Humans have scanned and searched the heavens for signs of other advanced civilizations in the universe. And we've found nothing. Absolutely nothing. So maybe we shouldn't be so focused on intelligent life, but on any sort of life whatsoever.
Sure, a tiny microbe may not be as exciting as swapping stories with distant aliens, but signs of non-intelligent life may be much more common, and much easier to find, in our galaxy.
Is anybody out there?
Life, including intelligent life, evolved on Earth. Yet there shouldn't be anything particularly remarkable about our planet; it's just another random world in the galaxy. So if intelligent life happened here, it must be pretty common — common enough that we should be seeing signs of alien civilizations all over the place.
So, where is everybody?
This is the heart of the infamous Fermi paradox, and it's the main argument used to fuel the search for extraterrestrial intelligence (SETI). At first glance, it seems immediately obvious that we should not be alone in the universe, and so if we look hard enough, we should see evidence for intelligence.
Perhaps aliens are blasting radio signals for us to listen to. Perhaps they're just generally blasting the radio, and we happen to pick it up. Perhaps they've left artifacts in the solar system, designed to monitor us or just hang around. Perhaps they'll engage in mega-engineering projects, like enclosing their star in a swarm of solar panels. Or perhaps they'll just play around and contaminate their star with heavy metals to announce their presence.
After over half a century of SETI, however, we've found nothing. No radio signals. No artifacts. No mega-engineering. To date, after over a hundred dedicated searches, we have absolutely no evidence for any intelligent life in our galaxy, or even in the universe.
Related: 13 ways to hunt intelligent aliens
The assumption behind SETI is that intelligent life should be easier to detect than regular, non-intelligent life, because intelligent creatures are capable of really making their presence known. If they've invented radio, then they can crank up the FM, creating a bubble of detectability racing away from their home system at the speed of light. If they are capable of harnessing a decent fraction of the energy coming from their star, they can transform their entire solar system. If they've cracked interstellar travel, then they can spread like (intelligent) weeds throughout the galaxy.
But something in this argument is going wrong. Either intelligent life isn't as common as we might have hoped, or it's not as detectable as we might have hoped. Either way, it doesn't look like SETI will bear fruit anytime soon.
So perhaps we should just search for extraterrestrial life, rather than focusing on advanced alien civilizations. That means any kind of life: single-celled organisms floating in oceans, moss clinging to rocks or the first hints of complex creatures moving around their environments.
Sure, these kinds of life-forms may not be as loud as intelligent life, but that doesn't make them invisible. Indeed, one of the key features of any kind of life is the ability to throw a planet out of equilibrium.
Take, for example, Earth's atmosphere. Our planet probably formed with a good deal of oxygen; there's plenty of it in the universe to go around. But oxygen is highly volatile and reactive, and it doesn't really last long on its own in an atmosphere; it either escapes into space or binds with other elements and turns into other things (like carbon dioxide or silicon dioxide).
But around 2 billion years ago, a planet full of single-celled photosynthetic organisms ate enough carbon dioxide and burped out enough oxygen to completely revamp Earth's atmosphere, giving it substantially more oxygen than it would in equilibrium. Life on Earth changed the very character of this planet's atmosphere.
And that's detectable.
When a distant exoplanet crosses in front of the face of its parent star, the light from that star filters through the planetary atmosphere before making its way to our telescopes. Different elements and molecules in the atmosphere alter the light in subtle, but measurable ways.
Our current technology limits us to studying the atmospheres of giant planets orbiting close to their parent stars. But NASA's Transiting Exoplanet Survey Satellite is cataloging a number of promising candidates for follow-up studies with the James Webb Space Telescope, which will have the capability to detect an overabundance of oxygen in the atmospheres of alien worlds.
As first-contact scenarios go, it may not be that exciting. There will be no exchange of prime numbers or fundamental constants. There will be no wondering about alien cultures or habits or bodies.
Most likely, our first evidence for life outside Earth will take the form of a wiggle in a line on a plot, telling us that living creatures have dramatically altered the equilibrium of their home planet. Non-intelligent life may not be as common as intelligent life (though, truth be told, we have no idea how common either is), but simple creatures are still capable of making themselves noticeable.
It may not be as exciting as the potential for epic space battles with alien androids, but finding any exterrestrial life at all would certainly be something to phone home about.
Learn more by listening to the "Ask A Spaceman" podcast, available on iTunes and askaspaceman.com. Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.
Get the Space.com Newsletter
Breaking space news, the latest updates on rocket launches, skywatching events and more!
Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.