Real Clues to Life Beyond Earth?

The star K2-18 is a small red dwarf located about 124 light-years from Earth, in the constellation Leo. In 2015, observations by the Kepler Space Telescope revealed a planet orbiting the star, which was designated K2-18b, following the standard naming convention for exoplanets. With a mass about eight times that of Earth, the planet falls into the “sub-Neptune” category—planets slightly smaller than Neptune.  It captured scientists’ interest because it orbits the star within the “habitable zone,” where temperatures could allow liquid water to exist on the surface.

What’s most intriguing, however, is its atmosphere. Initial measurements detected water vapor, and subsequent observations with the James Webb Space Telescope (JWST) identified the presence of carbon dioxide (CO₂) and methane (CH₄) - simple carbon-based molecules closely associated with life on Earth. These gases are released with every breath and burp we make, although they can also arise from non-biological processes.

New measurements published this week have taken scientific curiosity to a new level with the detection of two additional simple compounds that, to our current knowledge, are produced on Earth solely through biological processes.

An atmosphere containing molecules such as carbon dioxide and hydrogen. An artist’s rendition of the planet K2-18b, with its host star rising on the left  | Credit: NASA, CSA, ESA, J. Olmsted (STScI), Science: N. Madhusudhan, Cambridge University

“The Signal Came Through Strong and Clear”

The new measurements, published in Astrophysical Journal Letters, revealed that the atmosphere of K2-18b contains dimethyl sulfide (DMS, with the chemical formula CH₃–S–CH₃) and dimethyl disulfide (DMDS, CH₃–S–S–CH₃). Dimethyl sulfide is a common molecule on Earth, known for its strong, unpleasant odor. It is produced wherever bacteria break down biological matter and is strongly associated with the characteristic smell of the sea, where bacteria decompose vast quantities of phytoplankton and release large amounts of DMS into the atmosphere. Dimethyl disulfide, which contains an additional sulfur atom, is also known for its pungent smell and is also associated with various biological processes. It is produced by bacteria and fungi, as well as by plants and animals. For example, the notorious “carrion flower” from the arum family, known for emitting a foul smell that attracts pollinating flies, relies heavily on DMDS and DMS to generate its stench.

In earlier observations using the James Webb Space Telescope (JWST), scientists had already suggested the possible presence of DMS in K2-18b’s atmosphere. Those measurements were taken using two of the telescope’s instruments operating in the near-infrared range (wavelengths shorter than 5 microns). The new study used a different JWST instrument—MIRI—which operates in the mid-infrared range (6–12 microns)

To identify the components of a distant planet’s atmosphere, researchers use spectrometry—a technique that analyzes the distribution of light across different wavelengths. Each molecule interacts with light in a distinctive way, absorbing and emitting at specific wavelengths and producing a unique spectral signature. To determine a planet’s atmospheric composition, scientists use a telescope to observe the planet as it transits its host star, analyzing how the starlight is altered as it passes through the planet’s atmosphere. Comparing this filtered light to the star’s unfiltered spectrum allows them to identify the chemical constituents of the atmosphere. This is a challenging task even under optimal conditions—and it is all the more difficult when dealing with relatively small celestial bodies over 120 light-years away. As a result, achieving a high level of confidence is difficult, and reaching a clear conclusion requires careful analysis of both the observations and the measurements.

This time, however, the researchers had no doubts. “This is an independent line of evidence, using a different instrument than we did before and a different wavelength range of light, where there is no overlap with the previous observations. The signal came through strong and clear,” said lead researcher Nikku Madhusudhan of the University of Cambridge. “It was an incredible realisation seeing the results emerge and remain consistent throughout the extensive independent analyses and robustness tests,” added Måns Holmberg, a researcher at the US Space Telescope Science Institute in Baltimore and co-author of the paper.

The findings support a model in which the planet has a hydrogen-rich atmosphere and a surface ocean of liquid water—conditions that could theoretically support life. A planet of this type is known as a ‘Hycean’ planet—a blend of the words “hydrogen” and “ocean.” “Earlier theoretical work had predicted that high levels of sulfur-based gases like DMS and DMDS are possible on Hycean worlds,” said Madhusudhan. “And now we’ve observed it, in line with what was predicted. Given everything we know about this planet, a Hycean world with an ocean that is teeming with life is the scenario that best fits the data we have.”

The results remained consistent. Measurement data from the James Webb Telescope (yellow dots) overlaid on model-predicted values (blue band) | Credit: A. Smith, N. Madhusudhan, Cambridge University

Many Open Questions

Despite the research team’s optimism, several significant issues must be addressed before anyone can claim the discovery of life beyond our solar system. First, the researchers themselves note that the statistical significance of their findings is at the 3-sigma level—meaning there’s a 0.3% chance the results are due to random fluctuations. In studies of this nature, the gold standard is a 5-sigma level, which reduces the probability of error to just 0.00006%. In practical terms, this means the risk that the findings are incorrect is small—but not low enough.

Another issue is the reported concentration of DMS and DMDS in K2-18b’s atmosphere. According to the findings, these molecules are present at levels roughly 10,000 times higher than on Earth. While Earth—abundant with life—shows concentrations of about one part per billion, the estimate for K2-18b is around 10 parts per million.  Even the researchers note that, despite the alignment with theoretical models, the implications of such high concentrations—if confirmed—remain unclear and will require extensive further study. “The inference of these biosignature molecules poses profound questions concerning the processes that might be producing them,” said co-author Subhajit Sarkar of Cardiff University, UK.

Perhaps the most important point is this: even if the findings are accurate, the fact that we do not know of any non-biological processes on Earth that produce these molecules does not mean such processes cannot exist elsewhere.  After all, these are relatively simple molecules, and without direct knowledge of the planet’s surface conditions—beneath its atmospheric layer—or of any volcanic or geological activity that may be occurring there, we cannot rule out the possibility that they are being produced through entirely abiotic means.

In fact, just a few months ago, analysis of samples from the asteroid Bennu showed the presence of contained complex molecules, including some of the building blocks of life. This finding suggests that the spontaneous formation of simpler molecules on a distant planet may be far from unlikely.

There’s one more point worth mentioning. Although this planet is relatively close in cosmic terms, it remains unimaginably distant from any practical perspective. Even if we were to discover intelligent life there, send a message that will be understood and receive an immediate response, the reply would take 240 years to reach us. In other words, if we asked how they're doing, only the great-great-great-grandchildren of our great-grandchildren might live to hear the reply. With current technology, a spacecraft would take millions of years to reach K2-18b—longer than the entire span of human existence.

The researchers are fully aware of these limitations and make no attempt to downplay them. The University of Cambridge’s press release was appropriately cautious, titled “Strongest hints yet of biological activity outside the solar system.” As is often the case, many media outlets glossed over the nuances, and headlines such as “Clear signs of life found on another planet” began to circulate. The reality, of course, is far more nuanced. Still, the discovery is a compelling one—far from conclusive and requiring much more research, yet offering another small clue that we may not be alone in the universe.

Far from definitive evidence of life. Watch the University of Cambridge's video explaining the research: