Exoplanet 120 light years from Earth hints at possible life and rare oceans

An artist’s concept shows what exoplanet K2-18 b could look like based on data. Photo / NASA, ESA, CSA, Joseph Olmsted (STScI)

Chemicals that hint at life have been found in the atmosphere of an exoplanet 120 light years from Earth.

‌The James Webb Telescope spotted signs of methane, carbon dioxide and dimethyl sulphide on “K2-18 b” – a planet nearly nine times the size of Earth which sits in the Goldilocks zone of its star, where conditions are neither too hot, nor too cold for liquid water.

‌The discovery of dimethyl sulphide is particularly intriguing because it is a molecule that is only produced by life on Earth, and is mostly emitted from phytoplankton in marine environments.

‌Scientists believe that the planet may possess a hydrogen-rich atmosphere and a water ocean-covered surface that has the potential for life.

‌Dr Nikku Madhusudhan, associate professor in Astrophysics at the University of Cambridge and lead author of a new paper on the discovery, said: “For the first time we have detected carbon-bearing molecules in the atmosphere of a habitable-zone planet. This has never happened before.

‌”The atmospheric composition tells us that, of all the possible ways to explain it, the most plausible way is that there is an ocean underneath, it is very hard to get that composition otherwise.

‌”Planet-wide oceans and hydrogen atmosphere are just the right conditions to be able to host life similar to the conditions of what we see on Earth.

‌”Could this planet support life? We can’t say for sure. We did detect a molecule at a tentative level, dimethyl sulphide, and this molecule is unique to life on Earth and it has been predicted that it would be a very good biosignature for life on other planets.”

More Webb observations are planned

‌K2-18 b orbits the cool dwarf star K2-18 in the habitable zone and lies around 705 trillion miles from Earth in the constellation Leo.

‌Although the search for life has traditionally focused on rocky planets similar in composition to Earth, scientists are increasingly thinking that life could also exist on large watery worlds – dubbed Hycean exoplanets or sub-Neptunes.

‌”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,” said team member Subhajit Sarkar of Cardiff University.

‌”We have obtained the most detailed spectrum of a habitable-zone sub-Neptune to date, and this allowed us to work out the molecules that exist in its atmosphere.”

‌The abundance of methane and carbon dioxide, and shortage of ammonia, support the hypothesis that there may be an ocean underneath a hydrogen-rich atmosphere on K2-18 b.

‌Although there is only a hint of dimethyl sulphide, future Webb observations are planned to confirm the findings.

‌”These results are the product of just two observations of K2-18 b, with many more on the way,” added team member Savvas Constantinou of the University of Cambridge.

‌”This means our work here is but an early demonstration of what Webb can observe in habitable-zone exoplanets.”

Questions of life

‌The team now intends to conduct follow-up research with the telescope’s Mid-InfraRed Instrument spectrograph that they hope will further validate their findings.

‌”Traditionally, the search for life on exoplanets has focused primarily on smaller rocky planets, but the larger Hycean worlds are significantly more conducive to atmospheric observations,” added Dr Madhusudhan

‌”This hints at the core of the questions we have asked as a species for thousands of years. Why are we here? Are we alone? These are questions everyone can be found asking.”

‌The results have been accepted for publication in The Astrophysical Journal Letters.