If aliens exist, here’s how we’ll find them

And one day, molecules and cells and multicellular organisms copied themselves and reproduced, eventually becoming flying birds, swimming fish, and two-legged, complex beings known as humans.

We’ve been looking for extraterrestrial life for a long time. We sent twin spacecraft into the void, each armed with a record of Earthlings. We occasionally shoot out radio waves, hoping to get a response.

We could look for waste products of life, or we could look for molecular building blocks of life, planetary scientist Mike Malaska tells Inverse.

Somewhere in the universe, we might find the first stirrings of life. Something to tell us about our own origins.

On Earth, oxygen is a biosignature — it’s created by plants as they convert sunlight and water into nourishing sugars. Lipid molecules — long chains of hydrogen and carbon — are also biosignatures. They make up the walls of our cells.

For instance, amino acids make up proteins and are the building blocks of life. But amino acids form on the desolate — and lifeless — surfaces of comets.

When we look for biosignatures, it’s not only the presence of the molecule that matters, Morgan Cable, a planetary scientist, tells Inverse.

If we’re looking for an amino acid biosignature, the type of amino acid would matter, Cable says.

There are 20 kinds of amino acids. Simple amino acids, like glycine or alanine, can form abiotically, Cable says. But these simple amino acids can’t perform certain life-sustaining functions.

Life requires complex molecules. If we found a lot of big, complicated amino acids, in the right ratio, Cable says, that might be a biosignature. Or at least a clue we should look closer.

The Mars we know is a cold and dry world pummeled by radiation. Any life would have moved to the subsurface, planetary scientist Amy Williams tells Inverse.

This is why rovers and landers investigate Mars’s soil for biosignatures.

In 2018, NASA’s Curiosity rover found fragments of complex organic molecules in 3-billion-year-old rock. These molecules tell us that if life once existed on Mars, we can detect it.

Curiosity also measured seasonal methane spikes in Mars’s atmosphere. Scientists think the methane originates in the Red Planet’s subsurface, but they’re not sure what creates the methane.

On Earth, we know living things create methane. But chemical reactions underground can also make methane, so Mars’s methane isn’t necessarily a biosignature.

In 2006, NASA’s Cassini mission discovered an ocean of liquid water underneath the ice of Saturn’s tiny moon Enceladus... when the spacecraft unexpectedly flew through water vapor geysers spewing from the moon’s south pole.

In the geysers, scientists found organic molecules, carbon dioxide, methane, and molecular hydrogen. This revealed “something funky is going on in Enceladus,” Cable says.

Enceladus has an ice crust, internal ocean, and a rocky core. The hydrogen found in Enceladus’s plumes could mean life-sustaining chemicals form as the ocean interacts with hot spots on the rocky ocean floor.

On Earth, we call these spots hydrothermal vents — or fissures in the ocean floor — and they are thriving ecosystems teeming with life which feed on the chemicals produced in the reaction between the hydrogen gas and the rock. But that doesn't mean the same is true for Enceladus, Cable says.

Maybe someday we could land a spacecraft Enceladus or Jupiter’s moon Europa, which also is believed to have an internal liquid water ocean.

That lander could drill into the icy crust and look for pockets of pockets of large, complex amino acids, or traces of fatty acids indicating the presence of cells.

So scientists study their atmospheres using Earth-based and space-based telescopes instead.

Earth is cosmically lucky — not only are we in the “Goldilocks zone” of our solar system, where our temperature range allows water to be a liquid on our surface, we’ve also got a magnetic field that protects us from our Sun’s radiation.

A biosignature in the atmosphere of an exoplanet has to pass a lot of tests. Firstly, the biosignature must be in the form of a gas, Clara Sousa-Silva, an astrobiologist and planetary atmospheres expert, tells Inverse.

The gas must be high enough in abundance for telescopes to detect it. And it has to be distinguishable from other gases.

“It's never going to be one species producing one molecule. It's always going to be some form of ecosystem producing an array of biosignatures,” Silva-Sousa says.

It won’t just take one kind of scientist, either, Cable says. She and her colleagues have consulted marine biologists, cave scientists, and geochemists in their search for life.