Is someone really out there?

An artist’s impression, released by ESA/Hubble, of the K2-18b super-Earth. PIC/PTI

For as long as humans have gazed at the night sky, we've asked a question both simple and profound: Are we alone in the universe? Once the domain of philosophers and science fiction writers, that question has become the subject of serious scientific inquiry. And now, thanks to a faraway planet and the most powerful space telescope ever launched, we may be closer than ever to a real answer.

The planet in question is K2-18 b, orbiting a red dwarf star roughly 120 light-years away in the constellation Leo. It was discovered in 2015 and, at first, was just one among thousands of exoplanets - worlds outside our solar system - discovered over the past three decades. But recent findings using NASA's James Webb Space Telescope (JWST) have thrust this planet into the spotlight.

What makes K2-18 b so interesting isn't just its size - about 2.6 times the diameter of Earth and almost nine times its mass - but the fact that it orbits in the so-called "habitable zone" of its star, where conditions might allow liquid water to exist. More compelling still are the atmospheric chemicals detected by JWST: methane, carbon dioxide, and - most intriguingly - dimethyl sulfide, or DMS. On Earth, DMS is a compound produced almost entirely by living organisms, particularly marine plankton. No known non-biological process creates DMS in notable amounts on our planet. If confirmed, the presence of DMS in K2-18 b's atmosphere would represent the first possible biosignature - an actual clue to life - found beyond our solar system. While methane and carbon dioxide can be created through geological processes, DMS is different. It is not something that volcanoes or chemical reactions typically produce on their own. Its presence suggests, at the very least, that we may be looking at an environment that resembles Earth's oceans in some fundamental way.

To understand how scientists reached this conclusion, consider the technique used - transit spectroscopy. When K2-18 b passes in front of its star, a tiny fraction of starlight filters through the planet's atmosphere. The molecules in that atmosphere absorb certain wavelengths of light, leaving behind a spectral fingerprint. By analysing this fingerprint, astronomers can determine which gases are present. It's an unbelievable feat - analysing the atmosphere of a planet more than a hundred light-years away based solely on how it bends starlight.

The team behind this work was led by Professor Nikku Madhusudhan of the University of Cambridge, an Indian-origin astrophysicist who has emerged as a leading voice in exoplanet science. While methane and carbon dioxide detections are robust, the signal for DMS is still tentative, though statistically meaningful. Currently, the DMS detection stands at a 3-sigma confidence level - meaning there's about a 0.3% chance the signal is a statistical fluke. In scientific terms, this constitutes "evidence," but not a discovery. The gold standard is 5-sigma (a 0.00006% chance of error), which is the threshold that decided the discovery of the Higgs boson at CERN in 2012. This level of rigour isn't bureaucratic - it's foundational. Science works not just on excitement, but on evidence. A 3-sigma signal invites further scrutiny, while a 5-sigma signal demands acceptance. In acknowledging this limitation, the researchers embody an often-overlooked virtue in science - epistemic humility. Instead of making bold claims, they've opted for cautious optimism, refining their models, collecting more data, and inviting peer review. This is science at its best - self-correcting, careful, and grounded in evidence. The DMS signal could eventually reach 5-sigma, or it could fade with better data. Either way, it's the process, not just the outcome, that pushes our understanding forward.

Of course, in science, caution is essential. The excitement around DMS must be tempered with the understanding that this is not yet a confirmed discovery. The signal is intriguing, but not yet strong enough to rule out all other explanations. Another team, led by Dr Nicholas Wogan at NASA Ames Research Center, has proposed an alternative interpretation. According to their models, K2-18 b might not be an ocean-covered "Hycean" world, but rather a mini-Neptune with a deep, thick atmosphere and no solid surface. In such an environment, high temperatures and pressures deep inside the planet could drive complex chemical reactions, producing gases like methane and carbon dioxide without any biological input. These gases would then rise to the upper atmosphere, mimicking what we might expect from a life-bearing planet.

Temperature adds another layer of complexity. If K2-18 b lacks a reflective cloud layer, it may absorb too much heat from its star, pushing the temperature of its lower atmosphere above 647 degrees Celsius - the so-called critical point of water. At that point, water no longer behaves like a liquid or a vapour but instead becomes a supercritical fluid, a state of matter that is not known to support life as we understand it. Whether K2-18 b has such clouds - or any water at all - is still unknown.

Still, whether or not life exists on K2-18 b, these findings represent a turning point in the way we search for life beyond Earth. For the first time, we are not just cataloguing distant planets; we are analysing their atmospheres, searching for the kinds of chemical fingerprints that might indicate biology. And we are doing so with a level of detail and sophistication that was unimaginable just a decade ago.

It's also a moment of quiet pride for Indian science and for scientists of Indian origin working at the frontiers of discovery. Professor Madhusudhan's work is a reminder that the exploration of the universe is a global endeavour, one that draws on the talents and insights of researchers from every part of the world. In a recent interview, he remarked, "We are now in an era where we can not only find potentially habitable planets, but also explore their atmospheres. It is a truly transformative moment for science." That transformation is still unfolding.

More observations of K2-18 b are planned using JWST, and other telescopes are on the horizon. NASA's upcoming Roman Space Telescope and the European Space Agency's ARIEL mission will add new tools to the astronomer's kit, allowing even more detailed studies of exoplanet atmospheres. Meanwhile, scientists here on Earth are working in laboratories to recreate the conditions found on distant planets, testing whether molecules like DMS can be formed in lifeless settings.

If future data confirms the presence of DMS and rules out non-biological sources, K2-18 b could become the first serious candidate for a life-bearing planet beyond Earth. But even if that confirmation never comes, this moment matters. It marks the beginning of a new era - one where our search for life is guided not by speculation, but by data, precision, and evidence.

Maybe the universe is finally whispering back.

Nishant Sahdev is a theoretical physicist at the University of North Carolina in Chapel Hill, United States. He can be contacted at nishantsahdev.onco@gmail.com