New Delhi: Never in the history of human existence have we come so close to finding life beyond Earth, says professor Nikku Madhusudhan from the University of Cambridge, who has led a team that discovered chemical footprints of molecules associated with life on an exoplanet.
While still in early stages, he stressed that these observations would form the basis of future interplanetary studies.
On Thursday, the team led by the astrophysicist and exoplanetary expert published its findings in the peer-reviewed journal The Astrophysical Journal Letters, claiming to have detected the chemical fingerprints of dimethyl sulphide (DMS) and dimethyl disulphide (DMDS) in the atmosphere of the exoplanet K2-18b, which orbits its star in the habitable zone. This is the region around a star where liquid water can exist on the surface of a planet.
Professor Madhusudhan spoke to ThePrint about the significance of the findings and the road ahead.
Below are edited excerpts from the video interview:
Could you tell us what your team has found?
In a nutshell, we have found hints of DMS and DMDS—one or both of these two molecules—in the atmosphere of the planet K2-18b, which is about 124 light years away from Earth. We have been looking at this planet with theoretical studies and observations for the last five years.
We already have proof, through other studies, that this planet could be habitable—that it could be a Hycean world (proposed class of exoplanets characterised by planet-wide oceans beneath a hydrogen-rich atmosphere). This new evidence adds a bit more to the narrative that the planet could have life on it. But we have to be careful because these are very early signs.
This is a major step in the path of searching for life, but we cannot make a robust association with life just yet. What we are celebrating today is the ability to do so, and the first signs of it. But we should be careful not to stretch it too far and say we have conclusively found life.
Why DMS and DMDS? What association do these molecules have with life?
Both these molecules are known to be produced primarily by life here on Earth. Studies in the past have assessed various molecules that you would look for while finding life elsewhere, and these molecules have emerged as some of the best contenders. As far as we know, these two molecules cannot be produced in large quantities by any process other than life.
Though there have been experiments where tiny amounts of these have been produced in labs, large quantities of these molecules are not produced by any medium other than life. So, these are known to be strong biomarkers for life.
When we talk about large quantities, how large do we mean?
Our initial inference shows that it is a part in 100,000. That may sound like a small number, but when you observe how much of these molecules are present in the Earth’s atmosphere, it is in the order of a part in a billion or lower. So, very small quantities in the atmosphere.
We are inferring that these molecules are thousands of times higher than what we see in the Earth’s atmosphere. That is the significant bit.
Past studies have shown that if you see these molecules in such high quantities, the biological flux of sulphur-based bio-molecules (DMS and DMDS) from the ocean should be at least 20 times higher than what we see on Earth. That association had already been made before our observation, and that is an important point to note. There was a prediction already, and we have made an observation confirming those predictions. This is the big thing.
What was the process you followed to make these observations?
We have been observing this planet (K2-18b) for the last few years.
The first observation we made was two years ago in 2023, with the James Webb Space Telescope (JWST). Back then, too, we had seen significant evidence of carbon-bearing molecules, methane and carbon dioxide, and the absence of molecules, like ammonia and carbon monoxide.
That combination of molecules is what told us that this is most likely a Hycean world—a world with an ocean-covered surface and hydrogen-rich atmosphere.
We made these observations using the transit method. This is where you are looking at a star. You have a telescope and you can take a spectrum of the stars. Spectrum means light as a function of wavelength. But when you are looking at a star, as the planet transits in front of it, some of that starlight passes through the atmosphere of the planet before reaching the telescope. The molecules in the atmosphere absorb some of that starlight at different wavelengths. Light is slightly dimmer when the planet is moving in front of it.
Based on that absorption of light, and when you compare that to when the planet is not in front of the star, that difference can tell you how much absorption is happening in the planet’s atmosphere. That is what is known as the transmission spectrum.
Based on that, we can work out what molecules are in the atmosphere and how much of these molecules need to be in the atmosphere.
The observations are still at a nascent level, a sigma three (in the scientific discovery parameter). How long and how much more observation would it take to reach a sigma five (the acceptable standard for any scientific discovery)?
The three-sigma level is around 99.7 percent. It is a grey area, so it is telling you that there are hints, but we are not 100 percent sure. That is where we are right now. It is enough for us to be interested and invest resources.
With one observation, we could get to this level. We are hoping that with another two or three observations, exactly of the type that we have right now, we can get to a four or five sigma level. That is a big deal. This is why it is such a transformational moment in the search for life.
We got here with eight hours of observation. We would need another 16-24 hours with the JWST.
But even after we get to a sigma five, we will have to do our due diligence and follow up with a lot of experimental work to establish that there are no unknowns. Right now, we know that these molecules only come from life, but we need to be doubly sure that there are no other methods before we can say for sure.
(Edited by Nida Fatima Siddiqui)