There could be a flaw in the search for atmospheric oxygen as a signature of life around most of our nearby exoplanets. A team from the University of Washington suggests the light coming from low temperature M-dwarf stars, like TRAPPIST-1, would not support the energy-unlocking photosynthesis reactions required to build up O2 in the atmosphere to levels we could spot.
An artist’s impression of an Earth-sized exoplanet orbiting an M-dwarf star. Image credit: Sci-News.com.
In the search for extraterrestrial life, atmospheric oxygen has been discussed by experts as a prominent biosignature, which could also be relatively easy to detect with upcoming exoplanet-exploring facilities like the James Webb Space Telescope (JWST).
Certainly the oxygen that makes up 20% of our own atmosphere would give any alien astronomers clues of life on Earth. This is because there are so many ways to remove it (including reactions with volcanic gases, dissolving in rainwater or oxidative weathering of continental rocks), and yet only one straightforward way of replenishing it, through photosynthesis.
The scale of photosynthesis to maintain such an oxygen presence in the air we breathe is only possible due to the plentiful supply of light energy from our star. However, future exoplanet-exploring facilities like the JWST will focus instead on the far more common but lower temperature M-dwarf stars.
Planetary scientist Owen Lehmer wondered if the output of M-dwarfs, like the famous, seven planet-hosting TRAPPIST-1, could support a photosynthesizing biosphere of sufficient scale to alter an exoplanet atmosphere to an extent we could detect.
On Earth, most oxygen photosynthesizing organisms use light with wavelengths between 400-750 nm, where longer wavelengths means lower energy per unit of light.
Some organisms can use longer wavelengths, up to 1,020 nm, by modifying the photosynthetic process, however it seems unlikely this can be pushed much further when lab experiments have failed to detect any vital electronic excitations kickstarting photosynthesis reactions from photons beyond 1,100 nm.
“It seems like organisms on Earth have hit a similar limit that we have in the lab,” says Lehmer.
This might be a problem considering M-dwarf stars like TRAPPIST-1 emit largely in the less energetic infrared and near infrared part of the spectrum with longer wavelengths between 700 nm – 1,000 nm.
To investigate further, Lehmer and his colleagues took the modern living, breathing Earth and moved it 40 light-years in the direction of the constellation of Aquarius, dropping it into orbit around TRAPPIST-1.
They then used various models, including one from the 1970’s that showed why plants on Earth are green, to explore how orbital distance and star-type impact photon availability for photosynthesis.
The team considered four photon wavelength limits for oxygen production — including the current highest found in nature, and the higher lab-based figure for detectable electronic transitions.
They then applied these to their model Earth, positioned at different orbital distances around TRAPPIST-1, and other hypothetical M-dwarf stars of various temperatures.
Their results, published in the Astrophysical Journal, showed M-dwarf’s lower energy light output falls some way short of that required for photons absorption to kick start photosynthesis reactions in even the most efficient systems evolution has produced.
For the TRAPPIST-1 simulation, the team found all three of its habitable zone planets would have insufficient light to maintain Earth’s terrestrial biosphere.
While oxygen producing organisms on Earth use 8 photons in the 400-750 nm range per CO2 molecule during photosynthesis, organisms tuned to the emissions of a TRAPPIST-1-like star could require up to 24-36 photons per CO2, reducing their capacity for creating fuel, and oxygen as a byproduct.
This isn’t to say photosynthesis isn’t taking place around M-dwarfs, says Lehmer, or that life isn’t present releasing energy via some other process.
However, if you are looking for oxygen as a biosignature, you are making an assumption around the process forming it, and Lehmer’s paper suggests M-dwarfs light radiation would limit photosynthesis’ ability to change an atmosphere to the extent we would notice.
“For the past few years everyone has been saying oxygen is a great biosignature. This paper adds an asterisk. Oxygenic photosynthesis could evolve like it did on Earth and we would never detect it.”
“It’s interesting and highlights a reason I am holding out for finding and characterizing rocky planets orbiting Sun-like stars,” says Professor Sara Seager, a planetary scientist at MIT, who wasn’t involved in the paper.
“However, we have no idea what life will be like on another planet. I would not take a stand one way or another about life and its biosignature gases on planets orbiting M dwarf stars until we have actual observations. The reality is for now and possibly for the next decade or two, small planets orbiting M dwarf stars are the only ones accessible to astronomers.”
Limited to these lower temperature star systems, Lehmer’s believes we should focus our exploration of them to finding so-called disequilibrium biosignatures — evidence of gases around exoplanets that shouldn’t co-exist, indicating something replenishing one or both.
If we look at the Earth we find both methane and oxygen, which should react out of the atmosphere in just a few hundred years, but are still there because they are replenished by life.
“We may be unlikely to find oxygen so we shouldn’t put all our eggs in that basket. We should consider other options,” says Lehmer.
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Owen R. Lehmer et al. The Productivity of Oxygenic Photosynthesis around Cool, M Dwarf Stars. ApJ 859, 171; doi: 10.3847/1538-4357/aac104
