Spoiler alert: We haven’t found extraterrestrials yet, despite where a trip down the K-hole of alien internet might lead you. That doesn’t mean scientists have stopped looking. On the contrary, we’re closer to discovering life on other planets than ever before. (That is, assuming the aliens aren’t all dead.)

That excitement, however, raises the question: What kinds of clues are astronomers and other space researchers hunting for?

Before we dive in, it’s important to remember not all life is created equal. Extraterrestrials, should we ever stumble upon them, could arrive in the form of super-advanced creatures whose evolutionary leads to to mechanisms we can’t begin to fathom; or they could be the most primitive and basic of organic molecules, barely falling under the biological definition of life.

That’s a big range to work with. So here are the six signs of alien life scientists are most interested in investigating — with the understanding that there are plenty of other observations and data, too, that could determine whether life exists on a different world.

Water

Life on Earth fundamentally requires H2O. Where there is water, organic molecules can come together and form living systems. These, in turn, reproduce and pass down genetic material. That’s why astronomers are so obsessed with finding water on other moons and planets.

What makes water so vital? It has chemical properties that no other natural substance in the world can emulate. It takes a lot of energy to change the temperature of water — so it does a great job of insulating bodies from the cold while keeping them cool under heat. It’s excellent at carrying nutrients into cells while expelling waste and toxins. It can withstand sharp pressure shifts. It’s really good at dissolving other others substances. Simply put, life as we know it can’t exist without water.

Recurring slope lineae on Mars, formed by water. 

This is why the discovery of liquid water on Mars was so big. Though surface of Mars is likely lacking life, there’s hope we might find signs of ancient Martian organisms — or that the planet could be home to future forms of life.

Gas biosignatures

Liquid isn’t the only state of matter that matters. It’s not always pleasant, but it’s a reality that living things on Earth produce gas. The large amounts of specific gases in our atmosphere act as biosignatures of life. Inorganic geochemical processes can produce gas — but concentrations of certain gases would be a good sign of life on another planet.

Oxygen is the biggest signature on Earth, and methane is a close second. But other biosignatures include any kind of carbon-based gas. And really, besides the noble gases, life on Earth is produces every single gas known to man. Just imagine walking through a dense forest, or along the ocean. Everything you smell that’s a sign of life is technically a biosignatures.

If you have instruments that can analyze the chemical composition of another world’s atmosphere, you’re in a good position to deduce whether biosignatures are present and the likelihood there’s life. A big problem, however, is making sure that organisms produced those biosignatures. Technology, as always, is key.

Radio waves

Here’s where we distinguish the search for just any signs of alien life and the search for intelligence. If aliens are anything like us, chances are good they can harness radio waves for communication and scientific purposes. Nikola Tesla was one of the first people to suggest aliens might try to reach us through radio transmission. As our radio telescopes have improved, the possibility of stumbling on E.T.’s radio are better than ever.

One of the most promising radio telescopes is the Square Kilometer Array under construction in Australia and South America. When completed, it will be 50 times more sensitive than any other radio instrument, capable of scanning the sky 10,000 times faster than we can now. Wherever the radio waves passing through our solar system might originate — be it inside the Milky Way, or from a galaxy dozens of light-years away — this array could pick them up.

The antennas of the Australia Telescope Compact Array

That highlights the biggest problem with looking for radio waves — they may be coming from light-years away, potentially millions of years old. We’d be listening in on the ancient past. Successfully sending a response back would take much longer than humanity’s lifetime. Still, that hasn’t prevented new investments into SETI research.

Heavy elements

It stands to reason that intelligent life would rely on the same heavy elements we use to construct infrastructure and technology in our sentient civilization.

We’re not simply talking about metals like gold and iron and aluminum. We’re talking bigger. Nuclear. Stephen Hawking once observed that “when intelligent life gets smart enough to send signals into space, it is also busying itself with stockpiling nuclear bombs.”

In that case, that species needs to deal with nuclear waste. Nuclear material collected in unusually large concentrations on a planet — or even out in space — might be a sign of an intelligent civilization nearby. A fortuitous sign, but we would want to be a little cautious that introductions don’t inadvertently trigger an interstellar nuclear war.

Artifacts

If Mars was overflowing in vast oceans at some point in its ancient history, then perhaps some form of life existed on the red planet. And if this was intelligent life, there must be some sign it that still remains.

That’s the hope among some scientists looking to find alien artifacts sitting on Mars or some other planet or moon. These could be ruins of an ancient city or small tools hidden away in a cave. Or anything else in between. Looking for alien artifacts would actually not be too dissimilar from how archaeologists study early humans.

Furthermore, artifacts aren’t necessarily a sign that species has gone extinct. They may have migrated to another planet, and what remains are leftovers from a failed or lost colony.

Technostructures

Lastly, the best and most direct sign of intelligent life would be finding what are called “technostructures” — signs of technology that don’t include radio messages. These could be small, like the space probes we’ve also sent off into space — or incredibly massive, like alien megastructures. In essence, a technostructure would show us there’s a species of life out there that’s at least as smart as humans were in the 20th century.

An illustration of what an alien megastructure might look like.

The possibility of finding a technostructure with alien origins is extremely low. That might be a good thing — a species far smarter than we are could eradicate us or enslave us without great difficulty. Or, perhaps, it would be so advanced that it simply views us like ants moving through a mound of dirt.

We won’t be giving up the search any time soon.

Photos via NASA/JPL/University of Arizona, Stewart Duff, CSIRO, Wikimedia Commons, Jonas de Ro