Citizen Science #27 by Jamie Zvirzdin

Solving the Mystery of Life on a Faraway World

“Who could imagine that a little farther on in that quiet landscape I should meet and shake by the hand a man who […] had seen this Earth from where it looks like a mere point of green fire, and who had spoken face to face with a creature whose life began before our own planet was inhabitable?”

This line from “Perelandra,” C.S. Lewis’ 1943 novel and the second book of his Space Trilogy series, speaks to our endless fascination with the possibility of life beyond our home. Today, this fascination points us toward an exciting discovery: a distant planet with a rather forgettable name, K2-18b, and a story that could change how we see our place in the cosmos.

K2-18b isn’t exactly next door. At 124 light-years away, even our swiftest spacecraft would need nearly two million years to reach it. Despite this vast distance, we’re learning more about it with every passing year.

First discovered in 2015 by the Kepler Space Telescope, K2-18b quickly drew attention. It is roughly 2.6 times Earth’s diameter, making it unlike anything in our solar system—a so-called “hycean” world. Imagine a planet with a deep, global ocean beneath a huge, puffy atmosphere dominated by hydrogen and helium. It’s not unlike Lewis’ fictional world Perelandra: vast, watery, mysterious.

But is there truly an ocean there? The answer remains elusive. Earlier predictions suggested K2-18b’s water might exist in a strange supercritical state—neither purely liquid nor gas. Recent observations by NASA’s James Webb Space Telescope, however, lean toward the possibility of a clearer boundary between liquid water and the planet’s thick atmosphere.

Intriguingly, JWST detected carbon dioxide and methane in 2023, both considered promising signs for life’s potential. Then, in an article published in “The Astrophysical Journal Letters” on April 17, 2025, an even more startling discovery emerged: spectral signs of dimethyl sulfide, a molecule known on Earth exclusively from living organisms, mostly ocean-dwelling phytoplankton.

Could this be the “smoking gun” of alien life? As usual, good scientists urge caution. As popular science educator Hank Green explains, real science rarely unfolds in neat moments of instant revelation. Instead, discoveries come gradually, through a process riddled with doubt, repeated checks and careful re-analysis. For K2-18b, scientists face the complication that dimethyl sulfide behaves differently depending on atmospheric conditions—conditions we know well only from Earth. This leaves ample room for doubt and further investigation.

Adding complexity, K2-18b orbits closely around a red dwarf star—a cooler, smaller and notably temperamental type of sun. Such stars often unleash powerful flares that could scour nearby planets, stripping away protective atmospheres. Without clear evidence of a magnetic field protecting K2-18b, its atmosphere might be vulnerable, complicating its potential habitability.

Another strange twist: Planets orbiting this closely are usually tidally locked, meaning one side faces permanently toward their star. This creates an “eyeball planet,” with scorching permanent daylight on one side, potentially balanced by icy darkness on the other. Life, if present, might cluster around a narrow, habitable twilight zone.

Despite these complexities, why is K2-18b drawing so much attention? It’s precisely because of its unusual “poofy” atmosphere. When a planet with such a thick envelope passes in front of its star, more starlight filters through the atmosphere, allowing astronomers to capture clearer spectral fingerprints of the molecules within. This technique, known as spectroscopy, is our best hope for detecting extraterrestrial life outside our solar system. Scientists identify chemical signatures by noting which colors (wavelengths) of starlight vanish as the planet passes, indicating the presence of specific molecules.

The intriguing molecule, dimethyl sulfide, complicates the puzzle. Although some hypothetical non-living processes might create it, such scenarios are far too slow to keep pace with how swiftly the molecule breaks down—even in a hydrogen-rich atmosphere like that of K2-18b. Many scientists are therefore arguing that something must be rapidly replenishing this chemical—something potentially alive.

Of course, skeptics argue that the recent findings might be premature or overblown, pointing to uncertainties in interpreting new spectral data. As Green emphasizes, science often moves more slowly than headlines suggest.

Regardless, the mystery of K2-18b deepens our wonder, and I absolutely encourage us to enjoy this unfolding story. We are better people when we step outside our own perspectives and problems and embrace a wider cosmic narrative and its attendant uncertainties. With ongoing studies and new data due soon, we might eventually confirm signs of life—or learn something even stranger.

And perhaps, should we confirm life on this distant world, we might give it a better name. Rather than the cumbersome label K2-18b, I vote for “Perelandra” to honor this vast, watery mystery.

Jamie Zvirzdin researches cosmic rays with the Telescope Array Project, teaches science writing at Johns Hopkins University and is the author of “Subatomic Writing.”