Life on other worlds, whether in our own solar system or orbiting distant stars, might well have to survive in ice-covered oceans, like those on Jupiter’s Moon Europa, or in sealed, gas-filled caves, which should be plentiful on Mars. If you can figure out how to isolate and identify life-forms that thrive in similarly extreme surroundings on Earth, you’re a step ahead in searching for life elsewhere.
One key milestone was a meeting in November, 1961 where astronomers, chemists, biologists and engineers (including Carl Sagan) gathered to discuss astrobiology, the science of life beyond Earth. He was interested in radio telescopes and other ways to search. Frank Drake developed the Drake equation that might predict the number of civilizations there might reasonably be out there: Start with the formation rate of sunlike stars in the Milky Way (the only factor with a known number at the time) X fraction of such stars that have planetary systems X number of life-friendly planets on average in each such system – planets about the size of Earth and orbit at the right distance from their star to be hospital to life X fraction of those planets where life arises X fraction of those where life develops intelligence X fraction of those that might develop the technology t emit radio signals we could detect X the number of radio-savvy civilizations by the average time they’re likely to keep broadcasting or even to survive.
It took 34 years to fill in some of the answers. The first planet was discovered in 1995 (a gas blob half the size of Jupiter with a 4-day orbit and surface temperature of 2000 degrees F) and by 2014, astronomers have found at least 2,000 exoplanets. Thousands more discovered by the Kepler space telescope await confirmation. More than a fifth of stars like sun harbor habitable, Earth-like planets with the closest as near as 12 light years away. The sun though, is not an average star as about 80% of stars in the Milky Way are small, cool, dim, reddish bodies known as M dwarfs that could support life (the planet would have to be closer to its sun than earth). It is also thought that planets anywhere from 1-5 times the size of earth could be habitable. The range of temperatures and chemical environments where extremophilic organisms might be able to survive is also greater than anyone in 1961 could imagine. Hydrothermal vents nourish a rich ecosystem of bacteria feasting on hydrogen sulfide and other chemicals dissolved in the water. Life forms have been have been discovered in niche environments like hot springs, frigid lakes thousands of feet below the surface of Antarctic ice sheets, highly acidic or alkaline or extremely salty or radioactive locations and even in minute cracks in solid rock a mile or more underground. On another planet, these could all be dominant scenarios.
The one factor critical for life as we know is water in liquid form – water once flowed freely on Mars, life could have existed (at least bacterial) and its possible it could still exist underground. Microbes may have take refuge in caves when the planet lost its atmosphere and surface water, and then would have to survive on an energy source other than sunlight. A cave in Mexico has a chemotrophs that oxidizes hydrogen sulfide. Organisms in environments where some resource is in short supply often grow in patterns of lines called bioverms – this is one biosignature based on simple rules of growth and competition for resources that could be a universal signature of life.
Jupiter’s moon Europa has cracks in its young, ice-covered surface – evidence that beneath the ice lies an ocean of liquid water. At half a billion miles or so from the sun, Eruopa’s water should be frozen but it is constantly flexing under the tidal push and pull of Jupiter, generating heat that could keep the water below liquid. Although Europa is the size of our moon, it has more water than all of Earth’s oceans. Europa’s surface captures sulfur flung into space from its volcanic sister moon Io (sulfur can be converted into a source of chemical energy for life. Charged particles in Jupiter’s magnetic field bombard the surface, converting sulfur and other elements into energy-rich compounds that could reach the ocean as pieces of crust slide under each other or via fissures or plumes, even though the ice is probably 10-15 miles thick.
Saturn’s moon Enceladus has an underground source of water as well and Titan, its largest moon, has rivers, lakes and rain of methane and ethane, not water.
A lake in Arctic Alaska has microbes – methanogens – that generate methane from decomposed organic matter. Not all methane is of biologic origin though as the giant planets and Titan form methane naturally in their atmosphere. Living bacteria have been extracted from Lake Whillans, half a mile under the West Antarctic ice sheet.
Of all these, Europa may be the best bet – liquid water, hydrothermal vents, comets depositing organic chemicals, radiation from Jupiter splitting the water in the ice and forming a whole suite of molecules. The Europa Clipper Probe will orbit Jupiter not Europa to save money by using less propellant (2 billion rather than 4.7 billion), but will still make 45 flybys of the moon cruising within 16 miles of the surface. It could take place in the early 2020s and would take 2.7 years to get there with the new NASA Space Launch System with its big rocket. It won’t be able to find life but could scout landing sites for a lander that could penetrate the ice. An under-ice rover is being developed to explore the underside of ice. Other jobs for Clipper include: 1. Sniff the atmosphere with a neutral mass spectrometer for organic molecules 2. Sense chemical impurities on the surface with a shortwave infrared spectrometer 3. Measure salinity and depth with magnetometers 4. Determine the depth of the ice with ice penetrating radar 5. Map geology with a topographical imager 6. Langmuir probes analyze charged particles in Jupiter’s magnetic field 7. Detect plume vents and other warm active surface features with a thermal imager and 8. Find a smooth landing surface with a reconnaissance camera.
Other techniques will be necessary to fill in missing parts of the Drake equation. NASA’s new planet hunting telescope is known as the transiting Exoplanet Survey Satellite or TESS is scheduled for launch in 2017. The James Webb Telescope, for launch in 2018, will search for targets for astrophysicists searching planetary atmospheres for biosignature gases. This assumes that life on other worlds will be built from complex molecules that incorporate carbon as an essential part of their structure, and use water as a solvent, just like on Earth. But alternatives, like a sulfur cycle might replace the carbon cycle producing different atmospheric signatures. SETI searches for flashes of alien light sources rather than radio waves.
Our galaxy, the Milky Way, has at least 100 billion star systems that might support life. Estimates of the number of active alien civilizations range from 10,000 to one million. So far, we have examined only a few thousand star systems. But the rate at which we’re doing so doubles about every two years. At that rate, we will have examined 1 million by the mid-2030s and 10 million by 2040, making it highly probable we will find someone or something.