Life in space

What is life?

One of the greatest challenges in the search for life in space is that we are not entirely sure what we are looking for. It might seem simple to look at a friend, or a fish, or perhaps even a tree, and see that they are alive. What about an amoeba? Or a bacterium? Or a virus? When life forms become very small, the transition from non-living to living becomes harder to identify.  

Researchers still do not fully agree on what life is, but the best definition we have today is NASA's, which reads:

"Life is a self-sustaining chemical system capable of Darwinian evolution."

What does this actually mean? Let's try to break it down into smaller parts:

A “chemical system” means that beneath it all, every living thing is driven by chemistry. We can think of our cells as tiny balloons made of carbon and filled with water. Carbon is a remarkably social atom: it loves to bond with other atoms, and can form long chains and complex shapes that no other element can quite match. This makes it the perfect building block for the complicated molecules that life needs. Inside each of those tiny balloons, a constant flurry of chemical reactions is taking place: molecules breaking apart and joining together, releasing energy, building new structures, and passing information around. It is those reactions, happening in trillions of balloons all at once, that keep us alive. Every living thing on Earth, from the tallest tree to the smallest bacterium, runs on this same basic chemistry.

For the chemical system to be "self-sustaining" our cells must be able to keep themselves alive by drawing energy from their surroundings. Plants and many bacteria get their energy from sunlight, while animals get energy by eating food. To be able to use that energy, we also need to breathe, so all life breathes in different ways.

The final part of the definition of life is that it must be capable of "Darwinian evolution." Charles Darwin is the scientist who discovered that life changes over time, and that it adapts to its surroundings in order to survive. We believe that all life must be able to evolve and adapt to survive, which means that life will often be perfectly suited to the environment it lives in. Here on Earth, all life has a molecule called DNA in its cells, and it is by changing the DNA that life evolves and adapts.

Therefore, when searching for life in space, we look for three specific aspects:

• Liquid water and chemistry (preferably involving carbon), because that is what we use in our cells.
• Energy in the surroundings, such as sunlight from a star, so that life forms can sustain themselves, and air for breathing (or more generally, a way to release and use energy from their surroundings)
• Something that changes over time, and perhaps even something resembling DNA.

This means we are not only looking for little green men or other aliens like the ones you see in films. We are looking for all kinds of life, and we actually expect that much of the life we find will resemble bacteria more than it resembles us, or perhaps it will look completely different from anything we can even imagine!

Life in the Solar System

We know that life exists on Earth, which means that life in the Solar System is possible. But does that also mean there is life on the other planets in the Solar System?

That is a question we have long tried to answer, first by observing the planets through telescopes and later by sending space missions and rovers (robotic vehicles) to them. In the beginning, we thought we would find life everywhere! That Venus would be a tropical paradise and that Mars might be home to intelligent civilisations. That is certainly not what we found. In fact, we are yet to find any kind of life-form.

However, scientists from around the world are far from giving up! Even if there is no abundance of life as we know it from Earth, there might still be microscopic life, or we might find clues that life once existed on these planets, even if it doesn’t exist today.

The place where we search most actively for life is Mars. There have been and there will be a number of missions to Mars. You can read all about it in the section Missions to Mars, where you can also learn a lot more about the search for life on the red planet.

Some other exciting places where scientists are looking for life-forms in the Solar System are on some of the moons around Jupiter and Saturn.

Two of these moons are the ice moons Europa and Enceladus. Both are thought to have liquid saltwater oceans beneath their icy crusts, which is exciting because liquid water is one of the things we know is important for life.

Europa and its planet Jupiter have been visited by many missions, such as Voyager and Galileo, which photographed the moon and measured its surface temperature, chemistry, and magnetic field. It was from these measurements that scientists concluded there are probably liquid oceans beneath the surface, and some interesting chemistry has also been found on the surface that resembles the chemistry we associate with life on Earth.

The same is true for Enceladus, which together with its planet Saturn was visited by the Voyager missions, and later Cassini, which discovered that Enceladus has geysers shooting liquid water several hundred kilometres above the surface.

Although both Europa and Enceladus have liquid water, they lie very far from the Sun, and researchers are somewhat concerned about whether they have access to the energy that life needs to sustain itself. However, the geysers on Enceladus and possible similar activity on Europa suggest there may be volcanic processes at work on these moons, and we know that such activity can release a great deal of energy that life could make use of. It is therefore possible that these moons harbour enormous oceans filled with energy and rich chemistry, which could perhaps be the perfect environment for life to arise.

