No need to leave the planet to look for alien life — perhaps it's here, in peaceful coexistence with or complete isolation from the standard variety that permeates Earth.
"If life does form readily under Earth-like conditions, shouldn't it have formed many times over, right here on our home planet?" said Paul Davies, a theoretical cosmologist and astrobiologist at Arizona State University. "There's no planet more Earth-like than Earth itself."
The question of alien life is more commonly associated with the extraterrestrial variety, a quest that has NASA scratching its head over not only where to look, but how to identify it if found.
"If we find that life has happened in the solar system twice, from scratch, if we can be sure of that, then it's going to have happened all around the universe," Davis said. "The universe is going to be teeming with life, and there's a very good chance that we are not alone."
Davies, who spoke last weekend at the American Association for the Advancement of Science conference in Chicago, says the evidence for an alternative life form could be right under -- or even in — our noses.
Scientists already have some candidates for alien Earth life, including a population of arsenic-fed microbes living in a California lake and a colony of strange organisms near deep-sea vents.
Slideshow 12 photos
Month in Space: January 2014
"There's been no systematic search for life as we don't know it right here on Earth," Davies said.
The rapidly growing field of synthetic biology — the engineering of life forms — is helping scientists understand what biochemical alternatives for life are possible.
"Unless it happens to shoot at you with a ray gun, life you encounter off of Earth will not necessarily have the same biochemistry as us," said Steven Benner with the Florida-based Foundation for Applied Molecular Evolution. "You would not it necessarily be able to be recognize it as life."
Davies recalled that when he was attending school in the 1960s, the prevailing view was that life is "statistically improbable, a freak accident, a chemical quirk. It happened once in the observable universe, and we are it."
In the last few decades, however, the pendulum has swung the other way.
"It is now quite fashionable to say that the universe is teeming with life, but the scientific facts haven't changed very much," Davies said. "We're still completely ignorant about whether life pretty much automatically and naturally arises in Earth-like conditions, or whether it is just a fluke."
