The Drake equation, formulated in 1961, estimates the number of alien civilizations we could detect. Recent discoveries of ­numerous planets in the Milky Way have raised the odds.

The final step: Multiply the number of radio-savvy civilizations by the average time they're likely to keep broadcasting or even to survive. If such advanced societies typically blow themselves up in a nuclear holocaust just a few dec­ades after developing radio technology, for example, there would probably be very few to listen for at any given time.

The equation made perfect sense, but there was one problem. Nobody had a clue what any of those fractions or numbers were, except for the very first variable in the equation: the formation rate of sunlike stars. The rest was pure guesswork. If SETI scientists managed to snag an extraterrestrial radio signal, of course, these uncertainties wouldn't matter. But until that happened, experts on every item in the Drake equation would have to try to fill it in by nailing down the numbers—by finding the occurrence rate for planets around sunlike stars or by trying to solve the mystery of how life took root on Earth.

It would be a third of a century before scientists could even begin to put rough estimates into the equation. In 1995 Michel Mayor and Didier Queloz of the University of Geneva detected the first planet orbiting a sunlike star outside our solar system. That world, known as 51 Pegasi b, about 50 light-years from Earth, is a huge, gaseous blob about half the size of Jupiter, with an orbit so tight that its "year" is only four days long and its surface temperature close to 2000°F.

Nobody thought for a moment that life could ever take hold in such hellish conditions. But the discovery of even a single planet was an enormous breakthrough. Early the next year Geoffrey Marcy, then at San Francisco State University and now at UC Berkeley, would lead his own team in finding a second extrasolar planet, then a third. After that, the floodgates opened. To date, astronomers have confirmed nearly two thousand so-called exoplanets, ranging in size from smaller than Earth to bigger than Jupiter; thousands more—most found by the exquisitely sensitive Kepler space telescope, which went into orbit in 2009—await confirmation.

None of these planets is an exact match for Earth, but scientists are confident they'll find one that is before too long. Based on the discoveries of somewhat larger planets made to date, astronomers recently calculated that more than a fifth of stars like the sun harbor habitable, Earthlike planets. Statistically speaking, the nearest one could be a mere 12 light-years away, which is practically next door in cosmic terms.

That's good news for astrobiologists. But in recent years planet hunters have realized that there's no reason to limit their search to stars just like our sun. "When I was in high school," says David Charbonneau, an astronomer at Harvard, "we were taught that Earth orbits an average star. But that's a lie." In fact, about 80 percent of the stars in the Milky Way are small, cool, dim, reddish bodies known as M dwarfs. If an Earthlike planet circled an M dwarf at the right distance—it would have to be closer in than the Earth is to our sun to avoid being too cold—it could provide a place where life could gain a foothold just as easily as on an Earthlike planet orbiting a sunlike star.

Moreover, scientists now believe a planet doesn't have to be the same size as Earth to be habitable. "If you ask me," says Dimitar Sasselov, another Harvard astronomer, "anywhere from one to five Earth masses is ideal." In short, the variety of habitable planets and the stars they might orbit is likely to be far greater than what Drake and his fellow conferees conservatively assumed at that meeting back in 1961.

A microbe retrieved in 2013 from Lake Whillans, half a mile beneath the Antarctic ice, reveals life's ability to take hold even in the most extreme environments. TRISTA Vick-Majors and PAMELA SantibÁÑez, Priscu Research Group, Montana State University, Bozeman

That's not all: It turns out that the range of temperatures and chemical environments where extremophilic organisms might be able to thrive is also greater than anyone at Drake's meeting could have imagined. In the 1970s oceanographers such as National Geographic Explorer-in-Residence Robert Ballard discovered superheated gushers, known as hydrothermal vents, nourishing a rich ecosystem of bacteria. Feasting on hydrogen sulfide and other chemicals dissolved in the water, these microbes in turn feed higher organisms. Scientists have also found life-forms that flourish in hot springs, in frigid lakes thousands of feet below the surface of the Antarctic ice sheet, in highly acidic or highly alkaline or extremely salty or radioactive locations, and even in minute cracks in solid rock a mile or more underground. "On Earth these are niche environments," says Lisa Kaltenegger, who holds joint appointments at Harvard and the Max Planck Institute for Astronomy in Heidelberg, Germany. "But on another planet you can easily envision that they could be dominant scenarios."

