Hyderabad:

Published in The Astrophysical Journal Letters, have found the chemical fingerprints of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS) on the exoplanet— known to be produced only by life on Earth, primarily microbial life such as marine phytoplankton.

While scientists believe the discovery to be the strongest evidence yet that life may exist on a planet outside our solar system, they remain cautious as an unknown chemical process may also be the source of these molecules in K2-18b's atmosphere. The probability that they occurred by chance is just 0.3 per cent or three-sigma level of statistical significance. Researchers believe that follow-up observation with JWST may help them reach the all-important five-sigma significance, reducing the chance of DMS/DMDS occurring by chance to below 0.00006 per cent and qualify as scientific discovery.

The search for life beyond Earth may have taken a significant step forward as scientists believe they have found a possible biosignature outside the solar system. Using data from the James Webb Space Telescope (JWST), astronomers, led by the University of Cambridge and lead researcher Nikku Madhusudhan, have detected signs of life in the atmosphere of the exoplanet K2-18b, which orbits its star in the habitable zone.

K2-18b: A strong contender for life beyond Earth:

Another weaker signal of life prompted astronomers to take a look at K2-18b with JWST using a different instrument, said Professor Nikku Madhusudhan from Cambridge’s Institute of Astronomy, who led the research. The earlier, tentative, inference of DMS was made using JWST’s Near-Infrared Imager and Slitless Spectrograph (NIRISS) and Near-Infrared Spectrograph (NIRSpec) instruments, covering the near-infrared (0.8-5 micron) range of wavelengths. The new, independent observation used JWST’s Mid-Infrared Instrument (MIRI) in the mid-infrared (6-12 micron) range.

This is not the first time K2-18b has been found to have signs of life beyond Earth. The exoplanet is 8.6 times as massive and 2.6 times as large as Earth and lies 124 light years away in the constellation of Leo. Early observations of the K2-18b identified methane and carbon dioxide in its atmosphere, marking the first time when carbon-based molecules were found on an exoplanet in the habitable zone— suggesting the existence of a ocean-covered world underneath a hydrogen-rich atmosphere.

“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,” said Madhusudhan. “The signal came through strong and clear.”

The analysis of DMS/DMDS on K2-18b:

To determine the chemical composition of the atmospheres of faraway planets, astronomers analyse the light from its parent star as the planet transits, or passes in front of the star as seen from the Earth. As K2-18b transits, JWST can detect a drop in stellar brightness, and a tiny fraction of starlight passes through the planet’s atmosphere before reaching Earth. The absorption of some of the starlight in the planet’s atmosphere leaves imprints in the stellar spectrum that astronomers can piece together to determine the constituent gases of the exoplanet’s atmosphere.

"DMS and DMDS are molecules from the same chemical family, and both are predicted to be biosignatures. Both molecules have overlapping spectral features in the observed wavelength range, although further observations will help differentiate between the two molecules Astronomers analyse starlight passing through a planet’s atmosphere during its transit to determine its chemical composition.

When exoplanet K2-18b passes in front of its star, the JWST detects a drop in brightness, while some starlight filters through the planet’s atmosphere. By studying how the atmosphere absorbs certain wavelengths of light, scientists can identify its constituent gases.

The study noted that the concentrations of DMS and DMDS on K2-18b are estimated to be thousands of times stronger than they are on Earth. Madhusudhan says that while the results are exciting, it’s vital to obtain more data before claiming that life has been found on another world.

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