The James Webb Space Telescope (JWST), which launched in December 2021 and became fully operational in July 2022, has been making groundbreaking observations of exoplanets in recent months. One observation in particular of the exoplanet K2-18b has sparked excitement and debate in the scientific community about the potential detection of biosignatures – signs of life – in the atmosphere of this faraway world.
K2-18b is a “mini-Neptune,” an exoplanet larger than Earth but smaller than Neptune, orbiting the M-dwarf star K2-18 about 111 light years from Earth. JWST analyzed the atmosphere of K2-18b using spectroscopy, separating out the different wavelengths of light filtering through the planet’s atmosphere as it passes in front of its star. In the data, scientists identified the chemical signature of water vapor, lending support to the idea that K2-18b could have liquid water oceans on its surface – a key ingredient for life as we know it.
The Science Behind the Observations
“We detected water, and we can now determine how much water is in the atmosphere,” said Professor Björn Benneke of the University of Montreal, who led the observations. “Can it form liquid water droplets or ice? Can water cycle between the atmosphere and the surface? Understanding the water cycle helps us determine the composition and conditions at the surface.”
The JWST data provided the most precise measurements yet of K2-18b’s atmospheric composition. Scientists are also interested in methane levels, as methane combined with water vapor and carbon dioxide can be a tentative biosignature. “We see methane, and can measure how abundant it is, but we cannot say if it was created by life or geological processes,” Benneke said. More observations are needed to make that determination.
“It’s too early to speculate about the habitability of this planet or any others discovered by JWST,” said Dr. Nikole Lewis, an astronomer at Cornell University. “But these first observations of a rocky, habitable-zone exoplanet are a significant step forward in our search for life.”
History of K2-18b
K2-18b has fascinated astronomers ever since its discovery in 2015 by NASA’s Kepler spacecraft. About 2.6 times the size of Earth and over 8 times as massive, K2-18b orbits in the habitable zone of its small red dwarf star, at just the right distance for liquid water to exist.
“K2-18b receives a similar amount of energy from its star as Earth does from the Sun,” explained Dr. Björn Benneke. “If the planet has a similar atmospheric composition as Earth and contains surface liquid water, then its surface may have a moderate temperature, allowing liquid water to exist.”
After K2-18b’s initial discovery, follow-up observations detected water vapor, hydrogen and helium in its atmosphere. The ratio of hydrogen to helium provided clues that K2-18b has a large extended atmosphere, surrounding a dense water layer below. However, the early observations were not precise enough to determine much beyond this basic atmospheric composition. JWST’s advanced instruments have now enabled a much more detailed analysis.
The Significance of Exoplanet Atmospheres
Over 5,000 exoplanets have been confirmed to date, mainly by the Kepler mission, but studying their atmospheres remains challenging even with advanced telescopes. As starlight filters through an exoplanet’s atmosphere, signatures of molecules can be detected based on which wavelengths are absorbed. This allows scientists to chemically analyze the atmosphere.
“Atmospheres are extremely important because they impact planetary evolution and habitability,” said Dr. Natalie Batalha of the University of California. “Understanding the components, layered structure, and extent of an atmosphere leads to insights on both the origin and eventual fate of a planet.”
For small rocky planets like Earth, atmospheres provide gases essential for life – such as oxygen, nitrogen and carbon dioxide – as well as greenhouse gases to balance temperatures. They also contain biogenic gases generated by lifeforms, like methane, ozone, nitrous oxide and more. Detecting these biosignature gases around exoplanets is an active area of research.
“So far, no exoplanet atmospheres with clear biosignatures have been measured,” cautioned Lewis. “But JWST gives us that capability for the first time with small, rocky planets.” K2-18b may not be a true Earth twin, but serves as an important test case.
While many outlets rushed to claim a discovery of life on K2-18b, scientists involved are quick to rein that in. “There are false positives and false negatives inherent to searching for life with today’s telescopes,” Lewis explained. “We may find a planet that looks alive but isn’t. Or vice versa.”
To build confidence in any biosignatures found, Lewis says more observations are needed from JWST as well as other facilities like the upcoming Extremely Large Telescope. Multiple strong biomarker detections, showing gases out of chemical equilibrium, would make for a convincing case. There is also much work to be done modeling exoplanet environments and potential false positive scenarios.
“We have to approach this systematically and not jump to conclusions, either positive or negative, before doing rigorous hypothesis testing,” said Dr. Thomas Zurbuchen, NASA Associate Administrator. “That is the scientific way.”
At the same time, Zurbuchen knows the implications of discovering life beyond Earth would be immense. “If we ever get to the point where we think we have detected life outside the solar system,” he said, “we will proceed very carefully, questioning ourselves, before making any announcements.”
What’s Next for JWST
While the exoplanet observations have made headlines, JWST has a full slate of programs studying all facets of the cosmos. One surprising early result was the clearest image yet of the Cartwheel Galaxy, revealing new details on star formation. JWST’s infrared instruments can also peer back to within 100-200 million years of the Big Bang, when the first stars and galaxies lit up the young universe.
“JWST will revolutionize almost every area of astrophysics,” said Dr. Amber Straughn, Deputy Project Scientist for Webb. “We’ve only begun to scratch the surface of what it can do.”
Later this year JWST will turn its mirrors towards Mars, capturing weather dynamics and seasonal changes. More exoplanets are in the queue too, including GJ 367 b orbiting a red dwarf just 31 light years away. About the size of Mars, GJ 367 b resides on the hot inner edge of the habitable zone. Understanding how it has evolved over time serves as “a weather forecast model for what the fate of our planet could be in front of an increasing luminous Sun,” said Prof. Björn Benneke.
In other words – are we glimpsing our own future? Exoplanet atmospheres observed by JWST today could foretell what happens to worlds like Earth billions of years from now. It’s a sobering thought, but also underscores why this research matters. Finding evidence of life thriving in unexpected places, like K2-18b, would fundamentally transform how we view biology’s place in the cosmos.
JWST’s initial discoveries have shown its power to explore those timeless questions: Where did we come from? Where are we going? Are we alone? Whatever answers it reveals along the way, one thing is certain – our view of the universe will never be the same.