‘Strongest signs of extraterrestrial life’ are rather weak

It would, unique and without any competition, be the science news of the century: discovered extraterrestrial life! But if that head is unique, why do we see it so often?

Thursday reported the BBC That scientists from the University of Cambridge had found « the strongest evidence so far » of extraterrestrial life, with similar heads in other media. Colleague researchers are very skeptical.

Astrophysicist Nikku Madhusudhan and colleagues from the University of Cambridge announced that they had seen traces of the fabric dimethylsulfide (DMS) and dimethyl disulfide (DMDS) in the atmosphere of the exoplanet K2-18B traces. On Earth, those substances are mainly produced by microorganisms such as Plankton.

« Although an unknown chemical process can be the source of these molecules in the atmosphere of K2-18B, the results are the strongest evidence so far that life could exist on a planet outside our solar system, » the researchers write in a press releasethat belongs to one scientific article in the Astrophysical Journal Letters.

With that alien life you should not imagine green males, intelligent aliens or even simple animals or plants, but rather microorganisms such as algae, plankton or bacteria. Yet the find, in our cosmic backyard, would mean that life apparently is quite easy in the universe.

Planet with oceans

K2-18B is a hefty planet, 8.6 times heavier than the earth, who revolves around the star K2-18, a dwarf star at 124 light-year distance in the constellation Leeuw. According to Madhusudhan and his research group, a so -called « Hycean » world, a planet with oceans and an atmosphere that is rich in hydrogen.

In 2023 the group reported the detection of methane and carbon dioxide (CO₂) in the atmosphere of K2-18b. These are molecules that contain carbon, an ingredient of all earthly life. Also saw ‘possible signs of DMS’, which also led to headlines then.

Such detections, done with the help of the James WebB space telescope, are extremely difficult to perform. The light of the star is much more intense than the light of the exoplanet. The moment the exoplanet moves past the star, there is a small decrease in the light intensity of the star: the exoplanet was discovered in the first place.

If the planet has an atmosphere, a thin shell gas around the planet ball, the light spectrum of the star light changes a bit at that time, because molecules in the atmosphere partially stop that sterlicht. On the ‘fingerprint’ that the molecules leave on the spectrum of light, it is theory to determine which molecules they are.

In practice, that is difficult: there is quite a bit of noise in the data. The infrared spectrum is also not nearly as specific as a real fingerprint. Detections are therefore strongly linked to a chemical model of the atmosphere.

Critical about the model

Now the researchers have once again investigated the atmosphere on board JWST, and they are more secure of their business. « The signal came through strongly and clearly, » says Madhusudhan in the press release. The scientific article is more cautious: « The spectrum cannot be explained by most molecules predicted for K2-18B, with the exception of DMS and DMDS. »

The researchers claim a significance of « three Sigma », which means that there is a 0.3 percent chance that the signal was created by chance. The standard for such an important discovery is five Sigma, a chance of 0.00006 percent.

Other researchers are skeptical. “Madhusudhan has a theory that there is a new kind of planet, the Hycean Worldand he has been talking to this theory for some time, « judges Inge Loes Ten Kate, astrobiologist at Utrecht University. Ten Kate is also critical of the atmosphere model that the researchers use. » They have drawn up that themselves, and is not a model that other atmosphere researchers have thought about. « 

There are also chemical processes conceivable to make DMS and DMDS that have nothing to do with life. « And in the context of the life we ​​know, you get sulfur hydrogen, and we don’t see that here again. »

‘No final proof’

On Bluesky is also astronomer Chris Lintott critical: “With their own code they say they have a 3-sigma detection from DMS. Interesting, but not definitive evidence By Any Stretch of the Imagination. Let alone a ‘hint of life’. « 

Ten Kate also has more fundamental criticism. Scientists have often possible biosignures reported, measurements that indicate possible extraterrestrial life: such as phosphine in the atmosphere of Venus, methane production on Mars, and recently long carbon chains on Mars. Often tightened by media, but also by the scientists themselves in press releases and interviews. When measuring further, the peaks disappeared, or there was something else going on.

In order to prevent such a false alarm-and ultimately damage to trust in science-researchers of alien life have drawn up a kind of science communication escalation ladder.

You discuss finds with many ‘possible’, ‘hint of’ and other strokes as an interesting find with scientific colleagues. Against the press or audience you make all the messages and buts clear. As the certainty grows, and other explanations are excluded, you can then take larger terms such as « discovery » and « extraterrestrial life ». Ten Kate: « That clearly didn’t happen here. »

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