Astronomers announced Thursday that they had discovered the most promising “hints” of potential life on a planet outside our solar system, but other scientists remained skeptical.
Scientists have been debating whether the planet K2-18b, located 124 light years away in the Leo constellation, could be an ocean world capable of supporting microbial life.
Using the James Webb Space Telescope, a British-US team of researchers discovered signs of two chemicals in the planet’s atmosphere that were previously thought to be “biosignatures” indicating extraterrestrial life.
On Earth, only life, primarily microscopic marine algae known as phytoplankton, produces the chemicals dimethyl sulfide (DMS) and dimethyl disulfide.
The researchers emphasized caution, stating that additional observations were required to confirm these findings and that they were not making a definitive discovery.
However, the implications could be enormous, according to Nikku Madhusudhan, a Cambridge University astrophysicist and lead author of the study published in The Astrophysical Journal Letters.
Madhusudhan told BBC News that he hopes to obtain conclusive evidence soon.
“This is the strongest evidence yet there is possibly life out there,” according to him. “I can realistically say that we can confirm this signal within one to two years.”
However, outside experts pointed to disagreements over previous discoveries about the exoplanet, claiming that these chemicals could have been created through unknown means unrelated to life.
“Goldilocks” zone
K2-18b, which is more than eight times the mass of Earth and 2.5 times the size, is unique among the approximately 6,000 exoplanets discovered thus far in that it orbits its star in a habitable or “goldilocks” zone.
This means that it is neither too hot nor too cold to have liquid water, which is regarded as the most essential component of life.
Telescopes observe such distant exoplanets as they pass in front of their star, allowing astronomers to study how molecules block the light that passes through their atmosphere.
The Webb telescope detected methane and carbon dioxide in K2-18b’s atmosphere in 2023, marking the first detection of such carbon-based molecules on a habitable exoplanet.
It also detected weak chemical DMS signals, prompting astronomers to refocus Webb on the planet a year ago, this time using its mid-infrared instrument to detect different wavelengths of light.
They discovered much stronger signs of the chemicals, though they were still well below the “five sigma” threshold of statistical significance that scientists seek in such discoveries.
Even if the findings are confirmed, this does not necessarily imply that the planet is home to life.
Last year, scientists discovered traces of DMS on a comet, indicating that it can be produced in non-organic ways and may not be a “biosignature.”
However, the chemical concentration observed on K2-18b appears to be thousands of times higher than on Earth, indicating a biological origin, according to Madhusudhan.
“Hellishly hot”
K2-18b has long been regarded as the best candidate for a “hycean planet”—an ocean world larger than Earth with a hydrogen-rich atmosphere.
Hycean planets, like Earth, are covered in oceans and have atmospheres rich in hydrogen, an element required for life. Many planets are larger and hotter than Earth, with atmospheres reaching temperatures of nearly 200 degrees Celsius (392 degrees Fahrenheit).
These planets are unlikely to support intelligent alien life, but rather tiny microbes similar to those found billions of years ago in Earth’s oceans.
“Hycean planets open up a whole new avenue in our search for life elsewhere,” Madhusudhan stated previously.
Some researchers have questioned whether the currently proposed hycean planets, such as K2-18b, are too close to their stars to support liquid water.
Raymond Pierrehumbert, a planetary physics professor at Oxford University, conducted separate research that found K2-18b is too hot for life.
If the planet had water, it would be “hellishly hot” and uninhabitable, he told AFP, adding that oceans of lava were more likely.
Sara Seager, a planetary science professor at MIT, urged patience, citing previous reports of water vapor in K2-18b’s atmosphere that turned out to be a different gas.
Madhusudhan estimated that it would only take 16 to 24 more hours of Webb’s time to reach the five-sigma threshold, which could occur within the next few years.
Stephen Burgess, a statistician at Cambridge University, compared the current three-sigma to the odds of flipping a coin ten times and getting the same result every time.
Five sigma would yield that result after 20 flips, implying that “we can be very confident that this observation isn’t just a chance finding,” he explained.
Even beyond K2-18b, Madhusudhan believes Webb and future telescopes will allow humanity to discover life beyond our planet sooner than previously thought.
“This could be the tipping point, where suddenly the fundamental question of whether we’re alone in the universe is one we’re capable of answering,” he told me.