The list of candidates in our solar system most likely to harbor life or show signs of past life has narrowed in recent months. A hot debate now rages, inside NASA and throughout the science world, over where and how best to conduct the hunt.

Uniquely human, we cannot agree on how to answer the biggest questions in life.

So SPACE.com posed a handful of tough questions to three leading astrobiology experts, each of them in the thick of the debate. The answers are more varied than we expected, and they illustrate both how simple and how complicated it will be to conduct the search, and to ultimately find out whether or not we have company in the solar system.

The driving force

Because of the financial and philosophical implications, the search for life has, many researchers agree, become the primary driving force in science. That is certainly the case inside NASA, which is in the driver's seat, most literally, when it comes to deciding when, where and how our species will learn if we have cosmic cousins or astral ancestors.

"It's the prime directive," says John Charles of the Johnson Space Center in Houston. "It has brought a focus to our program unlike any focus since the Apollo days, when the goal was to beat the Russians to the Moon."

And that's a focus that tosses more than $14 billion dollars around every year. Throw in budgets of the European Space Agency members, as well as China, Japan and Russia, and the wildcard possibility of a privately funded search, and soon you're talking real money.

Next Page: What have we learned about ET in the past 30 years?