New tool helps interpret future searches for life on exoplanets

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

Since the first exoplanet – a planet orbiting a star other than the Sun – was discovered 25 years ago, more than 4,300 more have been identified. And the list is still growing: a new one is discovered every two or three days. About 200 of the exoplanets found so far are telluric, which means that they are mainly composed of rocks, such as Earth. While this is not the only requirement for a planet to be able to host life – it must also have water and be a certain distance from its sun – it is one criterion that astronomers use to focus their search.

In the coming years, the use of gas spectroscopy to detect biosignatures in the atmospheres of planets will become an increasingly important element of astronomy. Many research programs are already underway in this area, such as the CHEOPS Exoplanet Hunting Satellite, which launched in December 2019, and the James-Webb Optical Telescope, scheduled to launch in October 2021. launch.

Start with an unknown

While much progress has been made in detecting exoplanetary biosignatures, several question marks remain. What are the implications of this type of research? And how should we interpret the results? What if only one biosignature is discovered on a planet? Or what if no biosignatures are detected – what should we conclude? Such questions are what the scientists of EPFL-Tor Vergata tried to answer with their new model.

Her work tackles the problem from a new angle. Traditionally, astronomers have searched for life on another planet based on what we know about life and biological evolution on Earth. But with their new method, scientists began with an unknown: how many other planets in our galaxy have some form of life. Their model includes factors such as the estimated number of other stars in the galaxy equal to the Sun and how many telluric planets can orbit at a habitable distance from those stars. It uses Bayesian statistics – which are particularly well-suited for small sample sizes – to calculate the chance of life in our galaxy based on how many biosignatures are detected: one, multiple or none at all.

“Intuitively, it makes sense that if we find life on one other planet, there are probably many others in the galaxy with some sort of living organism. But how many?” says Amedeo Balbi, a professor of astronomy and astrophysics in Tor Vergata’s physics department. “Our model transforms that intuitive assumption into a statistical calculation, and lets us determine exactly what the figures mean in terms of quantity and frequency.”

“Astronomers are already using different assumptions to evaluate how believable life is on a given planet,” said Claudio Grimaldi, a scientist at EPFL’s Laboratory of Physics of Complex Matter (LPMC) who is also affiliated with the Enrico Fermi Research Center in Rome. . “So one of our research goals was to develop a method for weighing and comparing those assumptions in light of the new data that will be collected in the coming years.”

Distribution from one planet to another

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

The scientists also saw the theory of panspermia, which states that instead of rising independently on a given planet, life forms could be transferred from another planet – such as through organic matter or microscopic organisms carried on comets or distribution between adjacent planets. This implies that the chance of life on a planet also depends on how far it is from other planets and how easily different life forms – whose physical characteristics may be extremely different from those we are familiar with – are capable of the extreme conditions of space to resist travel and adapt to the new planet. Factoring in panspermia changes the number of inhabited planets delivered elsewhere in the galaxy.