Interpreting future searches for life on exoplanets

Is there life on a distant planet?

Are we alone in the Universe?

One of the primary goals for astronomy is the remote search for biosignatures in exoplanets. They are bringing various ideas to search for life outside Earth. This is a great intellectual adventure they are on.

One of the ideas includes analyzing the light that is scattered off a planet’s atmosphere. Some of that light emitted from the stars it orbits has interacted with its atmosphere and gives significant insights to the gases it contains. If gases like oxygen, methane, or ozone are identified, that could demonstrate living beings’ presence. Such gases are known as biosignatures.

EPFL scientists, in collaboration with the Tor Vergata University of Rome, have come with a model that can help astronomers interpret the results of the search for these “signs of life.”

With the discoveries of exoplanets, much progress has been made on detecting exoplanetary biosignatures. But, several questions remain unsolved. What are the implications of this kind of research? And how should we interpret the results? What if just one biosignature is detected on a planet? Or what if no biosignatures are detected – what should we conclude? 

A new model set out answers such kinds of questions.

Traditionally, astronomers have looked for life on another planet based on what we know about Earth’s growth and biological evolution. But with their new method, the scientists started with an unknown: how many other planets in our galaxy have some form of life.

The model incorporates factors like the evaluated number of different stars in the galaxy like the Sun and what number of telluric planets may be orbiting inside a habitable distance from those stars. It utilizes Bayesian statistics – especially appropriate to small sample sizes – to figure the likelihood of life in our galaxy dependent on how many biosignatures are identified: one, several, or none at all.

Amedeo Balbi, a professor of astronomy and astrophysics in Tor Vergata’s Physics Department, said, “Intuitively, it makes sense that if we find life on one other planet, there are probably many others in the galaxy with some living organism. But how many?”

“Our model turns that intuitive assumption into a statistical calculation, and lets us determine exactly what the numbers mean in terms of quantity and frequency.”

Claudio Grimaldi, a scientist at EPFL’s Laboratory of Physics of Complex Matter (LPMC), also affiliated with the Enrico Fermi Research Center in Rome, said, “Astronomers already use various assumptions to evaluate how credible life is on a given planet. One of our research goals was to develop a method for weighing and comparing those assumptions in light of the new data collected over the coming years.”

Given the small number of planets that will likely be examined in the near future, and assuming that life will emerge independently on any one planet, the EPFL-Tor Vergata study found that if even just one biosignature is detected, we can conclude with a greater than 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.

Journal Reference:
  1. Amedeo Balbia et al. Quantifying the information impact of future searches for exoplanetary biosignatures. DOI: 10.1073/pnas.2007560117

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