In a potentially groundbreaking discovery, scientists utilizing the James Webb Space Telescope have obtained what they are calling the strongest indications to date of possible life beyond our solar system, detecting in the atmosphere of an alien planet the chemical signatures of gases that on Earth are exclusively produced by biological processes.
The two gases – dimethyl sulfide, or DMS, and dimethyl disulfide, or DMDS – observed by Webb in the atmosphere of the planet designated K2-18 b are generated on Earth by living organisms, primarily microbial life such as marine phytoplankton – algae.
This finding suggests the planet could be teeming with microbial life, the researchers stated. However, they emphasized that they are not announcing the discovery of actual living organisms but rather a potential biosignature – an indicator of a biological process – and that the findings should be interpreted cautiously, pending further observations.
Nonetheless, they expressed excitement. “These are the first hints of an alien world that is possibly inhabited,” said astrophysicist Nikku Madhusudhan of the University of Cambridge’s Institute of Astronomy, the lead author of the study published in the Astrophysical Journal Letters.
“This is a transformational moment in the search for life beyond the solar system, where we have demonstrated that it is possible to detect biosignatures in potentially habitable planets with current facilities. We have entered the era of observational astrobiology,” Madhusudhan asserted.
Madhusudhan noted that various efforts are underway to search for signs of life within our own solar system, including several claims of environments that might be conducive to life on bodies like Mars, Venus, and various icy moons.
K2-18 b has a mass 8.6 times that of Earth and a diameter approximately 2.6 times larger than our planet.
It orbits within the “habitable zone” – a distance from its star where liquid water, a crucial ingredient for life as we know it, can exist on a planetary surface – around a red dwarf star smaller and less luminous than our sun, located about 124 light-years from Earth in the constellation Leo. A light-year is the distance light travels in a year, equivalent to 5.9 trillion miles (9.5 trillion km). Another planet has also been identified orbiting this star.
Since the 1990s, approximately 5,800 planets beyond our solar system, known as exoplanets, have been discovered. Scientists have theorized the existence of exoplanets termed “hycean worlds” – planets covered by a liquid water ocean potentially habitable by microorganisms and possessing a hydrogen-rich atmosphere.
Earlier observations by Webb, launched in 2021 and operational since 2022, had identified methane and carbon dioxide in K2-18 b’s atmosphere, marking the first detection of carbon-based molecules in the atmosphere of an exoplanet within its star’s habitable zone.
“The only scenario that currently explains all the data obtained so far from JWST (James Webb Space Telescope), including the past and present observations, is one where K2-18 b is a hycean world teeming with life,” Madhusudhan explained. “However, we need to be open and continue exploring other scenarios.”
Madhusudhan suggested that if hycean worlds exist, “we are talking about microbial life, possibly like what we see in the Earth’s oceans.” Their oceans are hypothesized to be warmer than Earth’s. When asked about the possibility of multicellular organisms or even intelligent life, Madhusudhan responded, “We won’t be able to answer this question at this stage. The baseline assumption is of simple microbial life.”
DMS and DMDS, both belonging to the same chemical family, have been predicted as significant exoplanet biosignatures. Webb found that one or the other, or possibly both, were present in the planet’s atmosphere with a 99.7% confidence level, meaning there is still a 0.3% chance that the observation is a statistical anomaly.
The gases were detected at atmospheric concentrations exceeding 10 parts per million by volume.
“For reference, this is thousands of times higher than their concentrations in the Earth’s atmosphere, and cannot be explained without biological activity based on existing knowledge,” Madhusudhan stated.
Scientists not involved in the study advised caution in interpreting the findings.
“The rich data from K2-18 b make it a tantalizing world,” commented Christopher Glein, principal scientist at the Space Science Division of the Southwest Research Institute in Texas. “These latest data are a valuable contribution to our understanding. Yet, we must be very careful to test the data as thoroughly as possible. I look forward to seeing additional, independent work on the data analysis starting as soon as next week.”
K2-18 b belongs to the “sub-Neptune” class of planets, with a diameter larger than Earth’s but smaller than Neptune, our solar system’s smallest gas giant.
To determine the chemical composition of an exoplanet’s atmosphere, astronomers analyze the light from its host star as the planet passes in front of it from Earth’s perspective, a technique known as the transit method. During a transit, Webb can detect a slight decrease in the star’s brightness. A small fraction of the starlight passes through the planet’s atmosphere before reaching the telescope, allowing scientists to identify the constituent gases.
Webb’s previous observations of this planet provided a tentative hint of DMS. Its new observations utilized a different instrument and a different wavelength range of light.
The “Holy Grail” of exoplanet science, Madhusudhan explained, is to find evidence of life on an Earth-like planet beyond our solar system. He added that humanity has pondered the question “are we alone” in the universe for millennia, and we might be just a few years away from detecting potential alien life on a hycean world.
However, Madhusudhan still urged caution.
“First we need to repeat the observations two to three times to make sure the signal we are seeing is robust and to increase the detection significance” to a level where the odds of a statistical fluke are below roughly one in a million, Madhusudhan said.
“Second, we need more theoretical and experimental studies to make sure whether or not there is another abiotic mechanism (one not involving biological processes) to make DMS or DMDS in a planetary atmosphere like that of K2-18 b. Even though previous studies have suggested them (as) robust biosignatures even for K2-18 b, we need to remain open and pursue other possibilities,” Madhusudhan concluded.
Therefore, the findings represent “a big if” regarding whether the observations are due to life, and Madhusudhan emphasized that it is in “no one’s interest to claim prematurely that we have detected life.”
