For humanity, the discovery of evidence that there is life beyond Earth is unlikely to happen in a dramatic way, for example with the landing of an alien spacecraft on the side of the Eiffel Tower in Paris. It is much more likely to occur through telescope observations of a distant planet whose atmosphere contains chemicals indicative of biological activity.
On Monday, researchers said methane could be the first detectable sign of extraterrestrial life, if discovered in the atmosphere of a rocky planet orbiting in the “habitable zone” — an area neither too cold nor too hot. too much for the existence of water in liquid form on the surface of the planet—from a Sun-like star.
Scientists are working to understand the indicators of life — biosignatures — that may be present in observations of planets in other solar systems, so-called exoplanets, knowing that telescopes of ever greater capacity will soon be available.
In a study published in the Proceedings of the National Academy of Sciences, researchers demonstrated that the abundance of methane in the atmosphere can be a persuasive sign of life, in the right planetary context. Methane is an important waste gas in the Earth’s atmosphere, present in volume of less than two parts per million.
Unlike other potential biosignatures such as atmospheric oxygen, methane is one of the few gases that could be easily detected using the James Webb Space Telescope, launched in December by the US National Aeronautics and Space Administration (NASA) and being ready to go into operation in a few months.
“On Earth, the vast majority of methane is produced by life,” said Maggie Thompson, the paper’s lead author, a graduate student in astronomy and astrophysics at the University of California at Santa Cruz.
Most of the gas is generated directly by life: methane-producing microbes in wetlands, rice paddies, or in the digestive tracts of large animals. Methane is also generated by human activities such as the burning of fossil fuels, including coal and oil, which are remnants of dead organisms. The proportion of non-biologically generated methane on Earth is minuscule.
The researchers presented a threefold argument in favor of methane as a promising biosignature.
“First, it would not be surprising that life produces methane in other environments. Even though the biochemistry of alien life is radically different from what exists in Earth’s biosphere, methanogenesis is an obvious and easy metabolic strategy for any carbon-based life, given energy sources likely to be present on rocky exoplanets,” said Joshua Krissansen-Totton, study co-author and Nasa-Sagan researcher in the department of astronomy and astrophysics at the University of California, Santa Cruz.
Second, they said, methane would not persist for long in the atmosphere of habitable rocky planets without constant replenishment, possibly through living organisms. On Earth, atmospheric methane is unstable — the chemical effects of light destroy it — but there is a constant biological replenishment.
Third, according to the researchers, it would be difficult for nonbiological processes, such as volcanism or chemical reactions in oceanic mountain ranges and hydrothermal vents, to maintain replenishment without leaving “fingerprints” indicating that the methane was not generated biologically.
Gas-emitting volcanoes, for example, would release carbon monoxide along with methane, but biological activity tends to devour carbon monoxide and reduce its atmospheric concentration. Thus, they said, non-biological processes are not able to easily produce atmospheres rich in both carbon and methane on rocky planets, as is the case on Earth, but with little or no carbon monoxide.
Scientists are looking forward to new insights into the atmosphere of exoplanets will be offered by James Webb and other new telescopes, examining the chemistry as these distant planets pass in front of the stars whose systems they occupy, from Earth’s perspective.
Oxygen, more abundant than methane in the Earth’s atmosphere, is another potential biosignature. It also enters the Earth’s atmosphere via biological processes — in this case, photosynthesis in plants and microbes. But the James Webb will not be able to detect oxygen.
“Methane is not a hypothetical biosignature. We know that life on Earth has been producing methane for essentially its entire history, and atmospheric concentrations of methane may have been high early on on Earth, before there was oxygen in the atmosphere,” Krissansen said. Totton.
“But it’s important to point out that the diversity of planetary environments in other systems is likely vast, and there may be other non-biological methane-producing processes that no one has considered until now.”
Translation by Paulo Migliacci
