Why do astronomers look for signs of life on other planets based on life on Earth? Couldn't completely different kinds of life exist on other planets?
Cole Mathis: Have you ever played hide and seek in a recent place? It's much more challenging than playing at home. You only know the obvious hiding places: under the bed, in the closet, behind the sofa. The trick is to come up with hiding places that you can't even imagine. How to search in places you never thought could be hiding places?
This is what scientists like me do when we look for alien life; we try to come up with recent ways of searching for life. Meanwhile, we are looking for life, looking for life like ourselves, because that is what we can imagine.
Looking nearby
The closest place to look for extraterrestrial life are the planets of our solar system.
NASA's Viking 1 mission launched in orbit of Earth's neighbor Mars in 1976. The search for life on Mars was one of the most critical scientific issues faced by the mission. The spacecraft contained a lander that could fly to the planet's surface to see if there were any life forms in the ground.
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 test what life does, not what it creates.
For example, plants and some other forms of life on Earth perform photosynthesis, a process that uses sunlight and carbon dioxide from the air to store energy and grow. Scientists on Viking 1 designed the lander to look for signs of photosynthesis taking place on Mars.
To do this, the lander collected some earth, illuminated it with lithe, and took measurements to see if any carbon dioxide from the air had entered the ground. This experiment showed no signs of photosynthesis in the dirt on Mars.
The lander conducted two other experiments to look for evidence of organisms growing in the soil on Mars. One used carbon dioxide gas and another used sugar and amino acid molecules that like to eat life forms on Earth.
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 none that photosynthesizes or eats sugar. However, we still don't know whether Mars harbors traces of archaic life or even present life deep below the surface.
The Viking lander experiments were the most direct test of life on other planets. But when it comes to hide and seek, these experiments were like looking in a closet: it's a pretty obvious hiding place, but you should look there just in case. Still, it took scientists a long time to interpret the results.
Looking far away
The search for life outside the solar system is even more challenging and requires different techniques.
The closest exoplanet – a planet orbiting a star that is not our Sun – is Proxima Centauri b, more than 2 million miles (or 2 followed by 13 zeros) from Earth. These distant worlds are so distant that scientists do not intend to send landers to conduct experiments on them for a long time.
Looking for life on exoplanets is like playing hide and seek with your neighbor, with the difference that you only have to look through the windows and you can't go inside. You might get lucky and find the right angle to spot someone hiding, but you can't know all the places you can't see.
Tools like the recent James Webb Space Telescope can reveal the size of exoplanets, their distance from stars, and perhaps the gases in their atmospheres. But this is it. How would you go about life with this?
Astronomers have thought about searching for life on exoplanets by searching for oxygen. They started with this strategy because on Earth, life forms produce most of the oxygen in our atmosphere. Maybe the oxygen on another planet was created by alien life.
However, we have learned that there are other ways to produce oxygen that do not require life. So now astronomers aren't just looking for oxygen – instead, they're hunting for a planet that contains oxygen along with water and other gases like methane and carbon dioxide. Together, these combinations could point to life, because we don't think planets without life would have them. But we're still not sure about that!
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 out the windows and imagine people hiding behind the couches. We might as well try – where else would we look?
What game are we playing?
There are two gigantic 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 is possible that there is no other life there and it is possible that there are aliens living right next to it. 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 believe that alien life is hiding from us; we just haven't noticed it yet. There are some ideas that more advanced civilizations could avoid detection, but researchers don't think this happens 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 alien presence that are truly different. But honestly, we've never detected aliens before, so it's strenuous to know where to start. If you don't know what to do, it's better to start somewhere than nowhere.
Searching for life through experiments like the Viking lander or the search for oxygen may not assist. But maybe we'll get lucky. And even if that doesn't happen, we'll cross some obvious possibilities off the list. Then we can focus on the more challenging issue, which is imagining something we have never thought about before.
Cole Mathis, assistant professor of complicated adaptive systems at Arizona State University
This article has been republished from Conversation under Creative Commons license. Read original article.
Image Source: Pixabay.com
