In what could be a groundbreaking discovery, scientists utilizing the James Webb Space Telescope have reported obtaining what they describe as the most compelling evidence yet of potential life beyond our solar system. They have identified chemical signatures in the atmosphere of an exoplanet that, on Earth, are produced exclusively through biological processes.
The two gases—dimethyl sulfide (DMS) and dimethyl disulfide (DMDS)—were detected in the observations of a planet known as K2-18 b. On our planet, these gases are primarily generated by living organisms, particularly by microbial life such as marine phytoplankton and algae.
This implies that the planet may be brimming with microbial life, according to the researchers.
However, they emphasized that they are not proclaiming the discovery of actual living organisms but rather pointing to a potential biosignature—an indicator of biological activity. They urged caution, noting that further observations are needed.
About 5,800 planets outside our solar system, termed exoplanets, have been identified since the 1990s.
“This is a transformative moment in the quest for life beyond our solar system, demonstrating that it is possible to detect biosignatures in potentially habitable planets using current technology. We have entered the era of observational astrobiology,” stated Mr. Madhusudhan.
Mr. Madhusudhan mentioned that various initiatives are underway to seek signs of life within our solar system, including multiple claims about environments potentially conducive to life on Mars, Venus, and several icy moons.
K2-18 b is approximately 8.6 times the mass of Earth and has a diameter around 2.6 times larger than our planet.
It orbits within the “habitable zone”—the distance where liquid water, a crucial element for life, can exist on a planetary surface—around a red dwarf star that is smaller and less luminous than our sun.
A light-year is defined as the distance light travels in one year, about 9.5 trillion kilometers.
Another planet has also been discovered orbiting this star.
A ‘Hycean world’
Since the 1990s, around 5,800 exoplanets have been discovered beyond our solar system.
Scientists have hypothesized the existence of exoplanets termed hycean worlds—planets covered by liquid oceans that could be habitable for microorganisms and characterized by a hydrogen-rich atmosphere.
Earlier observations by Webb—launched in 2021 and operational since 2022—had identified methane and carbon dioxide in the atmosphere of K2-18 b, marking the first instance of carbon-based molecules being found in an exoplanet’s atmosphere within a star’s habitable zone.
When asked about the potential for multicellular organisms or intelligent life, Mr. Madhusudhan responded, “At this stage, we can’t answer that question. Our baseline assumption is simple microbial life.”
DMS and DMDS, both part of the same chemical family, have been predicted to be significant biosignatures for exoplanets.
Webb detected one or both of these gases in the planet’s atmosphere with a confidence level of 99.7%, indicating a 0.3% chance that the observation could be a statistical anomaly.
The gases were identified at atmospheric concentrations exceeding ten parts per million by volume.
“For context, this concentration is thousands of times higher than what we observe in Earth’s atmosphere, and current knowledge suggests this cannot be explained without biological activity,” Mr. Madhusudhan commented.
Scientists not involved in the study advised caution.
Transit method
K2-18 b falls within the “sub-Neptune” category of planets, having a diameter larger than Earth’s but smaller than that of Neptune, which is the smallest gas planet in our solar system.
To determine the chemical makeup of an exoplanet’s atmosphere, astronomers analyze the light from its host star as the planet transits in front of it from Earth’s perspective—a technique known as the transit method.
As the planet transits, Webb can detect a decrease in stellar brightness, allowing a small portion of starlight to pass through the atmosphere of the planet before being recorded by the telescope.
This process enables scientists to identify the gases present in the planet’s atmosphere.
Previous observations of this planet suggested a tentative presence of DMS.
The recent observations employed a different instrument and a different wavelength range of light.
The “Holy Grail” of exoplanet research, Mr. Madhusudhan remarked, is to find evidence of life on an Earth-like planet beyond our solar system.
Reflecting on humanity’s longstanding curiosity about whether “we are alone” in the universe, Mr. Madhusudhan expressed optimism that we might be just a few years away from potentially discovering alien life on a hycean world.
Nonetheless, he called for patience.
“First, we need to replicate the observations two to three times to ensure the signal we are detecting is solid and to enhance the statistical significance of our findings,” Mr. Madhusudhan advised, noting that the odds of a statistical anomaly should be reduced to about one in a million.
“Second, we require further theoretical and experimental studies to determine if there exists any abiotic mechanism—one not involving biological processes—that could produce DMS or DMDS in a planetary atmosphere like K2-18 b’s.”
Despite previous studies proposing robust biosignatures for K2-18 b, Mr. Madhusudhan cautioned that we must remain open to alternative explanations and continue exploring other possibilities.
Thus, these findings represent “a big if” regarding whether the observations indicate life, and “it is in no one’s best interest to prematurely claim we have discovered life,” he concluded.
