geology and philosophy, he said.

Lindsay Coppock, a freshman general studies major, said the talk didn't make her want to change her major, but it did make her think about the possibility of life on other planets.

"It makes you wonder what life is evolving to," Coppock said.

Kevin Lee, a research assistant professor in physics and astronomy at UNL, said the talk would help students put current astrobiology research into context.

"Just this morning there was an article in (the paper) about crashing Galileo," Lee said. "This gives them a perspective to interpret the significance of that."