HomeSkyHow to Search for Extraterrestrial Life by Viewing Our World with Alien Eyes Charles March 13, 2017 Sky Most scientists believe that we’re not alone in the universe. Probability alone suggests that the chance of life only existing on Earth is vanishingly small. However, given the vastness of space, finding life beyond our own world is far from easy. Nonetheless, NASA and other space agencies around the world are pumping vast amounts of resources into the search for extraterrestrial life. But how do they hope to finally answer the age-old question of whether we’re alone or not? Why Do We Have Such an Earth-Centric Approach? NASA/JPL-CaltechThe estimated habitable zone of the TRAPPIST-1 system is a very vague habitability indicator and, of course, entirely Earth-centric. Many people criticise the search for extraterrestrial life as being too Earth-centric, helplessly biased towards what we know from our own experiences. It’s a fair argument too. After all, why should life breathe oxygen or be dependent on photosynthesis? In fact, right here on Earth, anaerobic bacteria can only live in environments without free oxygen, and deep-sea hydrothermal vent ecosystems thrive completely independently of the Sun. In other words, the incredible diversity and adaptability of life on Earth makes the definition of Earth-like life ambiguous to say the least. Unfortunately, we need to start somewhere, and that means searching for what we already know. Life on Earth might be extremely versatile, but that doesn’t mean it’s without its limits. For example, the scorching temperatures of the surface of Venus would sterilize any form of life on Earth, as would the near vacuum of the surface of Mars of the bitterly cold temperatures of the outer planets and moons. That’s not to say that completely different biochemistries could evolve in completely different environments to any of those found on Earth; but pure speculation and real science don’t sit well together. Tiny Specks of Light in an Unending Universe NASASeen from Mercury, around 48 million miles (77 million km) away, the Earth and Moon look more like a binary star system. Mankind has made its mark not just upon the Earth, but also upon the wider solar system and beyond. Radio broadcasts have been beaming out into space at the speed of light for well over half a century. Our space probes continue to travel on their eternal journeys waiting to be picked up by an alien species. However, searching for alien life that’s capable of communication is quite another matter. It’s highly likely that humankind is the only sentient species we will ever know of given the fact that we are just one among billions of species that have ever existed on Earth. From orbit, it’s obvious that the Earth is populated. Our city lights illuminate the night side, and our impact on the environment shows itself through atmospheric pollution. From the outer solar system, however, our planet would appear as nothing more than a point of light to the unaided eye. Move along to the next nearest solar system, Alpha Centauri, and our planet will be long gone from view. Now, only the Sun would appear as a point of light. Owing to the great distances between the stars, it’s notoriously difficult to view planets in other solar systems directly. As such, the overwhelming majority of them were discovered using indirect methods, such as by detecting tiny drops of light when they pass in front of their host stars. Viewing Earth from afar, we would have a hard time, at least with our current technology, determining anything about the planet’s surface or atmosphere. When determining the qualities of planets many trillions of miles away, we typically need to study their effects on their host stars. By studying their hosts instead, we’re able to determine things like approximate mass (an, in turn, composition) and orbital characteristics. Unfortunately, that gives us virtually nothing to go on in terms of planetary habitability. All we have is an estimated habitable zone based on the distance of the planet from the star which, in itself, is a rather superficial notion. Studying Alien Atmospheres from Light Years Away NASA Goddard Space FlightgSpectroscopy allows us to detect atmospheric signatures in the electromagnetic spectrum, potentially allowing us to find life-bearing atmospheres. The relatively few extrasolar planets that have been studied directly are all much larger than Jupiter. Since we still can’t image Earth-sized exoplanets directly, determining their atmospheric properties is extremely difficult. A good place to start is to see what we can learn about our own planet once it’s beyond physical view. What imprints does Earthly life leave on the Universe that may be discovered from afar? Fortunately, different elements leave evidence of their existence in the form of spectroscopic signatures that we can study directly. Since we’re looking for an Earth-like world that could support life as we know it, it makes sense to look for evidence of nitrogen in the atmosphere. In 2015, NASA revealed how observations from a spacecraft 17 million miles away could detect the presence of nitrogen in Earth’s atmosphere, thus giving us valuable insights into how we might be able to study the atmospheres of distant exoplanets. Finding an abundance of nitrogen in an alien atmosphere would be an important breakthrough, since it would point to the planet being able to host liquid water on its surface. Discovering oxygen in the same atmosphere would further increase the chances of the planet being able to host life. However, it is also worth considering that microbial life on the early Earth lived at a time when there wasn’t any free oxygen in the atmosphere. Will NASA’S James Webb Space Telescope Reveal All? NASAThe James Webb Space Telescope will launch next year and will be NASA’s premier space-based observatory for the next ten years. Set for launch in 2018, the James Webb Space Telescope will allow us to study the atmospheric characteristics of distant exoplanets like never before. One of the main tools in its impressive arsenal is a cutting-edge spectrometer. According to NASA, it will be able to detect things like water vapour, methane, oxygen and carbon dioxide in alien atmospheres, including those of the seven planets recently discovered in the TRAPPIST-1 system 39 light years away. NASA seems quite confident that we will find the first definitive evidence of the existence of alien life very soon. Just as the highly successful Kepler mission has revealed thousands of extrasolar planets since 2009, the James Webb Space Telescope will no doubt yield many more fascinating insights into our universe. With the ability to detect atmospheric signatures, it presents the next major step forward in our inexorable quest to find life among the stars. 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