The holy grail for finding worlds beyond Earth that are hospitable to life has been planets just the right distance from their mother stars where liquid water can exist on the surface — the so-called "Goldilocks" zone.
But scientists now say this elusive zone where conditions are not too hot and not too cold for life to exist is far bigger than originally thought.
"When people talk about 'habitable zones,' they mean where there's liquid water on the surface. But there's liquid water elsewhere in the solar system; it's buried under thick sheets of ice on moons," Francis Nimmo, a planetary scientist with the University of California at Santa Cruz, told Discovery News.
Nimmo is among a growing cadre of scientists who believe the search for life beyond Earth should be focused on selected moons of Jupiter and Saturn, where liquid oceans or lakes are believed to exist beneath the frozen ground.
In addition to water, organic molecules, which could have been deposited on the surface by crashing comets, somehow would have to get through the thick shells of ice for life to form, a situation that puts Saturn's geyser-spewing moon Enceladus at the top of Nimmo's list of potential spots for life.
"It looks like the geysers are coming from an ocean, which means that it's in contact with the surface," Nimmo said.
It also would be far easier to get a water sample from Enceladus, which has plumes of water vapor, ice and particles shooting more than 300 miles off its surface, than from other moons, such as Jupiter's Europa, where a massive ocean is believed to be buried beneath a thick icy crust.
"If we go to Enceladus, we essentially get a free sample," said Nimmo, who presented a paper at the American Geophysical Union conference earlier this month about how the properties of ice impact the habitability of moons in the outer solar system.
"There aren't any bad choices," added John Spencer with the Southwest Research Institute in Boulder, Colo.
"These places look habitable, but we don't know if there's some fatal flaw. Maybe there's not enough energy in the Europa ocean, or maybe the hotspots of Enceladus freeze over. Or maybe you can have perfect conditions but life requires molecules coming together at all the right moments," Spencer told Discovery News.
For now, the question about whether there is life beyond Earth is wide open.
"We can't disprove that we're alone in the universe, so it's a completely reasonable hypothesis given our complete lack of information at the moment," Spencer added.
Scientists are in the midst of assessments to choose a destination, or destinations, for NASA's next major exploration initiative to the outer solar system.
"We hope we'll be going to Europa, but nothing's definite at this point," Spencer said.