The Abiotic Baseline

M. Kornmesser / ESO

Most astronomers agree that life is likely common throughout the Universe. While Earth is the only world known to have life, we know that life arose early on our world, and the building blocks of life, including amino acids and sugars, form readily. We also know there are countless worlds in the cosmos that might be home for life. But just because life is likely, that doesn’t mean proving it will be easy. Many of the biosignatures we can observe can also have abiotic origins. So how can we be sure? 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 alien astronomers light-years away were to observe Earth’s atmosphere as it transited the Sun, they would find the presence of oxygen, water vapor, and methane, all of which can indicate the presence of life. This would suggest the presence of life but not prove it. However, if they compared Earth’s atmosphere to those of Mars and Venus, Earth would stand out. Our sibling worlds have dry atmospheres of mostly carbon dioxide. Since planets of a solar system have similar chemical compositions, the fact that Earth’s atmosphere stands out makes a strong case for the presence of life. If Earth, Venus, and Mars all had atmospheres rich in water and oxygen, that would weaken the case for life on Earth.

Constantinou, et al

This is the idea behind a new study on the arXiv.¹ The authors propose that rather than looking at the atmospheres of individual worlds, we should look at the atmospheres of several planets within a system. Since most of the worlds are likely barren, a world with life will stand out. The team looks at our solar system and the Trappist-1 system. For our system, the results are glaringly obvious. Earth’s atmosphere is so unique that life is easy to detect compared to the rest of the system.

For Trappist-1, things are likely more subtle. The system has seven Earth-sized worlds, and since they all orbit close to their red dwarf star, they are likely tidally locked. As the authors show, even if one of the Trappist worlds harbors life, its atmosphere won’t necessarily stand out the way Earth does. So the authors propose using the atmospheres of all seven worlds to form what they call an abiotic baseline. They consider several molecules that are relatively easy to detect in exoplanet atmospheres, including oxygen, methane, nitrous oxide, and phosphine.

Constantinou, et al

Taken individually, each of these molecules has both biotic and abiotic origins, so the fact that a planet’s atmosphere contains them is not conclusive. But with an abiotic baseline for the system, astronomers could identify a planet that is a statistical anomaly. If a Trappist planet has an anomalous amount of these molecules, it would be strong evidence for the presence of life.

The authors note that this approach would still not be conclusive. But by identifying unusually strong candidates for life, astronomers could then gather more data to prove the presence of life.