Why We Should Be Wary Of The Putative K2-18b Extrasolar Biosignature
This illustration shows what exoplanet K2-18 b could look like based on science data. K2-18 b, an ... More exoplanet 8.6 times as massive as Earth, orbits the cool dwarf star K2-18 in the habitable zone and lies 120 light years from Earth.
In the high stakes science of looking for signatures of life on a planet circling another sunlike star, understandably, there's pressure to produce results. Thus, news that a University of Cambridge-led team in the U.K. had detected dimethyl sulfide (DMS), a sulfur-rich potential biosignature on a hydrogen-rich ocean-world some 124 light years away, was music to many astrobiologists' ears.
With fresh data from NASA's James Webb Space Telescope, the team reported the detection of the spectroscopic chemical fingerprints of dimethyl sulfide (DMS) and/or its cousin, dimethyl disulfide (DMDS), in the atmosphere of the exoplanet K2-18b. The planet, which is some 2.6 times as large as Earth, orbits its star in the habitable zone, which as noted in a paper appearing in The Astrophysical Journal Letters is crucial to the argument that the planet might harbor some crude form of life.
But within 72 hours of this news, there was an online outcry among several cosmochemists who cast aspersions on the idea.
The bio-signature is dimethyl sulfide, a molecule known to be present in interstellar clouds and a comet, Sun Kwok, a former president of the International Astronomical Union's Commission on Astrobiology and an astronomer at the University of British Columbia in Vancouver, tells me via email. The reported detection of this molecule in a planet, if real, is most likely the result of abiotic processes and has nothing to do with life, he says.
On Earth, as the University of Cambridge notes, dimethyl sulfide is only produced by microbial life, such as marine phytoplankton.
Just because a volatile molecule is in a comet or the interstellar medium doesn't mean much for a planetary atmosphere, Nikku Madhusudhan, the paper's lead author and an astrophysicist at the university of Cambridge in the U.K., tells me via email. Several biomarkers including methane and molecular oxygen are also present in those environments but have very little implication for planetary atmospheres, he says. The key point is that dimethyl sulfide can be produced in such environments in very small quantities, but much less than what is required to explain our observations, says Madhusudhan.
Yet Kwok sticks by his assertion that abiotic process that can produce dimethyl sulfide are common and effective. So, one cannot conclude that its detection is biological in origin, when other explanations are possible, he says. One can only make such an extraordinary claim when other possibilities are eliminated, says Kwok.
But Madhusudhan disagrees.
Dimethyl sulfide is much more unstable in a planetary atmosphere than in a comet and hence has very short lifetimes unless replenished continuously, says Madhusudhan. To date, it has not been shown that this can be a viable mechanism to explain DMS on habitable planets, he says.
As for these hydrogen-rich ocean worlds potentially harboring microbial life?
There are many biochemical pathways to life, and we should not push Earth analogs too far, says Kwok. I am pessimistic about detecting life as we know it but optimistic about the wide presence of extraterrestrial life in other forms, he says.
What should we be doing that we aren't?
Before we seek biosignatures, I would like to see a satellite-based infrared spectrometer with high spectral resolution, says Kwok. This would enable us to identify the structures of complex organic compounds in space to gain a better understanding of the abiotic synthesis of organics, he says.
Is finding extrasolar biosignatures an impossible task?
Planetary scientists have found a large variety of organic compounds (including nearly 100 amino acids in meteorites, all formed abiotically, says Kwok. So, it is very difficult to find a biosignature in exoplanets that can be conclusively identified as biological in origin, he says. The best chance to find life beyond the Earth is through planetary exploration using in situ observations or sample return, says Kwok.
Yet even if this turns out to be a spurious detection, it's heartening to know that we now can potentially find complex molecules on worlds totally unlike our own, many light years away.
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