National Geographic : 2014 Jul
life beyond earth 33 But that’s a lie.” In fact, about 80 percent of the stars in the Milky Way are small, cool, dim, red- dish 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 va- riety 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. 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 oceanogra- phers such as National Geographic Explorer-in- Residence Robert Ballard discovered superheated gushers, known as hydrothermal vents, nour- ishing 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 thou- sands 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 environ- ments,” 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 dis- solved 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 wa- ter 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 Encela- dus; subsequent measurements by the space- craft 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 per- mit life to exist. The surface of Titan, Saturn’s Michael Lemonick’s latest book is Mirror Earth: The Search for Our Planet’s Twin. Mark Thiessen shot our story on the solar system in the July 2013 issue. N=R* × ƒp ×ne ×ƒl ×ƒi ×ƒc ×L 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.