Life can exist in what were previously thought to be impossible environmental conditions here on Earth, meaning life on other planets in our solar system and beyond could be more widespread than earlier believed, scientists say.
NASA scientist Christopher McKay has published a paper that includes an expanded checklist of possibilities that life could exist on distant planets or moons.
Studies on Earth have suggested not all forms of life require the conditions we most commonly experience around us, McKay says, and some are able to survive and even thrive while living in extreme conditions.
Since we have had to expand our consideration of the existence of such extreme life forms in extreme conditions on Earth, it is logical to do the same for other, more distant places as well, he says.
For example, he says, because some earthly micro-organisms survive in environments consistently above the boiling point or below freezing, the search for extraterrestrial life shouldn't rule out any planets simply because they're too hold or too cold.
Nor should we rule out a planet because it's too far from its star for light as an energy source to reach potential life, he says, since some creatures on Earth live so deep in the sea that sunlight never reaches them.
"Our understanding of life on exoplanets and exomoons must be based on what we know about life on Earth," McKay, who works at the space agency's Ames Research Center in California, writes in the Proceedings of the National Academy of Sciences.
Although scientists consider some minimum amount of liquid water is necessary for life to survive, McKay cites lichens and photosynthetic cyanobacteria that can survive in some of the driest conditions.
"You don't need a Pacific Ocean," he says. "A planet like the fictional world in "Dune" would be habitable, though you may not have sand worms."
And water may not be the only liquid that could support life, McKay notes, pointing to Saturn's moon Titan that has liquids on its surface -- but not water.
Titan's seas and ocean are ethane and methane, above which is an atmosphere of methane and nitrogen, yet even this extreme environment contains complex molecules, which could serve as the building blocks of life.
"Titan is a little reminder that there are perhaps more things in heaven and Earth than we can imagine, as Hamlet said. It's a cautionary tale," McKay says. "If we discover something new, we will have to rewrite this chapter."