Astrobiologists are closer than ever before to finding evidence of extraterrestrial life. But with millions of planets in the universe, they first must narrow down their search.
Are we alone in the universe? At this point, most scientists would agree that the chance of Earth being the only planet to have fostered life is vanishingly small. With new planets discovered nearly every week, the quest for extraterrestrial life is ramping up. Not to mention the myriad possibilities in our own solar system; Mars, Jupiter’s moon Europa, and Saturn’s two moons Enceladus and Titan, are all on the shortlist of habitable worlds.
But how exactly do scientists go about looking for alien life? The first order of business is finding planets that might make a cozy home for microbes. Liquid water is key, since all living organisms on Earth require it as a solvent. Bacteria also require warmth that can provide energy for chemical reactions.
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Once a planet is deemed capable of supporting life, there are a few ways to hunt for signs of said life. In the case of Mars, NASA plans to send another rover in 2020 that will take samples of soil. Scientists will analyze these samples for signatures of biological activity, such as methane produced by bacteria, or even for tiny fossils of bacteria and other microbes. In preparation for their next mission to Mars, NASA scientists are currently testing an instrument called Signs of Life Detector that will identify any biosignatures in the soil.
In addition to soil samples, water can also carry products of organic processes. While most of Europa’s liquid water is trapped beneath its icy crust, it seems to spew plumes of water vapor from its south pole. NASA scientists are currently trying to figure out how to detect biosignatures in water in time for their planned mission to Europa in the 2020s.
For planets outside our solar system, astronomers first check to see if the planet orbits within the habitable zone of its star — that is, whether the planet receives enough light and heat to support life without burning those lifeforms to a crisp. Since they are too far away for us to land rovers on the surface, astronomers rely on telescopes to determine the chemical composition of the atmosphere based on the different wavelengths of light emitted by certain elements. Oxygen is the most obvious gas to look for, since its mass production by photosynthesizing bacteria makes all other life possible. The next few years will see a host of new telescopes that will bring higher processing power to the detection of gases produced by organisms.
But this approach may be too narrow, and could cause us to miss other indicators of alien life. Scientists have recently realized that lifeforms on other planets could use different chemicals for fundamental biological processes. For example,a study in May of this year proposed that organisms living in an oily hydrocarbon medium, such as methane, woulduse DNA and RNA-like molecules composed of ethers instead of sugar and phosphate. In addition, if they lived in an atmosphere full of hydrogen instead of oxygen and nitrogen, microbes would produce different biosignature gases than the ones we look for based on terrestrial life.
After all, plenty of organisms on Earth have evolved to survive in extreme environments that seem alien to us. Entire ecosystems living around undersea volcanic vents depend on colonies of “chemosynthetic” bacteria that perform a version of photosynthesis using inorganic molecules instead of sunlight. Who’s to say that life couldn’t have evolved using a completely different chemical system than what we are familiar with on Earth?
While studying these “extremophiles” (organisms that can survive in seemingly inhospitable environments) can open up the search for possible ways of life, it also increases the chance of landing on a false-positive. Even with well established biosignatures like oxygen and organic molecules containing carbon, scientists must exercise caution in case the compounds resulted from normal geologic processes. One way to reduce false positives is by examining two or more biosignatures that cannot coexist without any living organisms there to replenish the supply. For example, oxygen and methane tend to destroy each other, so their concurrence in the atmosphere indicates there are organisms that constantly produce both of them.
For now, the best course of action will be to amass as much data as possible, and methodically analyze each planet’s atmosphere for things we know to look for — oxygen, methane, maybe nitrogen. With the rapid advances in detection technology and data analysis we’ve already made, scientists agree that we will find evidence of alien life within the next decade.
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