The one factor that biologists argue is critical for life as we know it is water in liquid form—a powerful solvent capable of transporting dissolved nutrients to all parts of an organism. In our own solar system we've known since the Mariner 9 Mars orbiter mission in 1971 that water once likely flowed freely on the red planet. So life might have existed there, at least in microbial form—and it's plausible that remnants of that life could still endure underground, where liquid water may linger. Jupiter's moon Europa also shows cracks in its relatively young, ice-covered surface—evidence that beneath the ice lies an ocean of liquid water. At a half billion miles or so from the sun, Europa's water should be frozen solid. But this moon is constantly flexing under the tidal push and pull of Jupiter and several of its other moons, generating heat that could keep the water below liquid. In theory, life could exist in that water too.

In 2005 NASA's Cassini spacecraft spotted jets of water erupting from Saturn's moon Enceladus; subsequent measurements by the spacecraft reported in April of this year confirm an underground source of water on that moon as well. Scientists still don't know how much water might be under Enceladus's icy shell, however, or whether it's been liquid long enough to permit life to exist. The surface of Titan, Saturn's largest moon, has rivers, lakes, and rain. But Titan's meteorological cycle is based on liquid hydrocarbons such as methane and ethane, not water. Something might be alive there, but what it would be like is very hard to guess.

Mars is far more Earthlike, and far closer, than any of these distant moons. The search for life has driven virtually every mission to the red planet. The NASA rover Curiosity is currently exploring Gale crater, where a huge lake sat billions of years ago and where it's now clear that the chemical environment would have been hospitable to microbes, if they existed.

Penelope Boston of the New Mexico Institute of Mining and Technology and the National Cave and Karst Research Institute captures a drop of bio­film from the Cueva de Villa Luz ("cave of the lighted house") in Mexico. The viscous goo—dubbed a snottite—harbors bacteria that derive energy from hydrogen sulfide within the toxic cave. Life-forms in such extreme ecosystems serve as earthly analogues for organisms that might thrive in extraterrestrial environments.

A cave in Mexico isn't Mars, of course, and a lake in northern Alaska isn't Europa. But it's the search for extraterrestrial life that has taken JPL astrobiologist Kevin Hand and the other members of his team, including John Leichty, to Sukok Lake, 20 miles from Barrow, Alaska. The same quest has lured Penelope Boston and her colleagues multiple times to the poisonous Cueva de Villa Luz, a cave near Tapijulapa in Mexico. Both sites let the researchers test new techniques for searching for life in environments that are at least broadly similar to what space probes might encounter. In particular, they're looking for biosignatures—­visual or chemical clues that signal the presence of life, past or present, in places where scientists won't have the luxury of doing sophisticated laboratory experiments.

Take the Mexican cave. Orbiting spacecraft have shown that caves do exist on Mars, and they're just the sorts of places where microbes might have taken refuge when the planet lost its atmosphere and surface water some three billion years ago. Such Martian cave dwellers would have had to survive on an energy source other than sunlight—like the dripping ooze that has Boston so enchanted. The scientists refer to these unlovely droplets as "snottites." One of thousands in the cave, varying in length from a fraction of an inch to a couple of feet, it does look uncannily like mucus. It's actually a biofilm, a community of microbes bound together in a viscous, gooey blob.

The snottite microbes are chemotrophs, Boston explains. "They oxidize hydrogen sulfide—­that's their only energy source—and they produce this goo as part of their lifestyle."

Snottites are just one of the microbial communities that exist here. Boston, of the New Mexico Institute of Mining and Technology and the National Cave and Karst Research Institute, says that all told there are about a dozen communities of microbes in the cave. "Each one has a very distinct physical appearance. Each one is tapping into different nutrient systems."

One of these communities is especially intriguing to Boston and her colleagues. It doesn't form drips or blobs but instead makes patterns on the cave walls, including spots, lines, and even networks of lines that look almost like hieroglyphics. Astrobiologists have come to call these patterns biovermiculations, or bioverms for short, from the word "vermiculation," meaning decorated with "irregular patterns of lines, as though made by worm tracks."