Dr. Rajkumar Banerjee
Chief Scientist, CSIR-Indian Institute of Chemical Technology Hyderabad
Scientists, especially astrophysicists/astronomers are lately elated for a reason. They find a hint that can quell the thirst for establishing the fact that if human being and other life forms in Earth are all alone in the vast expanse of space (and time) or not! The universe carrying this species with innumerable galaxies, each having uncountable solar systems carrying even bigger number of exo-planets and their satellites, subplanetary forms etc. Aren’t there habitable places for life elsewhere? Probability says ‘yes’, but practical explorative results said a big ‘NO’ since mankind started extensive space exploration in modern times. In contrast to usual pessimistic mood of society, NASA operated James Webb Space Telescope (JWST) has given an interesting hint that reignited and refueled the imagination of many others like us. What is it?
In the sea shore, besides fresh air, you sometimes get to smell as if some rotten stuff is around; a sulphur like low pungent odour. That light rotten smell is nothing but an excreting gas called dimethyl sulphide [(CH3)2S; DMS] from sea planktons, the drifter organism under the influence of sea tides and current sometimes crowd near the shore, thus giving the smell of sea air. These drifter organisms are microscopic except for jellyfishes and crustaceans, which when become bigger and can swim against the
tide (still they are called planktons). The figure as shown here is chemically called as dimethyl sulfide (DMS). But, there is something unique about this gas, DMS. DMS is the only gas or compound that is produced only by a living organism through natural metabolism within the life form in this planet.
I think you got the hint now, why some scientists are excited. Scientists discovered the presence of vast quantities of various carbon-bearing molecules, such as methane (CH4) and carbon dioxide (CO2) as well as a minute amount of DMS in an exoplanet, which is discovered in an astonishing distance of about 120 light years away. The exoplanet is called K2-18b which is 8.6 times the mass and 2.6 times the diameter of Earth. It is found to be in the habitable zone of a red dwarf star called K2-18 lying in the Leo constellation. Red (cool) dwarf stars are lesser energy (and heat) radiating than our yellow star (the Sun)present in our solar system and hence it is logical that this host star’s habitable zone will be much closer to the star than the habitable zone in our solar system. K2-18b gets similar amount of solar radiation as Earth and has Earth like comparable temperature (may be few 0C higher), as probed by JWST.
There is one more interesting thing that is interpreted from those data. Scientists found vast presence of CH4 and CO2 but find shortage of ammonia (NH3). This indicates that unlike in Earth, where nitrogen (N2) is abundant, in K2-18b the atmosphere may be Hycean, or hydrogen-rich atmosphere. So, it is believed now that underneath this Hycean atmosphere there may be a water ocean. As JWST also indicated a possible presence of DMS, which in Earth’s atmosphere is produced only by life, i.e., by planktons in marine environments, it is logically believable that some oceanic marine life form might be there which produces DMS. Remember, among the most abundant carbon-based molecules found in K2-18b, i.e., CH4 and CO2, the causes for their production in Earth are from both life forms (anthropogenic or human influenced) as well as by natural phenomenon or sources, but it is not the case for DMS. Methane (CH4) is produced in marshy land and is a primary component of natural gas and CO2 is produced by burning
of organic matter as well as it is exhaled by living organism. In contrast, bulk of DMS is produced only by a life form in Earth.
Now, from such a huge distance from where JWST is located in space, how can the abundancy and relative quantity of gaseous materials and compounds could be detected and quantified? Couldn’t the measurement be an artefact? What is the repeatability? How many times the experiment was done? A clear understanding and answer to each of these queries can give credence to the claim by these over enthusiast scientists, isn’t it?
In 2015, Kepler Space Telescope, NASA’s first planet-hunting mission called K2 mission, discovered K2-18b exoplanet. When this planet while transiting in its orbit passed across the face of its host star, the telescope could detect a transient drop in the brightness of the star. Thus, the discovery of the exoplanet was relatively easier because, to my understanding, the exoplanet is closure to the less brighter cool star as the habitable zone is expected to be closure to the host star. Anyway, characterizing the gaseous materials in the exoplanet’s environment is not easy as the detection can be challenged or nullified by the overtly glare of the much larger parent star in the vicinity. So, the scientists did a trick. They analyzed the star light that grazed through the atmosphere of exoplanet when it was transiting in front of the star before it reached the telescope boarded on JWST. It is understandable that when the light encounters the molecule of a gas it diffracts, and the light is absorbed by the molecule at a precise wavelength. Consequently, the otherwise usual, whole band wide spectrum of star light will have a flicker in the wavelength region where the molecule absorbs or light diffracts. Analyzing this information from the incredibly small amount of star light, the spectra of K2-18b atmosphere is generated using JWST’s Near-Infrared Imager and Slitless Spectrograph (NIRISS) and Near-Infrared Spectrograph (NIRSpec). The more the abundance of the molecule, bigger, wider and multiple signature peak patterns will appear in the spectra.
In the spectra above [Courtesy: https://webbtelescope.org/]obtained from K2-18b, the signature peaks of CH4 and CO2 appears much more frequently and wider than that of DMS, which came as few flickers. In fact this is the representation of two transit observations over a relatively narrow wavelength range and in precision this is far better than Hubble telescope-based output. Clearly, the signature pattern of DMS has to be obtained in many more transit observations over multiple years to substantiate that the
date is believable. Hence, the data may be premature to give a final statement. Now, can DMS be considered a biosignature for extra-terrestrial life?
Contrastingly, a report in April 2024 by a chemist, Nora Hanni from University of Bern shows that this complex organic molecule DMS could be detected by their specific weights using Rosetta spacecraft’s mass spectrometer from the plethora of molecules obtained as a cloud of dust and gas shed from the icy space rock, a comet (obviously a lifeless rock) namely 67P/Churyumov-Gerasimenko, which is chased and sampled for its thrown away dust for last two years by European Space Agency’s comet probe, Rosetta spacecraft. This data potentially debunks DMS as a biosignature.
This data has put in many questions:
A. How DMS could be available in that dust shed by the comet?
B. Could it be that the comet while grazing past a hypothetical DMS-producing planet might have acquired that molecule and it continued to shed all other molecules including DMS?
C. Is it possible that in hycean exoplanets the mechanism of DMS formation is likely very different than in N2 abundant planets like Earth?
D. Our carbon based life form in Earth requires abundant nitrogen, because nitrogen-based molecules are inevitably present in proteins and genes, the building blocks of a living creature. If in K2-18b, there is scarcity of N2, can we assume that the living creature, if any, in that planet will not bear the same kind of life form that is existing in Earth?
E. Does nature of law change in Universe from a zone to another zone which are lightyears away from each other?
With the recent data so far obtained from space explorations, it created more questions than answers for if life form is present in any corner of this vast Universe. Let’s sit tight, watch or be a part of this exploration without delay.
