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Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to curiouskidsus@theconversation.com.
Why do astronomers look for signs of life on other planets based on how life is on Earth? Couldn’t there be completely different forms of life on other planets? – Henry, age 13, Somerville, Massachusetts
Have you ever played hide and seek in a new place? It is much more difficult than playing at home. You only know the obvious hiding spots: under the bed, in the closet, behind the couch. The trick is trying to think of hidden spots that you can’t even imagine. How do you search in places you never thought might be hidden spots?
That’s kind of what scientists like me do when we look for alien life; we are trying to think of new ways to look at life. Meanwhile, we are looking for life by looking for life like us so what we can imagine.
A close look
The closest place to look for extraterrestrial life is on the planets within our solar system.
NASA’s Viking 1 mission began orbiting Earth’s neighbor Mars back in 1976. The search for life on Mars was one of the mission’s most important scientific questions. The spacecraft included a lander that could go to the surface of the planet to see if there were any forms of life in the dirt.
Scientists knew that life on Mars could be very different from life on Earth, so they did not look for specific life forms or molecules. Instead, they tried to design experiments to look at what life does, rather than what it does.
For example, plants and some other forms of life on Earth carry out photosynthesis, a process that uses sunlight and carbon dioxide in the air to gather energy and grow. The Viking 1 scientists designed the lander to search for signs of photosynthesis occurring on Mars.
To do that, the lander grabbed some dirt, shone a light on it and made measurements to see if any of the carbon dioxide in the air was transferred into the dirt. This experiment showed no signs of photosynthesis in Martian dirt.
The lander performed two other experiments that looked for evidence of organisms growing in the dirt on Mars. One used carbon dioxide gas and another used sugar and amino acid molecules that life forms on earth like to eat.
Because of the combination of these three experiments and other measurements most scientists have agreed that there is probably no life on the surface of Mars, at least life that does something like photosynthesis or eats sugar. But we still don’t know if there are signs of ancient life forms on Mars, or even current life deep below the surface.
The experiments on the Viking lander were the most direct tests for life on another planet. In hide-and-seek terms, though, these experiments were basically like looking in the closet: It’s a pretty obvious hiding place, but you should check there just in case. However, it took a long time for scientists to interpret the results.
Looking far away
Looking for life outside the solar system is even more difficult and requires different techniques.
The closest exoplanet – a planet orbiting a star other than our Sun – is Proxima Centauri b, and it’s more than 2 million, million miles (that’s 2 and 13 zeros after it) from Earth. These distant worlds are so far away that scientists are not going to send landers to experiment on them for a long time.
It’s like you’re searching for life on an extra-planet and you’re trying to hide in your neighbor’s house, but you can only see through the windows and you can’t go inside. You might get lucky and find the right angle to see someone hiding, but you never know all the spots you can’t see.
Instruments like the new James Webb Space Telescope can reveal the size of exoplanets, how close they are to their stars, and possibly the gases in their atmospheres. But that’s it. How would you view life with that?
Astronomers have thought about looking for life on exoplanets by looking for oxygen. They started with this strategy because on Earth, life forms made most of the oxygen in our atmosphere. Maybe alien life made oxygen on another planet.
However, we have learned that there are other ways to make oxygen that are not related to life. So now, astronomers don’t just look 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 they would have lifeless planets. But we’re still unsure about that, too!
Searching for life searching for these gases is like looking behind the sofa in our game of hide and seek. Do we know there will be anyone? But we only look through the windows, and we can imagine people hiding behind the beds. We might as well try – where else would we even look?
What game are we playing?
There are two big differences between playing hide and seek and searching for aliens.
At first, when you’re playing hide and seek, you usually know you’re playing with someone else. We have no idea if there are aliens out there to be found! It is possible that there is no other life out there, and it is possible that aliens are right next door. Until we find examples of life beyond our own, we will not know how common life is in the universe.
The second difference is that most scientists don’t think alien life is hidden from us; it’s just that we haven’t seen it yet. There are some ideas that more advanced civilizations could avoid detection, but researchers don’t think that’s happening in our solar system.
Most astronomers and astrologers know that if we only look for life that is similar to Earth life, we may miss the signs of aliens that are really different. But honestly, we’ve never felt aliens before, so it’s hard to know where to start. When you don’t know what to do, it’s usually better to start somewhere than nowhere.
It might not help to search for life using experiments such as the Viking landing or the search for oxygen. But we might be lucky. And even if not, we’ll have to cross a few obvious possibilities off the list. Then we can focus on the more difficult question of imagining something we haven’t thought of before.
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And since curiosity has no age limit – adults, let us know what you think too. We won’t be able to answer every question, but we’ll do our best.
This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of our complex world.
Written by: Cole Mathis, Arizona State University.
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Cole Mathis previously received funding from the NASA Postdoctoral Program.
