Representative image of an exoplanet surface.
Credit: iStock Photo
With more than 5,000 already detected, planets are abundantly present outside the Solar System. A 2012 NASA survey estimated that the Milky Way alone contained a staggering 100 billion of these extrasolar objects, called ‘exoplanets’. And that's just in one galaxy out of an estimated 200 billion galaxies in the observable universe.
Given this fact, hunting for exoplanets that may be habitable and capable of harbouring life is a daunting task, one made much more difficult by the vast distances that separate us from our targets of observation.
Yet, this painstaking but exciting work pays dividends—recently, a group of scientists from the University of Cambridge, using the James Webb Space Telescope (JWST), found for the first time carbon-based molecules in an exoplanet lying in the habitable zone of distant star, indicating the presence of a liquid water surface underneath a thin atmosphere.
The study not only found the presence of carbon dioxide and methane in the exoplanet’s atmosphere but also reported “possible evidence” of a molecule called dimethyl sulphide (DMS) that, on our planet at least, is produced primarily by microbial life such as marine phytoplankton.
The exoplanet in question—K2-18 b—lies 120 light-years away and orbits a cool dwarf star in the constellation of Leo, but what makes the findings particularly promising is its location in the habitable or ‘Goldilocks’ zone of the star system.
The findings, understandably, created a buzz around what could be, so we decided to get in touch with the person leading the study—Nikku Madhusudhan, Professor of Astrophysics and Exoplanetary Science at the Institute of Astronomy, University of Cambridge.
In a freewheeling conversation with DH, Prof Madhusudhan explained the robust science behind exoplanet remote-sensing and what it felt like to be one of the people at the forefront of the search for extraterrestrial life.
Observing the unobservable
Given the massive distances that separate us from the exoplanets we're observing, these objects are not directly visible and are instead observed through indirect methods.
An artist's impression of 51 Pegasi b, also called Dimidium, the first exoplanet discovered orbiting a star like our sun.
Credit: NASA
Studying an exoplanet’s atmosphere is a tad bit trickier: “When you want to study a planet’s atmosphere, there’s only a subset [of the 5,000+ exoplanets] that are observable, in the sense that they’re close enough, their stars are bright enough, and you get a good enough signal-to-noise [ratio] to be able to tell the small spectral features of a planet,” Prof Madhusudhan explains.
From this, scientists are able to extract crucial information that tells us more about what an exoplanet might be like.
“If you look at the spectrum of an exoplanet, you can glean a lot of information out of it—chemical information, temperature information, and hints about other physical processes such as clouds and things like that.”
What this information is used for depends on the fundamental question a particular study is looking to answer: “Say you want to answer how this planet was formed, the kind of atmospheric conditions, or say, [questions relating to] astrobiology,” Prof Madhusudhan explains.
And as it so happens, his team’s findings could potentially be intertwined with the latter.
A needle (?) in the cosmic haystack
An unassuming person with a friendly demeanour, Prof Madhusudhan had a tough time hiding his excitement at the preliminary findings on K2-18 b, something that was the culmination of over a decade of scientific research.
“We were just very amazed to see the first results. There was a lot of disbelief when we saw the first results…it was too good to be true, to be able to detect [these] molecules like this,” he said, beaming.
“The carbon-based molecules we found, especially methane…we have been looking for methane for over a decade using previous facilities, and we never found it in low-temperature exoplanets,” Prof Madhusudhan continued, emphasising the difficulty of the protracted search prior to the launch of the JWST last year.
An image showing the atmosphere composition of K2-18 b.
Credit: NASA
“So when you get such a strong detection of it in the first observation of a habitable zone planet, it was quite shocking to realise how easy it was to find, and then we ran a number of robustness tests to make sure that what we had found was actually right and then obviously we were quite happy at the end,” he said.
The Cambridge astronomer’s investigations into K2-18 b began three years ago when he and his team started studying the exoplanet to explore what possible features it could have.
Based on another, previous set of observations on K2-18 b, Prof Madhusudhan and his team had, at the time, predicted the possibility of a liquid water ocean existing on the planet: “We were exploring the conditions that could be possible in the interior, and we found that a subset of solutions allowed for liquid water surface on the planet under a thin atmosphere,” he said.
