Dimethyl sulfide signature may not indicate extraterrestrial life

Belief in alien life, having been forever devoid of any empirical basis pro or con, has been free to enjoy cycles of fashion. When both traditional astronomy and Christian theology placed the Earth at the centre of the cosmos, few imagined the stars and planets as anything other than the unchanging celestial backdrop to life in this vale of tears. But ‘A Man that is of Copernicus’s Opinion’, in the evaluation of Christiaan Huygens in the late 17th century, ‘cannot but sometimes have a fancy that it’s not improbable that the rest of the Planets have… their Inhabitants too’. 

Huygens’ casual acceptance of extraterrestrial life remained the norm at least until HG Wells warned of its hazards, and even in 1952 the Astronomer Royal Harold Spencer Jones was happy to tell us that ‘it is almost certain that there is some form of vegetation on Mars.’

When the Viking lander missions of the 1970s revealed a sterile Martian landscape, scientific opinion reverted to the notion that we are a lone oasis of life in the solar system and possibly in the wider cosmos. Some still consider the origin of life on Earth to have been an extremely rare and improbable event on potentially habitable worlds. But probably a more common view, now that we know how abundant extrasolar and even somewhat Earthlike planets are, is that life of some kind might be fairly widespread in the universe, but that intelligent life – the sort that we purport to be – could be very rare. 

Hard steps

One argument for that view came from cosmologist Brandon Carter, who in 1983 pointed out a peculiar coincidence whereby the timescale for humans to evolve – around 4.6 billion years – is of the same order of magnitude as the span over which our planet will be habitable at all: in around 5 billion years’ time the Earth will be fried by the ageing, swollen Sun before it wanes into a white dwarf.¹ Carter inferred that it would normally take much longer for intelligent life to appear on a planet like ours, and that we are the result of one or two rare and improbable events required for such evolution: ‘hard steps’ that usually preclude advanced, intelligent life but which will happen here and there throughout the cosmos. In this view, we’re either alone or nearly so. 

Carter’s hard-steps model has recently been challenged by a team who argue that the late arrival of humans need not imply our improbability, but might rather reflect the fact that the environmental conditions needed for big, complex life forms like us – not least, an oxygen-rich atmosphere to support our energy-hungry cells – can’t evolve quickly from the inhospitable setting of the Hadean (Earth’s oldest geologic eon).² 

All this context adds intrigue to the recent claim by a team of astronomers at the University of Cambridge, UK, to have detected a potential ‘biosignature’ – a compound that, on Earth, has only a biogenic natural source – on an extrasolar planet called K2-18b, 2.6 times the diameter of Earth and orbiting a red-dwarf star 124 light years away.³ They report the spectral signature of dimethyl sulfide (DMS) in K2-18b’s atmosphere, based on measurements by the James Webb Space Telescope (JWST). The team made the same claim more tentatively two years ago from data collected by a different JWST instrument.⁴ 

Uncertainty

The most credulous (or most sensationalist) media sources reported this claim as a likely (‘99.7% certainty’) detection of alien life. Many experts were more sceptical. For one thing, the DMS detection itself may not be as convincing as the authors claim. They said there is only a 0.3% chance of the signal being mere noise, but others have questioned that number. 

On Earth, DMS in the atmosphere is produced by marine plankton, and the Cambridge team says the compound has been considered a ‘robust biosignature’ for planets like K2-18b, which is thought to have an ocean beneath an atmosphere rich in hydrogen. But an abiotic source of DMS has been reported from photochemical reactions involving hydrogen sulfide and methane.⁵ Some planetary scientists argue that it seems unlikely any putative biosignature gas in a planetary atmosphere can be considered compelling evidence for life, given how little we know about the geological conditions of any extrasolar planet. 

All the same, it’s interesting to contemplate what a world with DMS-generating microbes might imply. On Earth, these are relatively recent: the metabolic pathway leading to DMS is thought to have appeared only around 250 million years ago.⁶ So it’s a fairly sophisticated bit of biochemistry. Such life might then be supposed already to have completed some of Carter’s hard steps – in which case they wouldn’t look so hard after all.