Led by professor and astrophysicist Lars A. Buchhave, a group of researchers at DTU Space are on the hunt for life out in the Milky Way. This is done by studying exoplanets—planets outside our own solar system. Find out more about what the researchers are looking for and when they can say something about whether life on Earth is an exception.
Is it likely that there is life elsewhere?
That is precisely the question we would like to answer. On the one hand, life on Earth is the result of a chain of countless events, some of which are completely random. On the other hand, there are so many possibilities out there in the universe. Our galaxy—the Milky Way—alone has 3-400 billion stars, most of which have planets orbiting them, and many of these are planets like Earth. And bearing in mind that, in addition to the Milky Way, there are another 1,000 billion galaxies in the universe, there are truly many possibilities where life could, in principle, arise.
What are you looking for?
In our research group, we look for Earth-like planets, i.e. small terrestrial (or rocky) planets located in the habitable zone of their solar system. It is the zone where the temperature allows liquid water to exist on the planet. In other words, the planet is not so far away from its star that it is freezing cold and the water is frozen, nor is it so close that everything is boiling.
We are currently searching for these planets in solar systems with a star that is smaller than our own sun, because this gives us the most favourable conditions for observing the planets’ atmospheres. This type of star is one of the most common in the Milky Way. And we can see that almost half of these stars have one or more planets in the habitable zone. In other words, Earth-like planets in the habitable zone are extremely common.
How can you tell if there is life?
The atmosphere can reveal whether there is life on a planet. We know this from the only planet with life that we know of—Earth. Our atmosphere is very different from the atmospheres on Venus and Mars, for example, where there is no life.
Where their atmospheres consist mainly of CO2, and almost no oxygen, the Earth’s atmosphere contains a great deal of oxygen—approximately 21 per cent. This is due to photosynthesis, which converts CO2 into oxygen. So the oxygen in our atmosphere is caused by the biology we have on Earth, and that is why we consider oxygen, along with methane, to be an extremely strong biosignature—that is, a sign of life.
Can life be anything other than what we know on Earth?
Yes, that’s definitely possible. However, the earliest traces of life on Earth date back 3.8 billion years. Our Earth is ‘only’ 4.5 billion years old, and it probably took a couple of hundred million years before the planet cooled down enough for life to begin. So life originated relatively soon after that.
The fact that this can happen in such a short time—from an astrophysicist’s perspective—may be because the building blocks of life here on Earth consist of the most common elements in the universe, such as hydrogen, oxygen, carbon, and nitrogen. These substances are also capable of forming the complex organic molecules that are part of our biology, such as proteins, enzymes, DNA, etc.
Furthermore, these elements are extremely common throughout the universe, which reinforces the argument that if we are to search for life, it makes sense to look for something that has emerged from the most common materials, just like ourselves.
But we are very careful not to become too ‘Earth centric’—i.e. believing that all life is similar to what we know. Therefore, we will also pay attention to planets with atmospheres that reveal a surprising composition of substances far from chemical equilibrium, which we cannot immediately explain through, for example, geological or photochemical processes.
How do you determine the composition of the atmosphere on a distant planet?
We are utilizing the James Webb Telescope, which was launched in 2021. We measure the exoplanet when it moves in front of its own star. We can measure the light that passes through the planet’s atmosphere and out to the other side, i.e. out to us. Not all light will pass through the planet’s atmosphere, because some of it will be absorbed by the substances in the atmosphere. Different substances absorb light at different wavelengths (i.e. different colours). So by analysing the light at different wavelengths, we can determine which substances make up the exoplanet’s atmosphere.
When will we know if there is life out there?
We are still in the early stages of exploration. In our group, we are currently investigating the Trappist-1 planetary system, which is located approximately 40 light-years away and has seven Earth-like planets, at least two of which are in the habitable zone. It has not yet been possible to observe the atmosphere of an Earth-like planet, but the Trappist-1 system is one of the best options to do so for the first time. In addition, we are looking at nine other exoplanets to find out if they have an atmosphere.
I believe that within perhaps 20 years, when we expect new space telescopes to be launched, we will find out whether there are planets nearby where we can see signs of life. So within a few decades, we will have an idea of whether life is common in the universe or whether we are an exception.
