Free Astronomy Magazine November-December 2023

NOVEMBER-DECEMBER 2023 S pectra of K2-18 b, obtained with Webb’s NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec (Near-Infrared Spectrograph) displays an abundance of methane and carbon dioxide in the exoplanet’s atmosphere, as well as a possible detection of a molecule called dimethyl sulfide (DMS). The de- tection of methane and carbon dioxide, and shortage of ammonia, are consistent with the presence of an ocean underneath a hydrogen-rich atmosphere in K2-18 b. K2-18 b, 8.6 times as massive as Earth, orbits the cool dwarf star K2-18 in the habitable zone and lies 110 light years from Earth. [NASA, ESA, CSA, Ralf Craw- ford (STScI), Joseph Olmsted (STScI) − Nikku Madhusudhan (IoA)] “This result was only possible be- cause of the extended wavelength range and unprecedented sensitiv- ity of Webb, which enabled robust detection of spectral features with just two transits,” said Madhusud- han. “For comparison, one transit observation with Webb provided comparable precision to eight ob- servations with Hubble conducted over a few years and in a relatively narrow wavelength range.” “These results are the product of just two observations of K2-18 b, with many more on the way,” ex- plained team member Savvas Con- stantinou of the University of Cambridge. “This means our work here is but an early demonstration of what Webb can observe in habit- able-zone exoplanets.” The team now intends to conduct follow-up research with the tele- scope’s MIRI (Mid-Infrared Instru- ment) spectrograph that they hope will further validate their findings and provide new insights into the environmental conditions on K2-18 b. “Our ultimate goal is the identifi- cation of life on a habitable exo- planet, which would transform our understanding of our place in the universe,” concluded Madhusudhan. “Our findings are a promising step towards a deeper understanding of Hycean worlds in this quest.” planet can support life. The planet’s large size — with a radius 2.6 times the radius of Earth — means that the planet’s interior likely contains a large mantle of high-pressure ice, like Neptune, but with a thinner hy- drogen-rich atmosphere and an ocean surface. Hycean worlds are predicted to have oceans of water. However, it is also possible that the ocean is too hot to be habitable or be liquid. “Although this kind of planet does not exist in our solar system, sub- Neptunes are the most common type of planet known so far in the galaxy,” explained team member Subhajit Sarkar of Cardiff University. “We have obtained the most de- tailed spectrum of a habitable-zone sub-Neptune to date, and this al- lowed us to work out the molecules that exist in its atmosphere.” Characterizing the atmospheres of exoplanets like K2-18 b — meaning identifying their gases and physical conditions — is a very active area in astronomy. However, these planets are outshone — literally — by the glare of their much larger parent stars, which makes exploring exo- planet atmospheres particularly challenging. The team sidestepped this challenge by analyzing light from K2-18 b’s parent star as it passed through the exoplanet’s atmosphere. K2-18 b is a transiting exoplanet, meaning that we can detect a drop in bright- ness as it passes across the face of its host star. This is how the exoplanet was first discovered in 2015 with NASA’s K2 mission. This means that during transits a tiny fraction of starlight will pass through the exo- planet’s atmosphere before reach- ing telescopes like Webb. The starlight’s passage through the exoplanet atmosphere leaves traces that astronomers can piece together to determine the gases of the exo- planet’s atmosphere. !