Life in space

What is life?

One of the biggest challenges in the search for life in space is that we aren’t entirely sure what we are looking for. It may seem simple to look at a friend, a fish, or even a tree and recognise that it is alive. But what about an amoeba? Or a bacterium? Or a virus?

When life forms are very small, the boundary between non-living and living becomes harder to detect.

Scientists still don’t fully agree on what life is, but the best definition we have today comes from NASA:

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

So what does that actually mean? Let’s break it down into smaller parts:

A “chemical system” means that all life forms are made of chemistry. Like all other life on Earth, humans are made of cells, which are essentially tiny balloons filled with chemistry. The “balloons” are made of carbon and filled with water. A multitude of chemical reactions occur within the water, and it is these reactions that keep us alive.

That the chemical system must be “self-sustaining” means that the small carbon-water balloons we talked about before (our cells) must be able to keep themselves alive by obtaining energy from their surroundings. Plants and many bacteria get their energy from sunlight, and we animals get ours by eating food. To be able to use this 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 researcher who discovered that life changes over time and 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 in which it lives. Here on Earth, all life has a molecule called DNA in its cells, and it is by altering the DNA that life evolves and adapts.

When we search for life in space, what we are looking for is therefore:

• Liquid water and chemistry (preferably with carbon), because that is what we use in our cells.
• Energy in the surroundings, such as sunlight from a star, so that living things can keep themselves alive, and air for respiration.
• Something that evolves over time, and perhaps even something resembling DNA.

This means that we are not just looking for little green men or other aliens like those seen in films. We are searching 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 it might look completely different from anything we can imagine!

Life in the Solar System

We know that there is life on Earth! This shows that it is possible for life to exist elsewhere in the Solar System. But does that also mean there is life on the other planets?

This question has long been one we have tried to answer, first by observing the planets with telescopes and later by sending space missions and rovers (robotic vehicles) to them.

At first, we thought we would find life everywhere! That Venus would be a tropical paradise and Mars could be inhabited by intelligent civilizations.

But that was definitely not what we found. In fact, we have not found any life at all…

However, we do not give up, because even though there is not an abundance of life as we know it on Earth, there might still be microscopic life, or we could find evidence that life once existed on these planets, even if that life is now extinct.

The place we search for life most intensively is Mars. In fact, we have so many missions to Mars that we have written an entire page on Missions to Mars, where you can read much more about the hunt for life on the red planet.

Some other exciting places where we search for life in the Solar System are on certain moons of Jupiter and Saturn.

Europa and its planet Jupiter have been visited by many missions, such as the Voyager and Galileo missions, which have taken images of the moon and measured the temperature, chemistry, and magnetic field of its surface. It was from these measurements that scientists inferred that there are likely liquid oceans beneath the moon’s surface, and they have also found interesting chemistry on the surface that somewhat resembles the chemistry we know from life on Earth.

The same has happened for Enceladus, which, along with its planet Saturn, has been visited by missions such as Voyager and later Cassini. Cassini discovered that Enceladus has geysers that shoot liquid water hundreds of kilometres above the surface.

Although both Europa and Enceladus have liquid water, they lie very far from the Sun, so scientists are somewhat concerned that they may not have access to the energy that life needs to sustain itself. The geysers on Enceladus, and similar geysers on Europa, suggest, however, that there could be volcanic activity on the moons, and we know that volcanoes contain a lot of energy that life could use. It is therefore possible that the moons have enormous oceans filled with energy and chemistry, which could potentially be the perfect environment for life to arise.

We have not found life on these moons yet, but ESA and NASA have missions planned for both. The first, Europa Clipper and JUICE, have already been launched and are expected to arrive in 2030. Later missions are expected to land on the moons to search for signs of life on the surface, and perhaps even reach the deep oceans beneath the ice crusts.

Life around distant stars

As you can read on our “Exoplanets” page, the Solar System is not the only planetary system. Today, we know of more than 6,000 planets orbiting distant stars, and scientists are discovering new planets every day. Many of these exoplanets appear to resemble Earth both in size and in their distance from their star.

The distance from the star is important because it can determine whether there can be liquid water 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 into ice. The perfect distance from the star is what we call the habitable zone, and it is there that water can remain liquid.

Once we have found a planet in the habitable zone, we want to look for life on it. But this is difficult with exoplanets because they are so far away that we cannot send space missions to them in the same way we can with planets in the Solar System. It would take a whole 80,000 years to send a mission to the nearest exoplanet.

So instead, we observe these planets from afar using our telescopes, and telescopes such as the James Webb Space Telescope have been particularly important. James Webb can see the layer of air that surrounds many planets - what we call the atmosphere.

As we described earlier, life uses energy in the form of sunlight or food to keep itself alive, and in order to do this, a living organism must breathe. When humans breathe, we take one type of air into our lungs (oxygen) and expel another type of air (carbon dioxide). In this way, we change the air, or atmosphere, around us. Different types of life use different types of air, but all of them change the atmosphere around them. That is why we can detect life on a planet by studying its atmosphere, because the atmosphere looks very different if life is present.

If distant aliens looked at Earth’s atmosphere, they would see that the air that is most abundant is nitrogen, and the air that comes second is oxygen. If Earth had no life, there would be no oxygen at all. The aliens would therefore notice that something is unusual, because there is a lot of oxygen in the atmosphere that should not be there, and they could deduce that there might be life on Earth that is producing oxygen.

Our scientists try to do the same with exoplanets, and although they have not yet found definite signs of life, they have already discovered several exciting planetary systems. Among these is the TRAPPIST-1 system, which has seven Earth-sized planets, several of which lie in the habitable zone. And there is the water planet K2-18b, which is twice the size of Earth, but has an atmosphere, liquid water, and interesting chemistry.

Intelligent life

It is unknown how many different forms of life have ever existed on Earth, but scientists estimate 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 cosmos with telescopes and building 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 that most life in space will resemble us. Not even most life on Earth resembles us, and we are still related.

We still hope, however, that there could be aliens 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 we started the SETI project, which stands for “Search for Extra-Terrestrial Intelligence”, or “Søgen Efter Udenjordisk Intelligens”.

SETI listens out into space for signals from intelligent life trying to communicate with us. They are essentially listening for the sound of technology, particularly for so-called radio waves, which are the same waves we use in our radios, phones, and televisions. This is because we have discovered that radio waves appear to be the most effective signals to send across space, and we assume that intelligent aliens would reach the same conclusion and therefore also choose radio waves to communicate.

We have not yet received any signal, and whether this is because there are no other technologically intelligent life forms, or simply because we have not found them yet, is hard to say.

But here at the Planetarium, we believe it is important to remember that there are an incredible number of ways to evolve and many different forms of intelligence. Humans have a tendency to place ourselves on a pedestal, and it can be hard 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 oh, how wonderful it would be to finally meet those with whom we share the universe.