The sensational news about obtaining the most convincing evidence of biological life on the exoplanet K2-18b has become one of the most discussed in the fresh press. “James Webb” discovered two molecules in the spectrum of K2-18b that are produced on Earth exclusively by living organisms – plankton and some bacteria. This is the best biomarker of alien life known to us. But, as always, everything is not so clear-cut.
An artist’s impression of a distant habitable world. Image credit: University of Cambridge
The discovery of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) molecules in the atmosphere of the exoplanet K2-18b, 124 light years away from Earth, was reported by a team of scientists from the University of Cambridge’s Department of Astronomy. A year ago, Webb had already collected data on these substances in the atmosphere of K2-18b, but at that time they were extremely inconclusive.
New observations of the planet have allowed us to gather much more information, bringing the reliability of the discovery closer to three sigma (the probability of error is 0.3%). This is not enough to unconditionally confirm the result (for this, a statistical significance level of five sigma is required), but it is enough to seriously consider the discovery.
Planet K2-18b is a super-Earth or sub-Neptune. It is about 2.5 times larger than Earth. Moreover, there is a possibility that it is a gas giant like our Jupiter. The planet orbits a red dwarf star in or near the habitable zone, which allows for the presence of liquid water. So far, everything is consistent with the findings of British scientists. The planet’s atmosphere consists mainly of hydrogen and does not contain any signs of gaseous ammonia. This is quite possible if the planet has a global ocean that absorbs ammonia. In addition, the abundance of water is a favorable environment for the emergence of biological life.
The problem is that the ammonia in the atmosphere could also be absorbed by the ocean of liquid magma. In that case, life on the planet’s surface would be out of the question. Scientists have no data on the surface temperature of K2-18b. The exoplanet’s proximity to its star should also be taken into account (how else would the Kepler observatory have detected it using transit photometry?). Even the water ocean could evaporate from the surface of K2-18b if a significant portion of the star’s energy reached its surface.
Another recent paper suggests that the reflectivity of K2-18b’s atmosphere is very low, and the cloud layer on this exoplanet is not able to reflect a significant part of the energy falling on it. Modeling shows that the ocean on K2-18b could not have survived and was evaporated by the star long ago. So the news about the habitability of the exoplanet K2-18b is greatly exaggerated.
Life could theoretically exist in the upper atmosphere, however. Similar scenarios are occasionally discussed for Venus, which has lead melting on its surface but microorganisms possibly living high above. Similarly, biological life could exist high above the surface on K2-18b. Meeting intelligent brethren is tempting, but it does not seem to be the case on K2-18b.