How can you tell if a planet has life? The light coming to us from the planet’s star would show signs of oxygen, water, or some other chemical that we associate with life.
The first problem with trying to find life on other planets is that we tend to look for the signs that our planet would give off because we know that there is life here and we know what kind of signs that life makes. It is hard for us not to do that because we can obviously see that life has evolved here and we know that life on Earth needs certain things, like water and sunlight. The problem is that just because life here needs that, doesn’t mean that life on other planets would need it as well. Still, until we know otherwise, looking for the things we need for life is a good way of trying to find life on other planets.
So, how can we search for life on other planets? It is very clever. It is not possible to look at a planet because ever our most powerful telescopes are nowhere near powerful to do that. The only thing that astronomers can see are the stars in other galaxies. So, they have to use that light to work out what the atmospheres on the planets are made of. The first thing they need to do, though, is to work out if there are actually any planets there. They do this by watching to see if the light from the star wobbles or blinks. If it does, then that is probably because a planet is passing in front of the star. The blink comes because the planet is blocking the star’s light and the larger the blink, the larger the planet. The wobble comes because the gravity of the planet will act upon the star and the larger the wobbler, the larger the planet. By timing the frequency of the blinks, astronomers can work out how quickly the star is orbiting and use all of this information to guess at its size and distance from the star. Everything is an educated guess because there is no way of actually knowing.
Once they have identified stars with planets, the next step is to work out what the planet’s atmospheres are made of. This can be done because different elements absorb different wavelengths of light. Light from the exoplanet’s sun passes around the planet and heads our way, where it can be picked up by a space telescope like the James Webb telescope. As the light from the sun goes through the exoplanet’s atmosphere, if there is one, different wavelengths of the light spectrum are absorbed by the chemicals. The rest of the light comes to our telescope and a spectrometer can break it down into wavelengths and work out what is missing. From that, a very good guess can be made at what the exoplanet’s atmosphere is made of. The trouble is that knowing the makeup of the atmosphere doesn’t mean they have found life.
To decide if there might be life on those planets or not, astronomers look for two things. They look for chemicals or combinations of chemicals that wouldn’t normally be produced naturally and they look for an ecosystem that is out of balance.
We know that oxygen is a sign of life here on Earth, but oxygen can be made naturally when the sun breaks down water in a planet’s atmosphere into hydrogen and oxygen. Methane is also a sign of life, but methane can be produced by volcanoes. Methane breaks down very quickly and if a planet had a lot of methane in its atmosphere for a long period of time, it might show that there is life because bio organisms produce methane here on Earth and it would take something like that to be replenishing the methane in the exoplanet’s atmosphere. Combinations of gases, like methane and oxygen, might show there is life, but they can be produced naturally. The chances are slim, but there is still a chance.
A better plan seems to be looking for environments that are out of balance. In any atmosphere, the amount of gases and elements tend to reach a balance. However, if the planet is very volcanically active, there may be far more of one gas than of another. The same is true if the planet has life. Life producing one gas over another would swing the planet out of balance. Having an unbalanced atmosphere doesn’t mean that there is life, but it is a sign that something could be going on there. The only real way to know if there is actually life there is to make larger and larger space telescopes, and then possibly send probes there. And this is what I learned today.
Photo by Felix Mittermeier: https://www.pexels.com/photo/blue-and-purple-cosmic-sky-956999/
Sources
https://knowablemagazine.org/content/article/physical-world/2023/how-to-detect-life-on-exoplanets
https://www.pnas.org/doi/full/10.1073/pnas.2117933119
https://science.nasa.gov/exoplanets/is-there-life-on-other-planets