A new tool makes it possible to interpret future research on life on exoplanets

Is there life on a distant planet? Astronomers try to find out by analyzing the light scattered on a planet’s atmosphere. Some of this light, which comes from the stars it orbits, has interacted with its atmosphere and provides important clues about the gases it contains. If gases like oxygen, methane or ozone are detected, it could indicate the presence of living organisms. These gases are known as biosignatures. A team of scientists from EPFL and Tor Vergata University in Rome have developed a statistical model that can help astronomers interpret research results for these “signs of life.” Their research has just been published in Proceedings of the National Academy of Sciences (PNAS).

Since the discovery of the first exoplanet – a planet that revolves around a star other than the sun – 25 years ago, more than 4,300 more have been identified. And the list continues to grow: a new one is discovered every two or three days. About 200 of the exoplanets found so far are land-based, meaning they are mostly made up of rocks, like Earth. Although this is not the only condition required for a planet to be able to harbor life – it also needs to have water and to be at a certain distance from its sun – it is a criterion that the astronomers use it to focus their research.

In the years to come, the use of gas spectroscopy to detect biosignatures in the atmospheres of planets will become an increasingly important part of astronomy. Numerous research programs are already underway in this area, such as the CHEOPS exoplanet hunting satellite, put into orbit in December 2019, and the James-Webb optical telescope, scheduled for launch in October 2021.

Starting with a stranger

While much progress has been made in the detection of exoplanetary biosignatures, several question marks remain. What are the implications of this type of research? And how to interpret the results? What if a single biosignature is detected on a planet? Or what happens if no biosignatures are detected – what should we conclude? These are the kinds of questions that the scientists at EPFL-Tor Vergata wanted to answer with their new model.

Their work approaches the problem from a new angle. Traditionally, astronomers have looked for life on another planet based on what we know about life and biological evolution on Earth. But with their new method, scientists started from an unknown: how many other planets in our galaxy have some life form. Their model incorporates factors such as the estimated number of other stars in the galaxy similar to the sun and the number of terrestrial planets that could be orbiting within a habitable distance of these stars. It uses Bayesian statistics – particularly well suited to small samples – to calculate the probability of life in our galaxy based on the number of biosignatures detected: one, several, or none.

“Intuitively, it makes sense that if we find life on another planet, there are probably many more in the galaxy with some type of living organism. But how many?” says Amedeo Balbi, professor of astronomy and astrophysics in the physics department of Tor Vergata. “Our model turns this intuitive assumption into a statistical calculation and allows us to determine exactly what the numbers mean in terms of quantity and frequency.

“Astronomers already use various hypotheses to assess the credibility of life on a given planet,” explains Claudio Grimaldi, scientist at EPFL’s Complex Matter Physics Laboratory (LPMC), also affiliated with the Enrico Fermi Research Center at Rome. “One of our research objectives was therefore to develop a method to weigh and compare these hypotheses in the light of new data that will be collected in the years to come.”

Spread from one planet to another

Given the small number of planets that will likely be examined in the near future, and assuming that life will emerge independently on a single planet, the EPFL-Tor Vergata study found that if even a single biosignature is detected, we can conclude with a greater over 95% probability that there are over 100,000 inhabited planets in the galaxy – more than the number of pulsars, which are objects created when a massive star explodes at the end of its life. On the other hand, if no biosignature is detected, one cannot necessarily conclude that other forms of life do not exist elsewhere in the Milky Way.

Scientists have also looked into the theory of panspermia, according to which instead of emerging independently on one planet, life forms could be transported from another planet, for example by organic matter or transported microscopic organisms. on comets or propagating between neighboring planets. This implies that the probability of life on a planet also depends on its distance from other planets and the ease with which various forms of life – whose physical characteristics might be vastly different from those we know – are able to withstand extreme conditions of space. travel and adapt to the new planet. The inclusion of panspermia changes the inferred number of inhabited planets elsewhere in the galaxy.