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Why do astronomers look for signs of life on other planets based on what life is like on Earth? Couldn’t there be completely different types of life on other planets? – Henry, age 13, Somerville, Massachusetts
Have you ever played hide and seek in a new place? It’s much harder than playing at home. You only know the obvious hiding places: under the bed, in the closet, behind the couch. The trick is to come up with hiding places you can’t even imagine. How do you search in places you never thought could be hiding places?
That’s pretty much what scientists like me do when we look for extraterrestrial life; we try to think of new ways to look for life. In the meantime, we look for life by looking for a life like us, because that is what we can imagine.
Looking close
The closest place to look for alien life is on planets in our solar system.
NASA’s Viking 1 mission began in 1976 in orbit of Earth’s neighbor, Mars. Searching for life on Mars was one of the most important scientific questions for the mission. The spacecraft contained a lander that could go to the planet’s surface to see if there were life forms in the dirt there.
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, rather than what it makes.
For example, plants and some other life forms on Earth engage in photosynthesis, a process that uses sunlight and carbon dioxide in the air to gather energy and grow. Viking 1 scientists designed the lander to look for signs of photosynthesis on Mars.
To do that, the lander scooped up some dirt, shined a light on it and took measurements to see if any carbon dioxide from the air got into the dirt. This experiment showed no sign of photosynthesis in the Martian dirt.
The lander conducted two other experiments looking for evidence of organisms growing in the soil on 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 led most scientists to agree that there is probably no life on the surface of Mars, at least not life that does anything like photosynthesis or eats sugar. But we still don’t know if there are signs of ancient life forms on Mars, or even of current life deep beneath the surface.
The Viking lander experiments were the most direct tests for life on other planets. In terms of hide and seek, however, these experiments were basically like looking in the closet: it’s a pretty obvious hiding place, but you should check there just to be sure. Still, it took scientists a long time to interpret the results.
Looking far 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 Proxima Centauri b, and it is more than 2 million, million miles (that’s 2 followed by 13 zeros) from Earth. These distant worlds are so far away that scientists won’t send landers to conduct experiments on them for a long time.
Searching for life on exoplanets is like playing hide and seek in your neighbor’s house, but you can only look through the windows and can’t go inside. You might get lucky and find just the right angle to spot someone hiding, but you can’t know all the places you can’t see.
Tools like the new James Webb Space Telescope can 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 approach life with that?
Astronomers have thought about looking for life on exoplanets by looking for oxygen. They started this strategy because life forms on Earth made most of the oxygen in our atmosphere. Perhaps oxygen was created on another planet by alien life.
However, we have learned that there are other ways to make oxygen that do not involve life. So now astronomers aren’t just looking for oxygen; instead, they are hunting for a planet that contains oxygen, along with water and other gases, such as methane and carbon dioxide. Together, these combinations could indicate life, because we don’t think planets without life would have it. But we are still unsure about that too!
Searching 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 can only look through the windows, and we can imagine people hiding behind benches. We might as well try – where else would we look?
What game are we playing?
There are two major differences between playing hide and seek and looking for aliens.
First of all, when you play hide and seek, you usually know that you are playing with someone else. We have no idea if there are aliens to be found! It’s possible that there is no other life, and it’s possible that there are aliens right next door. Until we find examples of life besides our own, we won’t know how common life is in the universe.
The second difference is that most scientists don’t think alien life is hiding from us; we just haven’t seen it yet. There are some ideas that more advanced civilizations may not be discovered, but researchers don’t think this is happening in our solar system.
Most astronomers and astrobiologists know that if we only look for life that is similar to Earth life, we might miss signs of aliens that are truly different. But honestly, we’ve never detected aliens before, so it’s hard to know where to start. If you don’t know what to do, starting somewhere is usually better than nowhere.
Searching for life using experiments such as the Viking lander or searching for oxygen may not help. But maybe we’re lucky. And even if not, we can cross some obvious possibilities off the list. Then we can focus on the harder question: imagining something we’ve never thought of before.
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This article is republished from The Conversation, an independent nonprofit organization providing 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.