The idea of finding life in space is something that has fascinated humans for a very long time. It has been depicted in books and movies endlessly, and there are even religions based on the idea of life in space.
With that in mind, it is no wonder that we are looking for life in space now that the technology allows us to do so. We have space telescopes looking to find exoplanets that are hopefully within what is known as “the habitable zone” of their star, and we have sent several rovers to Mars in our quest to find life.
However, according to Kai Finster, professor of Astrobiology at Aarhus University, the life we are looking for doesn’t exactly spell E.T.:
“It is unlikely that there is no other place with life, but no one is looking for a green man anymore.”
Life as we know it can be incredibly small. That’s why we have to look for other indicators first when we look for life in space.
There is general agreement that water is necessary to all life. Professor Finster explains that this is because water is not only a solvent, but also universal:
“There is plenty of water all around; it is made up of very common element. It is the most common solvent in the universe. It also has a lot of properties which are favorable to life. It has a very high heat capacity, so it would regulate climate in a perfect way. It has properties that allow to dissolve salts, and stabilize ions, which is necessary in many biochemical reactions. It also supports the formation of membranes, which are necessary for all cellular life, as we know it from Earth.”
Therefore, to look for life in space, we start by looking for water. Again, water can come in many shapes and sizes, and not all of them would show up in a picture taken by a telescope. Instead, we rely on mathematics to calculate the probabilities of water’s presence on a planet.
We know our galaxy is full of stars. We also know that there is no star without at least one planet. Based on the size and temperature of the star, we can calculate a zone around any given star in which we believe it will be possible for water to exist in a liquid form on a planet. This is what is called the “habitable zone”.
There are many other factors to take into consideration when looking for life in space, but the first rough sorting of candidates is done by looking at the planets that are in their star’s habitable zone.
For our own Solar System, Earth is right in the middle of the zone, while Mars is at the edge. This is why we are currently looking for life on Mars; it is the closest planet to us that is also in the habitable zone, so it is the easiest to get to (read: the second nearest planet inside a habital zone would be “Proxima d” which is in our next-door neighbor solar system some 4.2 light years-away. In a modern rocket that journey would take more than 150,000 years)
Aside from the fact that Mars is in the habitable zone, photographic evidence shows what is believed to be ancient river deltas – in fact, the Perseverance Rover’s landing site was in one such deltas. This is a clear indication that water has once been present on Mars, meaning that life can have existed there.
According to professor Finster, any life we may find in space will most likely be unicellular. This is based on what we know of life’s evolution on Earth.
First, we know that there has been life on Earth for 4 billion years now, and that for 3 billion of those years, life was unicellular. Of course, we may find a planet somewhere far away that has a much longer timeline of life, which would increase the chances of multicellular life, but based on the statistics of our own timeline, unicellular life is more likely.
Second, we know that multicellular life only forms and survives based on a lot of coincidences. Professor Finster explains:
“One of the major coincidences is to develop the evolution of oxygenic photosynthesis as the photosynthesis that produces oxygen, because without oxygen, you could not have complex life.”
Somehow, those coincidences happened on Earth, but the chances of a similar string of coincidences leading to multicellular life happening on another planet are slim. The chance is so mathematically insignificant that scientists have judged it to be unlikely.
“If there is any life in space, it will be unicellular.”
When searching for life in space, we base our definition of life on what we find on Earth. Obviously, life in space might be completely different from life on Earth – it might actually be so alien that we cannot recognize it as life. However, it will be very difficult to look for something we don’t know what is, so it makes sense that we transfer our definition of life to the expectations we have for life in space.
Because of this, scientists try to prepare for looking for life in space by looking for what is called analogues on Earth: places that are similar and have similar properties to what we see in Space.
Professor Finster explains that for Mars, we know that it is a dry, cold place, so to find an analogue on Earth we turned our attention to Antarctica:
“In Antarctica, we have the McMurdo dry valleys where it is extremely cold and extremely dry, with very little biomass. We would then go there and see how organisms can survive under these conditions.”
Based on those findings, suppositions are made on where and how life might be found on Mars. For example, scientist now agree that the chances of finding life on Mars is greater underground than on the surface. Of course, you then have to take the differences into account too – the atmosphere and the levels of radiation from the Sun are different on Mars, both things that would also affect possible life there.
However, a lot of our preparations for rover missions for example, come from finding these analogues and using the knowledge about them to predict what will be necessary to ensure the missions have the best possible conditions to succeed.
Several rovers are currently on Mars and more are planned to be sent there. It is safe to say that Mars will be thoroughly searched for life before we turn our attention to any exoplanets also in the habitable zone.
The expectation is unicellular life, if any, and that won’t change when looking at other planets outside of our own Solar System.
Currently, we do not have the technology to reach even the nearest exoplanet to us that is also in the habitable zone of its star within a reasonable timeframe, but the technology is constantly developing and pushing the limits for what we thought was possible.
Someday we are going to find life in space. Whether it will be on Mars or a distant exoplanet farther in the future is yet to be seen.