Our astronomical knowledge increases on a daily basis, but we still have no tangible proof that extraterrestrial life exists elsewhere. Although the universe is a vast place, where planets are common and water is abundant, there is still no sign of aliens. The frustration and surprise at our inability to detect alien life is perfectly summarized in the Fermi paradox: If the conditions for life are so common across the cosmos, where is everybody?
Although fruitless so far, the search for alien life might be at a turning point. With the discoveries of potentially habitable terrestrial planets, the imminent launch of new dedicated telescopes, and other promising space missions, we might soon find definitive proof of extraterrestrial life. The following are the areas through which we are aiming to find ET.
The search for alien life has been on the mind of scientists, inventors, and the general public since at least the late 19th century, but it has been mostly limited to the hope of hearing radio signals emitted by other civilizations and the discovery of life in our Solar System.
The Search for Extraterrestrial Intelligence (SETI) is probably the most famous organization dedicated to monitoring electromagnetic radiation for signs of alien transmissions. Most of SETI’s work is focused on analyzing radio waves in the hope of finding a signal of clear artificial origin. They do this by observing thousands of stars in our galaxy for a clearly artificial signal, but so far nothing has been found.
The Wow! Signal Big Ear Radio Observatory and North American AstroPhysical Observatory (NAAPO)
So far, only one detection in 1977 – which was not by SETI – couldn’t definitively be explained as a natural phenomenon (its exact origin is still unclear). The mysterious signal – also known as the "Wow! signal" – was a 72-second-long narrow-band radio detection. It came from an area in the sky devoid of stars and planets, but it was never detected again, despite 50 more observations of the same area in the following decades.
Simple life forms in the Solar System
The investigation for life in the Solar System has not always been at the forefront of space programs, but it has been present in the objectives of space agencies around the world. The Viking program in the 1970s, for example, was the first to have the search for life on Mars as one of its primary scientific objectives. Although it didn't find any clear evidence of life, discoveries since then have suggested that life might have developed in the past.
With the recent discovery of briny water seasonally flowing on the Red Planet, and the confirmation of dynamic oceans under the surface of Europa and Enceladus, the attention is shifting to more life-focused exploration. ExoMars, the joint European-Russian Mars program due to launch in 2018, will have the specific intention of detecting biosignatures in the atmosphere and ground of Mars. For other missions, such as those to the oceans of Europa, we will have to wait, although NASA is planning to study Jupiter's moon in more detail soon with a new spacecraft. The technology necessary for an autonomous drilling probe capable of penetrating the icy crust of Europa or Enceladus, though, is still beyond our current capabilities.
A realistic color Galileo mosaic of Europa. NASA/JPL/SETI Institute.
Biosignatures in exoplanet atmospheres
It has been 20 years since we discovered the first exoplanet orbiting a main-sequence star, and we now know of almost 2,000 planets beyond the Solar System. With new planets being detected almost every day, more and more scientists believe that the first sign of alien life will come by detecting biological or artificial chemicals in the atmospheres of distant worlds.
A technique that could potentially detect the presence of photosynthetic plants on an exoplanet was developed by an American team in 2014. They suggested using oxygen dimers (two O2 molecules bound together by intermolecular forces) as a signpost for life, as they only form at normal pressures in oxygen-rich atmospheres.
Many researchers are curious as to what we will find by studying terrestrial planets. "Exoplanet atmospheres may have a number of telltale signs that could indicate life, or conditions that may be suitable to sustain life, should it have arisen," Dr. Louisa Preston, an astrobiologist, and TED fellow, told IFLScience. "If, for example, we observe water as H2O, then one of the three key ingredients for life is present on that world. Should we see oxygen then we call that a biosignature. Oxygen on Earth is produced by life and is only seen in our atmosphere due to the actions of life.
Soon we might be able to see the atmosphere of planets thanks to the light of their stars. NASA Goddard
"Methane is also an exciting possibility. Even though it is produced on Earth through both biological and abiotic processes, it could still tell us a lot about the conditions for life on that world. Therefore finding methane and oxygen, and also ozone, in another planetary body atmosphere would be very exciting."
Technological waste products
Another technique in the works is the observation of unintentional signals from an alien race. The level of technological advancement (also known as the Kardashev scale) is thought to be proportional to the energy needs of a civilization. No matter how careful they might be, energy usage always produces wasted heat, and planned telescopes – such asColossus – aim to look at these energy signatures. In a similar way, we might be able to detect artificial illumination on an alien planet from their lights at night, akin to how Earth is lit up by artificial lights.
Spectrographic analysis of planets’ atmospheres could also find industrial waste products. Carbon dioxide levels beyond what is naturally possible, or more complex byproducts of industrialization, would definitely raise some red flags and suggest the possible presence of an intelligent species.
"There is also one idea that we may one day detect the actions of alien civilizations in exoplanet atmospheres, such as an abundance of CFC’s [chlorofluorocarbon, the aerosol responsible for ozone depletion]," said Dr. Preston. "These do not occur naturally, they are manmade compounds, so finding these would be a huge discovery."
Artistic interpretation of the ESA gravitational wave detector LISA. NASA/JPL, via Wikimedia Commons.
Alien civilizations might be significantly more advanced than ourselves, and thus might be using technologies that we can’t detect yet. Suggested communication technologies include gamma rays, neutrinos, and gravitational waves.
Gamma rays allow the transmission of large data sets more quickly than traditional methods, and the technology to allow this is currently in development. Artificial neutrino emissions and gravitational anomalies could be detected in the next 10 years, but identifying the subtleties of the signal might be beyond our capability and we might not recognize them as definite proof of alien life. And of course, that's just speculation. If they do exist, who knows how they communicate.
Aliens and capitalism
While investigating for direct signals from possible alien civilizations, it is important to remember that they might not be emitting much. Through the use of satellite and cable communication, we have reduced the radio signals we send out into space because it's a waste of energy and money. Aliens might have done the same. We always consider extraterrestrials to be significantly different from us, but maybe we are more alike than we think; across the galaxy, we might all be waiting and listening but nobody is talking.