US scientists said they made the discovery while searching for signs of life on planets with atmospheres rich in methane and oxygen.
Professor David Catling, of Washington University, said: “Our study shows that this combination would be a compelling sign of life.
“What’s exciting is that our suggestion is doable, and may lead to the historic discovery of an extraterrestrial biosphere in the not-too-distant future.”
The boffins said it was hard to make oxygen without life of some form.
Located in a barren patch of desert in the Bolsón de Mapimí region in Durango, Mexico
Co-author Joshua Krissansen-Totton, a doctoral student at the university, said: “We don’t want to put all our eggs in one basket.
“Even if life is common in the cosmos, we have no idea if it will be life that makes oxygen. The biochemistry of oxygen production is very complex and could be quite rare.”
The stunning discovery will give hope to ufologists that there is intelligent life beyond Earth.
This week extraordinary images emerged of what is claimed to be an “alien photographed inside a real alien flying spaceship”.
This coin was discovered in Egypt – could that be an alien being?
“We don’t want to put all our eggs in one basket”
In the grainy black and white images, you can see the “inside a cockpit with seating, control panel and other objects”.
There is also what looks like a person wearing sunglasses that is said to be a “real alien visitor”.
The rare pictures of the “human-like alien” will be published for the first time by Flying Disk Press on February 1, according to Express.co.uk.