"Answering the question 'are we alone' is a top science priority," says Thomas Zurbuchen, head of the NASA Science Mission Directorate. He participated in NASA's announcement of the discovery of seven exoplanets in the TRAPPIST-1 system by the Spitzer Space Telescope in February 2017. Three of these exoplanets are in the so-called habitable zone: the area in any planetary system where liquid water, and potentially life, can exist.
A mere two months later, Zurbuchen once again headed a NASA newscast announcing a new discovery made by the Cassini probe: a possible chemical food source for alien life in the form of molecular hydrogen had been detected on Saturn's sixth moon, Enceladus. The energy equivalent of 300 pizzas per hour, no less! That's a lot of satisfied astro grad students... "This is the closest we've come, so far, to identifying a place with some of the ingredients needed for a habitable environment," proclaimed Zurbuchen.
Recently, it feels like NASA has come out with a new announcement every few weeks hinting that we are not very far from reaching the holy grail of space science: finding signs of extra-terrestrial life. Scientists have been careful to temper their published results, specifying that we have only begun to see evidence of environments that may be suitable to harbour life rather than evidence of life itself. Nevertheless, some media outlets such as Newsweek have succumbed to temptation and egregiously reported clickbaity headlines such as 'NASA just found signs of life on a Saturnian moon', and have suffered a huge backlash for it. I would link the initial tweet here, but it has since been deleted. The system works!
The search for extra-terrestrial life has been at the forefront of scientists' minds for decades and, beyond being a purely scientific concern, is a matter of philosophy and making sense of our existence in the Universe. Exoplanets have been detected by astronomers since 1988, and they continue to be discovered at a breakneck pace. Among the several thousands already confirmed, those deemed to be in their parent star's habitable zone are of special interest. Astronomers are looking for Earth-like planets — of a similar size and mass — that have the capacity to retain liquid water, the element we've deemed essential for life. Ultimately, we have been forced to list a set of desirable characteristics for these exoplanets solely based on the only example of life we know: life on Earth.
That being said, lifeforms on our very own home world continue to surprise us by their resilience and robustness. We are always discovering new organisms that thrive in extreme conditions we would have otherwise thought completely deadly. In 2010, NASA made the breakthrough discovery of a microorganism, an "Arsenic Bug", the first of its kind, that feeds off the toxic chemical arsenic in Mono Lake, California, a large saline soda lake. ``The definition of life has just expanded," said Ed Weiler, NASA's then associate administrator for their Science Mission Directorate. ``As we pursue our efforts to seek signs of life in the solar system, we have to think more broadly, more diversely and consider life as we do not know it."
These incredibly sturdy organisms known as extremophiles may even tie back to the very origins of life on Earth. In 1977, scientists made a stunning discovery at the bottom of the Pacific Ocean: hydrothermal vents were heating the frigid deep sea waters and ejecting impressive amounts of chemicals into their surroundings. These dissolved chemicals provide the necessary energy to support bacteria that form the base of the food chain of an impressively diverse underwater ecosystem. Scientists now suggest that ancient hydrothermal vents found on Earth billions of years ago may have populated our world with its very first blips of life. This ecosystem may be very similar to the one hidden at the bottom of Enceladus' oceans!
As we continue our tireless search for someone or something out there in the great dark void, we need to stop and ask ourselves: ``What are we even looking for?". A survey of the general population will show that we often rely on pieces of science fiction such as Star Trek and Star Wars to guide our expectations of alien life. The great majority of these works portray aliens as humanoid beings (I'll admit that Star Wars does a better job of mixing it up, as much as it pains me to say as a Trekkie myself). This makes for compelling story-telling; can you imagine Capt. Jean-Luc Picard having an intellectual showdown with a colony of bacteria (yes, I remember the episode where Wesley unleashes his nanobots colony, but you get my point)? Perhaps not as interesting as discoursing with an intelligent bipedal reptilian creature who happens to speak English. But is this how we should expect our first contact with aliens to play out?
If the molecular hydrogen detected on Enceladus indeed comes from hydrothermal vents not unlike our own, we could potentially find bacteria inhabiting this ocean world. But how can we expect life to evolve exactly as it did on Earth when these alien landscapes are so different from our own? The theory of evolution has showcased with what skill life manages to adapt itself to its surroundings. Intelligent octopus-like creatures may thrive on a purely aquatic planet. Alien life may solely exist in the form of viruses with which communication would be impossible. It may even take a form we have not yet obtained the capacity to imagine (my favourite, BEINGS OF PURE ENERGY-ERGY-ERGY-ERGY-ERGY). At the end of it all, when we do finally reach the point of the first hello, it may not be as simple as a handshake.