Fourteen years ago, NASA scientists triumphantly proclaimed that they’d found proof of life on Mars. Then-president Bill Clinton gave a press conference and postulated about all of the future wonders that awaited us. And then, for over a decade, we heard almost nothing about the telltale life-containing Antarctic meteorite. While the matter may have fallen away from the public’s immediate concern, NASA scientists haven’t forgotten about that promising discovery. In fact, they’re more convinced than ever that Mars at one time harbored microscopic life.

The original 1996 discovery centered on the discovery of microbial fossils in a 4-billion-year-old Mars meteorite that fell in Antarctica thousands of years ago. Scientists were able to identify the origin of the meteorite because the gases in them match gases found in Mars’ atmosphere. And the microbe fossils in the meteorites suggest that Mars was once a very different place than it is today: warmer, water-bearing, and with a protective atmosphere. Critics said that the fossils weren’t necessarily proof of life, leading an inter-disciplinary team of scientists to embark on a whole new kind of scientific journey: one based on simulations, inferences and the kind of detective work that the astronomy community loves to engage in. The result wasn’t definitive proof that Mars supported life at one time – but the report showed that the microbes couldn’t have come from non-biological sources. So while the scientists aren’t saying definitely that there is life on Mars, they are slowly making progress in disproving the critics.

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The search for other hospitable planets is nothing new. We’ve been speculating about where Earthly life forms might be able to survive since we realized that there were other worlds out there. But until now, we haven’t had a standardized way to measure the habitability of other planets for microorganisms such as the ones that flourish here at home. Recently, a team at the University of Puerto Rico at Arecibo compiled the first-ever comprehensive and quantitative evaluation of the habitability of other worlds.

Basing the suitability for microscopic life support on a scale of 0 to 1 (think of it statistically, where 0 = impossible and 1 = certain), the report identified the three worlds most likely to be able to sustain microscopic life as we know it. The most promising contender was Enceladus, Saturn’s sixth-largest moon. The satellite scored a 0.4 on the Standard Microbial Habitability index, the same as Earth. Though it hasn’t been explored thoroughly, scientists think that a vast ocean of water may exist under Enceladus’ icy crust.

Because of its proximity to Earth, Mars has often been pointed to as possibly being able to support Earth life. Until the 1960s, it was widely assumed that the red planet’s atmosphere was very similar to Earth’s. While we’ve since been able to disprove that theory, tests for life on Mars have come up either negative or inconclusive. However, the planet scored 0.3 on the Standard Microbial Habitability index. The ideal place for life on Mars wouldn’t be on the surface, though – it would be approximately 4 miles underground, where the temperature and pressure are welcoming for microbial life forms.

Europa, the sixth moon of Jupiter, also scored a 0.3 on the index. The moon’s surface, like that of Enceladus, is covered with ice. But also like Enceladus, Europa may have a vast ocean hiding under the surface that can support microorganisms. The fact that these worlds may be able to support microorganisms doesn’t mean that there is life present on them, just that the conditions may be favorable for habitation.

Five worlds were tested with the new index. The remaining two, Venus and Saturnian moon Titan, scored 0.4 and less than .0001, respectively. While Venus’ score may appear promising, it didn’t make the top three because any life forms would have to reside in the unstable atmosphere due to the surface’s high temperatures. Titan’s atmosphere, surface and subsurface are simply inhospitable to life – at least any life forms we currently know of.

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