Updated: Oct 20
Are we alone in this universe? This question has bugged humans since the Epicurean philosophers of ancient Greece. With the rise in popularity of science fiction and popular movies like Alien, an increasing number of the general population have wondered if there are other planets in our universe capable of supporting life. St Andrews even has an entire module, ES1006, dedicated to answering that question, or in the students’ case, learning how others have previously attempted to find those planets. There has been varying success. Finding planets outside our solar system, known as exoplanets, is hard. Finding those capable of supporting life is even harder. Since the first exoplanet discovery in 1992, we have continued to search for other planets outside of our solar system. With help from NASA and their space telescopes, we have now found over 5,300 exoplanets. Of those, only 59 have been deemed potentially habitable. Of these, planet K2-18b has caused quite a stir in the scientific community. The reason: potential signs of life.
If you’re picturing little green men right now, sorry to disappoint. The James Webb telescope, designed to, amongst other functions, detect the composition of exoplanet atmospheres, has detected a compound in the atmosphere of K2-18b called Dimethyl Sulphide (DMS). For many of us, when asked to name compounds produced by life, we immediately turn to carbon dioxide and oxygen. Unfortunately, these can also be produced abiotically, such as by volcanoes, weathering and other processes. Therefore, we have no way of knowing if or when they are being produced solely by life. Dimethyl Sulphide, alongside a few other compounds like Methyl Chloride and Nitrous Oxide, is one of the few that has no known abiotic source: as far as we know, only life can produce it. So, does this mean we have confirmed extraterrestrial life? Unfortunately not. The detection of DMS is less robust than other findings and more data is needed to confirm the findings. However, there is hope due to the many other characteristics that make K2-18b a good possibility for hosting life.
Planet K2-18b first became of interest when astronomers found potential signs of water vapour in its atmosphere. Water is thought to be essential for life. Its ‘universal solvent’ property allows it to help cells transport and use substances like nutrients and oxygen. Water also helps cells maintain their structure and is involved in many chemical reactions necessary for sustaining life. The abundance of methane and carbon dioxide in the atmosphere, along with the shortage of ammonia, suggests there may be even more water than that: the planet may have oceans. Oceans are incredibly important for life – we should know since we came from ours. The likelihood of an ocean on K2-18b is increased by the fact that the planet sits in the ‘habitable zone’ of its star.
The habitable zone is the range of distance from a star where liquid water could exist and hence conditions for life may be just right. K2-18b orbits a red dwarf star, K2-18, within the Leo constellation, a massive 124 light years away. The planet K2-18b has a 33-day orbit within the star’s habitable zone, meaning it receives about the same amount of starlight as the Earth does from the sun. As Earth is the only planet that is confirmed to have life, similar conditions to Earth elsewhere bring great promise in the hunt.
The search for life has often been focused on rocky planets like Earth. However, research has recently turned to Hycean planets – water-covered planets with a hydrogen atmosphere – as contenders for life. Looking for biosignatures is the primary way to test for life and Hycean planets like K2-18b are more conducive to atmospheric observation. The unconfirmed presence of DMS in K2-18b’s atmosphere could eventually lead to two outcomes: a paradigm shift in the world as we know it if life is confirmed on another planet, or scientific advancement in the study of worlds unlike anything we find in our solar system. With both outcomes favourable, all we can do is eagerly wait for the results in the hope of answering that time-old question: are we alone?
Image from WikiCommons