Scientists reveal 'most promising yet' signs of alien life on planet k2-18b
Scientists have discovered evidence of large quantities of
biosignature chemicals — only known to be made by life on Earth — on an
exoplanet more than 100 light-years away. It could be the most promising sign
yet of alien life.
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Astronomers have detected the most promising signs yet of a
possible biosignature on the exoplanet K2-18b, which orbits its star in the
so-called habitable zone. (Image credit: A. Smith, N. Madhusudhan (University
of Cambridge)) |
Chemical fingerprints of life have been found on a distant
exoplanet by NASA's James Webb Space Telescope (JWST). It is the "most
promising" evidence yet for alien life, scientists say.
These chemicals were detected in the atmosphere of an
exoplanet named K2-18b, which is located 124 light-years away from Earth and
orbits its star in the habitable zone — the region around a star where liquid
water can exist on a planet's surface — according to a new study published in
The Astrophysical Journal Letters on April 17.
On Earth, these molecules — dimethyl sulfide (DMS) and
dimethyl disulfide (DMDS) — are only produced by life, such as marine algae and
other microbes. Before now, DMS and DMDS hadn't been definitively detected in
the atmospheres of any other planets or moons. While it is theoretically
possible for these chemicals to be created without the presence of life, they
are considered potential biosignatures on other worlds.
"Our findings provide new independent evidence for the
possibility of a biosphere on K2-18 b" and "present an important step
forward in the search for signatures of life on exoplanets," the
researchers wrote in the paper.
K2-18b, which is 2.6 times the size of our planet and 8.6
times the mass, is suspected to be a "hycean world," meaning that it
potentially has a planet-wide ocean and an atmosphere rich in hydrogen.
In a previous paper published in 2023, the same team of
researchers detected methane (CH₄) and carbon dioxide (CO₂) in the planet's
atmosphere, marking the first time that carbon-based molecules had been found
in the atmosphere of an exoplanet in its star's habitable zone. The scientists
also discovered potential signs of DMS. However, the levels of DMS had
"low statistical significance," so the researchers couldn't be
certain that it was indeed present.
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The graph shows the observed transmission spectrum of the
habitable zone exoplanet K2-18 b using the JWST MIRI spectrograph. (Image
credit: A. Smith, N. Madhusudhan (University of Cambridge)) |
"We didn't know for sure whether the signal we saw last
time was due to DMS, but just the hint of it was exciting enough for us to have
another look with JWST using a different instrument," Nikku Madhusudhan, a
professor of astrophysics at the University of Cambridge and lead author of
both studies, said in a statement.
In the latest study, the researchers found that new
measurements of the planet's atmosphere taken by JWST's Mid-InfraRed Instrument
(MIRI) show specific features that can only be explained by the presence of
either DMS or DMDS. Due to the similarities of these two molecules, they appear
very alike in the measurements that JWST takes from the atmospheres of
exoplanets, meaning it is hard to tell which molecule is present in greater
quantities.
"This is an independent line of evidence, using a
different instrument than we did before and a different wavelength range of
light, where there is no overlap with the previous observations,"
Madhusudhan said. "The signal came through strong and clear."
The researchers noted that levels of DMS and/or DMDS in the
atmosphere may be as high as 10 parts per million by volume, which is much
higher than the levels seen here on Earth, which are below one part per billion
by volume.
The researchers said that these observations have reached a
"three-sigma" level of significance. This means there is just a 0.3%
probability that they occurred by chance. To confirm a discovery, scientists
typically require a five-sigma significance level, where there is below a
0.00006% probability of occurring by chance.
DMS and DMDS are not known to be produced in large
quantities through non-biological processes on Earth, meaning that their
detection in such large quantities on K2-18b is a major indicator for the
potential presence of life.
"Given everything we know about this planet, a Hycean
world with an ocean that is teeming with life is the scenario that best fits
the data we have," Madhusudhan said.
The researchers said that more measurements need to be taken
to achieve five-sigma significance and also differentiate between the presence
of DMS and DMDS. They hope to take these measurements soon, whenever they can
get the JWST to look at this exoplanet for a few more hours. They also noted
that the presence of DMS and/or DMDS could have arisen from as-yet-unknown
chemical reactions not involving any life at all, which they hope to test
experimentally.
"It's important that we're deeply sceptical of our own
results, because it's only by testing and testing again that we will be able to
reach the point where we're confident in them," Madhusudhan said.
"That's how science has to work."