If methane were detected in the atmosphere of an exoplanet located in the habitable zone of its star, it would be a decisive sign to establish that there is life outside the solar system, if it can be confirmed that it has a biological origin. The James Webb telescope could identify it.
Researchers at the University of California, Santa Cruz have found that methane may be the first detectable sign of extraterrestrial life if it is discovered in the atmosphere of a rocky planet orbiting in the “habitable zone” and proven to come from biological sources.
In astrophysics, it is called habitable zone to the region around a star in which a temperature that is neither too hot nor too cold would allow the presence of liquid water on the surface of any rocky planet (or satellite) that orbits it and also has a mass suitable for life, among other conditions.
Scientists have long been investigating biosignatures, or indicators of life, that might be present in observations of planets from other solar systems.
In a study published in the journal Proceedings of the National Academy of Sciences (PNAS), researchers led by Maggie Thompson conducted an in-depth assessment of the planetary conditions necessary for methane to be considered a reliable biosignature, showing that atmospheric methane abundant could be a determining sign for life in the right planetary context.
Methane, key to life
Methane is a major gas in Earth’s atmosphere, with a concentration of 1,774 parts per billion (ppb). It is one of the few gases that should be easily detectable with the James Webb Space Telescope, launched by NASA in December and which will be operational in a few months, the researchers point out.
On Earth, most methane is generated directly by life: methane-producing microbes in wetlands, rice fields, or in the intestines of large animals.
Methane, a very powerful greenhouse gas, is also generated through human activities, such as the burning of fossil fuels, including coal and oil.
The most important clue as a sign of life is probably the amount and origin of methane in an atmosphere. On Earth, the proportion of non-biologically generated methane on Earth is minuscule: much less methane is added by volcanic eruptions than by human activities, let alone by natural biological sources.
outside the solar system
The researchers note that it would not be surprising if life elsewhere in the universe also produced methane. Even if the biochemistry of extraterrestrial life were radically different from that of Earth’s biosphere, methanogenesis is an obvious and easy metabolic strategy for any carbon-based life, given the energy sources that would likely be present on rocky exoplanets, they add.
Second, they believe that methane would not last long in the atmospheres of habitable rocky planets without constant replenishment, possibly by living organisms. On Earth, atmospheric methane is unstable, destroyed by the chemical effects of light, but is in constant biologically generated replenishment.
Third, they conclude, it would be difficult for non-biological processes, such as volcanism or chemical reactions at mid-ocean ridges and hydrothermal vents, to sustain replenishment without leaving a “fingerprint” indicating that the methane was not biologically generated.
new telescopes
Gas-releasing volcanoes, for example, would release carbon monoxide along with methane, but biological activity tends to gobble up carbon monoxide and reduce its atmospheric concentrations so it would be easily detectable.
Therefore, they explain, non-biological processes cannot easily produce rocky planet atmospheres rich in methane and carbon dioxide, as on Earth, so their conclusion is that methane is a safe biosignature for exploring exoplanets.
Scientists hope to further understand exoplanet atmospheres using the new James Webb Telescope and other new ones, examining their chemistry as these distant worlds pass in front of their host stars from Earth’s perspective.
Oxygen too, but…
Oxygen, more abundant in Earth’s atmosphere than methane, is another potential biosignature. It also enters the Earth’s atmosphere through biological processes, in this case, photosynthesis by plants and microbes. But the oxygen could elude detection by the Webb telescope, the researchers say.
They conclude that methane is not, therefore, a hypothetical biosignature, since life on Earth has been producing methane for virtually its entire history, and atmospheric methane concentrations may have been high on the early Earth, before there was oxygen in the atmosphere.
They note, however, that the diversity of planetary environments elsewhere is probably enormous, and that there could be other non-biological methane production processes that no one has yet considered.
Reference
The case and context for atmospheric methane as an exoplanet biosignature. Maggie Thompson et al. PNAS, March 28. DOI:10.1073/pnas.2117933119