A team of researchers, in a new study based on observations made with the James Webb space telescope, has confirmed the presence of carbon dioxide and methane in the atmosphere of exoplanet K2-18 b (located 120 light years away).
K2-18 b is a planet almost 9 times more massive and with a radius 2.6 times larger than Earth. It orbits its star (a red dwarf) at a distance that places it within what is known as the habitable zone, a region around the star in which water, if it exists, could remain in a liquid state on the surface of the star. a planet.
The discovery of carbon dioxide and methane, along with the absence of ammonia signals in the data, is precisely what scientists would expect to find in the analysis of a Hycean-type planet, a hypothetical world completely covered by an ocean of water. liquid and with a hydrogen-rich atmosphere. Precisely, K2-18 b had become, since its discovery in 2015, a possible candidate for this type of planets.
In addition to the clear signals of carbon dioxide and methane, the data from the James Webb telescope also point to the possible detection of dimethyl sulfide. The fact that on Earth this compound is only generated naturally by the activity of living organisms (especially by phytoplankton in the marine environment), turns dimethyl sulfide into a potential biomarker, that is, an indicator of the possible presence of life on exoplanets.
However, the authors of the study are cautious in warning that the data analyzed do not allow them to categorically confirm the detection of dimethyl sulfide in the atmosphere of K2-18 b. “The new Webb observations should be able to confirm whether dimethyl sulfide is actually present, at significant levels, in the atmosphere of K2-18 b,” said Nikku Madhusudhan, an astronomer at the University of Cambridge and lead author. of the study.
In order to carry out the analysis of the atmosphere of K2 18-b, scientists have taken advantage of the fact that this exoplanet passes (transits, in scientific language) in front of the disk of its star according to our perspective of vision. This allows a small portion of the star's light to cross the planet's atmosphere before reaching the telescopes' instruments.
Each chemical compound absorbs certain wavelengths of incident light. These absorptions generate characteristic patterns in the light received by the most sensitive instruments, and function as the fingerprint that reveals the presence of the chemical compound in question.
The observations of K2-18 b have been made with the James Webb NIRSpec and NIRISS instruments, specialized in the capture and decomposition of infrared light. The fact that only two observations were required highlights the great sensitivity of the space telescope. According to the study's authors, the Hubble Space Telescope would have needed to make at least eight similar observations over a few years to match the level of precision of just one of the measurements taken by Webb.
Since its discovery, mass, size and temperature data suggested that K2 18-b could be compatible with three possible exoplanet models, all with a hydrogen-rich atmosphere. It could be a Hycean planet (with a surface covered by water), a super-Earth (with a rocky surface), or a mini-Neptune (basically with gaseous compositions).
Now, the data collected by the James Webb seem to point towards the first of the possibilities, since the detections of carbon dioxide and methane, together with the absence of other components such as ammonia and carbon monoxide, agree with the composition expected atmospheric temperature of Hycaean planets.
The non-detection of water vapor would also be in accordance with theoretical models, since it is believed that on this type of planets the water would condense in the lowest part of the atmosphere and would not generate a signal in the Webb instruments.
