Most astronomers agree that life in the entire universe is probably common. While the earth is the only world that is known that it has a life, we know that life in our world was created early, and the building blocks of life, including amino acids and sugar, are easy. We also know that there are countless worlds in the cosmos that could be at home for life. But just because life is likely does not mean that it will be easy. Many of the biosignatures we can observe can also have abiotic origins. So how can we be safe? One way is to compare our observations of a habitable world with other worlds in the system.
Our own solar system is a good example of this. If extraterrestrial astronomers were removed, if the earth's atmosphere is overrising the sun, they would find the presence of oxygen, water vapor and methane, all of which can indicate the presence of life. This would indicate the presence of life, but would not prove. However, if you compared the atmosphere of the earth with that of Mars and Venus, the earth would stand out. Our siblings have dry atmospheres from mainly carbon dioxide. Since planets of a solar system have similar chemical compositions, the fact that the earth's atmosphere emerges is a strong argument for the presence of life. If the earth, Venus and Mars all had atmospheres rich in water and oxygen, this would weaken the case for life on earth.
This is the idea behind a new study on the Arxiv. The authors suggest that we look at the atmospheres of several planets within a system instead of the atmospheres of individual worlds. Since most worlds are probably sterile, a world will notice life. The team looks at our solar system and the Trappist 1 system. The results are clearly obvious to our system. The earth's atmosphere is so unique that life is easy to recognize compared to the rest of the system.
How an abiotic baseline strengthens observations. Credit: Konstantinou et al.
For Trappist-1, things are probably more subtle. The system has seven earth -sized worlds, and since they all circle near their red dwarf star, they are probably neatly locked up. As the authors show, his atmosphere, even if one of the Trappist worlds contains life, will not necessarily drop as the earth does. So the authors suggest using the atmospheres of all seven worlds to form an abiotic baseline. They take into account several molecules that are relatively easy to grasp in exoplanet atmospheres, including oxygen, methane, laughing gas and phosphine.
Each of these molecules has both biotic and abiotic origins, so that the fact that the atmosphere of a planet contains is not conclusive. With an abiotic baseline for the system, however, astronomers could identify a planet that is a statistical anomaly. If a trappist planet has an anomal amount of this molecules, it would be strong proof of the presence of life.
The authors find that this approach would still not be conclusive. But through the identification of unusually strong candidates for life, astronomers could then collect more data to prove the presence of life.
Reference: Konstantinou, Tereza et al. “Comparative biosignatures.” Arxiv Preprint Arxiv: 2505.01512 (2025).
