Why do astronomers look for signs of life on other planets based on what life is like on Earth?

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Have you ever played hide and seek in a new place? It’s much more difficult than playing at home. You only know the obvious hiding places: under the bed, in the closet, behind the couch. The trick is to try to think of hiding places you can’t even imagine. How do you search in places you never thought could be hiding places?

This is more or less what scientists like me we do when we look for alien life; we are trying to think of new ways to look at life. Meanwhile, we look for life looking for a life like ours, because that’s what we can imagine.

Searching nearby

The closest place to look for extraterrestrial life is the planets in our solar system.

NASA’s Viking 1 mission began orbiting Earth’s neighbor Mars in 1976. Searching for life on Mars was one of the most important scientific questions for the mission. The spacecraft included a lander that could go to the planet’s surface to see if there was any life on Earth.

Scientists knew that life on Mars could be very different from life on Earth, so they didn’t look for specific life forms or molecules. Instead, they tried to design experiments to look for what life does, not what it produces.

For example, plants and some other life forms on Earth do not photosynthesisa process that uses sunlight and carbon dioxide in the air to gather energy and grow. Viking 1 scientists designed the probe to look for signs of photosynthesis happening on Mars.

To do this, the lander picked up some dirt, shined a light on it, and took measurements to see if any carbon dioxide from the air was transferred to the dirt. This experiment did not show any signs of photosynthesis in Martian dirt.

The probe carried out two other experiments that looked for evidence of organisms growing in the dirt of Mars. One used carbon dioxide gas and another used sugar and amino acid molecules that life forms on Earth like to eat.

The combination of these three experiments and other measurements has led most scientists to agree that there probably is no life on the surface of Mars, at least life that does something like photosynthesis or eats sugar. But we still don’t know whether there are signs of ancient life forms on Mars, or even of current life deep below the surface.

The Viking probe experiments were the most direct tests for life on other planets. However, in terms of a game of hide and seek, these experiments were basically like looking in a closet: it’s a pretty obvious hiding place, but you should check there just in case. Even so, scientists took a long time to interpret the results.

Looking away

Searching for life outside the solar system is even more difficult and requires different techniques.

The closest exoplanet – a planet orbiting a star that is not our Sun – is Next Centauri b, and is about 25 million and million miles (that’s 25 followed by 12 zeros) away from Earth. These distant worlds are so far away that scientists won’t send probes to experiment on them for a long time.

Looking for life on exoplanets is like trying to play hide and seek at your neighbor’s house, but you can only look through the windows and can’t go in. You might get lucky and get the right angle to spot someone hiding, but you can’t know every spot you can’t see.

Tools like new James Webb Space Telescope it could reveal the size of exoplanets, how close they are to their stars and perhaps the gases in their atmospheres. But that’s it. How would you face life with that?

Astronomers thought about looking for life on exoplanets looking for oxygen. They started with this strategy because on Earth, life forms produced most of the oxygen in our atmosphere. Perhaps the oxygen on another planet was produced by alien life.

We have learned, however, that there are other ways to produce oxygen that do not involve life. So now, astronomers aren’t just looking for oxygen — instead, they’re looking for a planet that has oxygen along with water and other gases like methane and carbon dioxide. Together, these combinations could indicate life because we don’t think lifeless planets would have them. But we’re still not sure about that either!

Looking for life by looking for these gases is like looking behind the couch in our game of hide and seek. Do we know someone will be there? No. But we only look through windows and we can imagine people hiding behind sofas. We might as well try – where else could we look?

What game are we playing?

There are two big differences between playing hide and seek and looking for aliens.

First, when you are playing hide and seek, you usually know that you are playing with someone else. We have no idea if there are aliens out there to encounter! It’s possible that there is no other life out there, and it’s possible that there are aliens right next door. Until we find examples of life beyond our own, we won’t know how common life is in the universe.

The second difference is that most scientists do not think alien life is hiding from us; It’s just that we haven’t seen it yet. There are some ideas that more advanced civilizations can avoid being detectedbut researchers don’t think this is happening in our solar system.

Most astronomers and astrobiologists know that if we only look for life similar to life on Earth, we may miss signs of aliens that are actually different. But honestly, we’ve never detected aliens before, so it’s hard to know where to start. When you don’t know what to do, starting somewhere is usually better than starting nowhere.

Searching for life using experiments like the Viking probe or looking for oxygen may not help. But we might be lucky. And even if we aren’t, we’ll cross some obvious possibilities off the list. Then we can focus on the more difficult question of imagining something we’ve never thought of before.

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This article has been updated to correct a typo in the distance to Proxima Centauri b.

This article was republished from The conversation, an independent, nonprofit news organization that brings you trusted facts and analysis to help you understand our complex world. It was written by: Cole Mathis, Arizona State University

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Cole Mathis has previously received funding from the NASA Postdoctoral Program.