The James Webb Space Telescope spectroscopic analysis of K2-18b, an exoplanet 124 light years from Earth, shows signs that the atmosphere may contain dimethyl sulphide (DMS). This finding is more impressive when you know that DMS on Earth is only produced by living organisms, not by any geological process. The atmosphere of K2-18b also contains methane and CO2, which could be compatible with liquid water on the surface. Methane is also a possible signature of life, but it can also be produced by geological processes. This is pretty exciting, but the astronomers caution that this is a preliminary result. It must be confirmed by more detailed analysis and observation, which will likely take a year.

According to NASA:

K2-18 b is a super Earth exoplanet that orbits an M-type star. Its mass is 8.92 Earths, it takes 32.9 days to complete one orbit of its star, and is 0.1429 AU from its star. Its discovery was announced in 2015.

This planet was discovered with the transit method, so we have some idea of its radius and therefore density. It’s surface gravity is likely about 12 m/s^2 (Earth’s is 9.8). It has a hydrogen-rich atmosphere, which would explain the methane without the need for life. It orbits a red dwarf, and is likely tidally locked, or in a tidal resonance orbit. It receives an amount of radiation from its star similar to Earth. The big question – is K2-18 b potentially habitable?

There are a number of ways to model conditions on the surface of this exoplanet. Some do include liquid water on the surface with conditions compatible with life. So it is not out of the question. The planet is just inside the habitable zone of its system, but this depends on the atmosphere and how much heat it retains. There would not be Earth-like conditions on the surface of this world, but there could be life in its oceans.

This makes the possible detection of DMS more intriguing. But it is too early to get excited. First, scientists need to confirm the presence of DMS in the atmosphere of K2-18 b. If this is confirmed, then they need to figure out if there is any possible non-biological origin for this molecule. This is always difficult to determine, to demonstrate a negative. We can say there is no non-biological origin on Earth, but conditions on K2-18 b are different. But still, if we confirm the existence of DMS and we cannot model a non-biological origin, it would be time to get a little excited. That would make K2-18 b the first possible exoplanet with life we have detected, and the first sign of life off Earth. That would be a huge discovery. But for that reason, we need to be as certain as possible the finding and interpretation are real.

Past history urges caution. It was not that long ago that scientists announce the possible discovery in the clouds of Venus of sulphine in 2020, a molecule also considered a sign of possible life. On Earth only living organisms produce sulphine is substantial amounts. This lead to speculation that the clouds of Venus might harbor bacteria-like organisms. That would be a truly exciting discovery. However, the excitement did not last long. However, about two years later, in November 2022, data from the SOFIA (Stratospheric Observatory for Infrared Astronomy) found no signal for sulphine.

A separate analysis also found that there is too little water in the atmosphere of Venus to support even the most extreme of extremophiles found on Earth. This was a discouraging finding, but not as definitive as the lack of sulphine. The analysis found that there is only 1% of the water concentration in the clouds of Venus than the lower limit of water concentration tolerable by Earth extremophiles. The researchers who did this analysis concluded that this would be an unbridgeable gap for life to cross, and that this low a water concentration was simply incompatible with life.

While this is probably true, there are two important caveats. This conclusion is based on life on Earth. That does not make it worthless, and it probably does say something about the probability of life under Venus conditions, but life also has a way of surprising us. I would not consider this conclusion absolute, therefore. But perhaps more importantly, other scientists pointed out that while the water content is extremely low it is not necessarily evenly distributed. There may be pockets of relatively higher water content, allowing life to bloom, and then going dormant again as it seeks out other pockets of water.

But given all of this, I am not holding my breath for the discovery of life on Venus. I would not rule it out entirely, but it is reasonable to no longer consider the clouds of Venus as a likely life candidate (barring some future surprising discovery). There are more likely places for life in our own solar system, most notably in the oceans of Europa or some other ice world. There may also be some layer in the clouds of Jupiter that could support life, but this is still highly speculative.

The number of potential exoplanet candidates is much greater, because it’s a big universe out there. For now we are limited to looking for interesting molecules in the atmospheres of exoplanets, looking for the signatures of life and then inferring the probability of life from that data. The right combination of molecules in the right conditions, however, could prove compelling.