Researchers have not yet found life on these moons, but both ESA and NASA already have missions on their way. Europa Clipper, launched in 2024, is expected to arrive at Jupiter in 2030, while ESA's JUICE mission will follow in 2031. These will be the first dedicated explorations of Europa and the broader Jupiter system in decades. Looking further ahead, ESA is also planning a dedicated mission to Enceladus itself, with an orbiter and lander, though that journey is still many years away with a launch targeted for the 2040s.

Life Around Distant Stars

As you can read in our section Exoplanets, the Solar System is not the only system with planets. Today we know of more than 6,000 planets around distant stars, and researchers are discovering new planets every day. Many of these exoplanets appear to resemble Earth both in their size and in how far they lie from their star.

The distance from the star matters because it can be crucial for whether liquid water can exist on the planet: if the planet is too close to its star, it will be too hot, and the water will begin to boil and evaporate. If the planet is too far from its star, it will be too cold and the water will freeze. The just-right distance from the star is what we call the habitable zone, and it is there that water can remain liquid.

Once a planet in the habitable zone is found, the natural next step is to search for traces of life. This is however quite difficult with exoplanets, since they are so far away that we cannot send space missions in the same way we can with the planets in the Solar System. It would take as long as 80,000 years to send a space mission to the nearest exoplanet!

So what scientists do instead is observe the planets from a distance using telescopes. One of the main protagonists in the hunt for exoplanets is  the James Webb Space Telescope. James Webb can detect the atmosphere, or in other words, the layer of air surrounding many planets (including Earth!).

As described earlier, life uses energy in the form of sunlight or food to sustain itself, and to do this, life forms need to breathe. When humans breathe, they breathe in one kind of air into the lungs (oxygen), and breathe out another kind (carbon dioxide). Through this process we are able to change the air, or the atmosphere, around us. Different kinds of life use different kinds of air, but all of them affect the atmosphere around them. The result of this process gives us remarkable information: life leaves its mark on the air around it, and by studying a planet's atmosphere from afar, we can look for those telltale chemical signs.

If distant aliens were to look at Earth's atmosphere, they would see that the most abundant gas is nitrogen, and the second most abundant is oxygen. If Earth had no life, there would be no oxygen at all, so the aliens would immediately notice that something was different, and that there must be something releasing oxygen into the atmosphere.

Our researchers are trying to do the same with exoplanets, and although they have not yet found definitive signs of life, they have already identified several exciting planetary systems: the TRAPPIST-1 system, which has seven planets the size of Earth, several of which lie in the habitable zone, and K2-18b, which has both an atmosphere, liquid water, and intriguing chemistry (see the Exoplanets section for additional info on these systems).

Intelligent Life

It is unknown how many different forms of life have existed on Earth, but researchers have estimated anywhere between two million and five trillion different species. What we know for certain is that only ONE of them developed technological intelligence. Only one species began observing the universe with telescopes and built rockets. One technological species is really not very many, if there have been five trillion different species on Earth.

This is why we do not expect most life in space to resemble us. Because not even most life on Earth resembles us, and we are even related.

We still hope, however, that there might be extra-terrestrial life forms who have evolved in a similar way to us, and who have also invented technology, because that would mean we might be able to communicate with them. This is why scientists started the SETI project, which stands for "Search for Extra-Terrestrial Intelligence."

SETI listens out into space for signals from intelligent life trying to communicate with us. They search for signals that nature alone could not produce, the kind that only technology can generate, and especially for so-called radio waves (the ones we use in our radios, phones, and televisions). Radio waves appear to be the most effective signals to send through space, and we can hope that intelligent aliens would arrive at the same conclusion, and therefore also choose radio waves to communicate with.

We have not yet received any signal, and whether that is because there are no other life forms with technological intelligence, or simply because they are too far away and we have not found them yet.

However, we at the Planetarium believe it is important to remember that there are an unimaginable number of different ways to evolve, and many different forms of intelligence. We humans have a slight tendency to place ourselves on a pedestal, and it can be difficult for us to accept that we are just one of many life forms on Earth, each excelling in its own survival strategy. So even if the life we find in space does not look or think like us, and even if it does not use technology as we do, it should still be met with the greatest respect and curiosity. And how wonderful it would finally be to meet those we share the universe with!