How biosignatures could indicate life on distant planets

Oxygen, ammonia and other gases in exoplanet atmospheres could be an indicator of biological processes.

Gases - or biosignatures - indicating life may be detectable on exoplanets orbiting M-class red dwarfsCredit: Darryl Fonseka/iStock/Getty Images
Published: March 22, 2022 at 9:33 am
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Over the coming years, as astronomers use spectroscopy to read the atmospheres of Earth-sized, habitable exoplanets, detecting the presence of one gas will be an important discovery: oxygen.

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On Earth, oxygen is released by life – specifically, by organisms using sunlight for energy.

Oxygen is a very reactive gas. Early in Earth’s history any oxygen released into the atmosphere was rapidly removed.

It reacted with rocks, or was destroyed by photochemical reactions driven by ultraviolet rays in sunlight.

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The moment human beings saw the disc of Earth against the blackness of space with their own eyes. This image was captured during Apollo 8, 21 December 1968. Credit: NASA / restored by Toby Ord
Credit: NASA / restored by Toby Ord

Such processes are known as ‘sinks’, and oxygen only started to accumulate in Earth’s atmosphere once its production had overwhelmed these sinks.

An oxygen-rich atmosphere is thought to be a sign of flourishing life on a world, which is why astronomers would be so excited about discovering one on an exoplanet.

But is oxygen the only ‘biosignature’ gas that would indicate the presence of life on an exoplanet?

Might other gases produced by biochemistry also be able to overwhelm the sinks on their exoplanets and accumulate to detectable levels?

Sukrit Ranjan, at Northwestern University, and his colleagues have been investigating this.

This artist’s impression shows the exoplanet LHS 1140b, which orbits a red dwarf star 40 light-years from Earth and may be the new holder of the title “best place to look for signs of life beyond the Solar System”. Using ESO’s HARPS instrument at La Silla, and other telescopes around the world, an international team of astronomers discovered this super-Earth orbiting in the habitable zone around the faint star LHS 1140. This world is a little larger and much more massive than the Earth and has likely retained most of its atmosphere.
Artist’s impression of exoplanet LHS 1140b, which orbits a red dwarf star 40 light-years from Earth.

The best candidate worlds for detecting such biosignature gases, they argue, are exoplanets orbiting small, cool M-class red dwarf stars.

Such stars emit less ultraviolet radiation than larger, hotter stars like the Sun, and so the sinks on those planets are much weaker.

Exoplanets orbiting M-class red dwarfs offer favourable conditions for the accumulation of reactive gases to levels that we could hopefully detect with space telescopes.

This is one of the reasons why the James Webb Space Telescope (JWST) will be targeting these stars.

Artist's impression of the James Webb Space Telescope. Credit: ESA, NASA, S. Beckwith (STScI) and the HUDF Team, Northrop Grumman Aerospace Systems / STScI / ATG medialab
Artist's impression of the James Webb Space Telescope. Credit: ESA, NASA, S. Beckwith (STScI) and the HUDF Team, Northrop Grumman Aerospace Systems / STScI / ATG medialab

Ranjan’s team considered an exoplanet with a hydrogen/nitrogen atmosphere in orbit around a M-class red dwarf.

The biochemistry of life on such a world might produce ammonia (like on Earth, where Rhizobacteria help nitrogen-fixing plants to draw nitrogen gas from the air).

This scenario has been nicknamed a ‘Cold Haber World’, after the process that uses heat, pressure and a catalyst to convert atmospheric nitrogen into ammonia for fertilisers.

The research team simulated a Cold Haber World with a climate suitable for oceans of liquid water, Earth-like volcanism and an atmosphere with the same surface pressure as Earth, but made up of 90% hydrogen and 10% nitrogen.

Ammonia in the atmosphere of a rocky exoplanet could be an important biosignature in the search for signs of life beyond the Solar System. Credits: NASA/JPL-Caltech
Ammonia in the atmosphere of a rocky exoplanet could be an important biosignature in the search for signs of life beyond the Solar System. Credits: NASA/JPL-Caltech

They simulated how quickly life on such a planet would release ammonia and the rate at which atmospheric ammonia would be destroyed by the dwarf star’s sunlight.

They found that for realistic biological ammonia production rates, the gas can overwhelm the sinks on the planet and accumulate to notable levels in the atmosphere.

They claim the JWST could detect such an atmospheric biosignature with two transits over two months.

Such a world would be different to Earth, but this result gives astronomers hope that we’ll be able to remotely detect atmospheric biosignatures on more diverse exoplanets.

Lewis Dartnell was reading Photochemical Runaway in Exoplanet Atmospheres: Implications for Biosignatures by Sukrit Ranjan et al. Read it online at: arxiv.org/abs/2201.08359.

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This article originally appeared in the April 2022 issue of BBC Sky at Night Magazine.

Authors

Astrobiologist Lewis Dartnell University of Westminster
Lewis DartnellAstrobiologist

Dr. Lewis Dartnell is an astrobiologist and science author based at the University of Westminster.

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