“That is what led us to propose this new class of objects called Hycean worlds, and we predicted that they should be out there.”
Three years since their hypothesis, the team may have finally found evidence of the same—
The main implication, which we actually stand by in our paper, is that the detection of CH4 and CO2 in a Hydrogen-rich atmosphere without the detection of ammonia… that combination, based on all we know about atmospheric chemistry, can be explained by the presence of a liquid water ocean, and that is what we would call a Hycean world.Prof Madhusudhan, lead study author
An artist's impression of the Hycean world K2-18 b.
Credit: NASA
Notably, these Hycean worlds may not be too uncommon: “Not just K2-18 b but [there should be] other planets as well and we should be able to detect their atmospheric signatures,” the astronomer added.
Searching for life in the dead of space
Water as they say is the elixir of life, but that has yet to be put to the test at the cosmic scale.
While the existence of liquid water on a distant planet may, based on our understanding of life on Earth, indicate its ability to support life, whether it actually can is a different question altogether.
Quizzed about the “possible detection” of DMS in K2-18 b’s atmosphere and its bearings on questions surrounding alien life, the professor was quick to urge caution.
He went on to explain that when it comes to “detecting” certain molecules in an exoplanet’s atmosphere, there are certain criteria relating to statistical confidence that have to be met before one can be sure of their findings: “Over 99 per cent is typically what you expect your confidence to be before you can claim that you have a reasonable detection.”
While carbon dioxide and methane satisfied said criteria comfortably, DMS did not.
“The reason for that is because the signature is faint…it’s expected to be faint compared to the prominent molecules—methane and CO2. But, if you take the data at face value… then we get DMS at 2 sigma (95 per cent) or even lower if we introduce offsets between data from different instruments,” Prof Madhusudhan explained.
“We’re actually not sure if it’s there, but if we take the signal at face value, there is something there, and that is worth following up is all we are saying,” he said candidly, with his team expecting a new set of follow-up observations within a year.
However, he emphasised the need to eliminate all other possible explanations before a claim relating to ‘life’ can be made.
“Then the question is whether it is directly indicative of any biological activity. Now it motivates us to consider that possibility and we should do theoretical calculations about what sort of biomass would give us such levels [of DMS], but that’s not enough,” Prof Madhusudhan said.
“What you want to prove beyond reasonable doubt is that no other abiotic process can produce the molecule in that environment—in the sense that on Earth you cannot get it in any other way apart from life, but in another environment, you just don’t know", the astronomer explained.
“We want to ask as many questions as we can to disprove its [DMS’] links to life, and that’s what we’re doing. The idea is…as much as possible, try to explain it without life, and only when all options fail can we have some reasonable explanation that involves life…that’s just about scientific robustness,” he stressed.
Yet, despite his cautiousness informed by a scientific approach, the professor himself is of the belief that there’s more likelihood of life existing elsewhere than not.
“If we look at 100 habitable planets in the next decade or so and never find life, that would be a major result in itself, [indicating the relative] uniqueness of life. Because just from the number of planets that are out there and the diversity in nature that we’re seeing, and all other aspects, atmospheric properties and so on, it’s very difficult to imagine that there’s no life out there,” the professor said.
A note of caution
The question of life beyond Earth has tormented humanity since antiquity, and with rapid scientific advancements, we seem to be inching closer towards having the capability to answer it.
With more observations on K2-18 b expected soon, Prof Madhusudhan is acutely aware of the implications of what he and his team may find and the ramifications of the same on society.
“You no longer think of it as a result that is coming from you or the research group, but you feel like it’s a result for the species, for our civilization, so there’s a lot more at stake than writing a paper confirming it,” he says.
“It’s a result for all of us together, so we all need to be extremely careful about the result, from the scientific side as well as the reporting side,” he cautions.
“So right now, there’s less excitement and more caution—[what we have to determine is] how do we go about finding the truth in the most robust way possible, given the implications that would have on science and society,” the professor says.
According to Cambridge, the team's next round of observations using the JWST will use the telescope's Mid-InfraRed Instrument (MIRI) spectrograph to analyse K2-18 b's atmosphere for chemical signatures called biomarkers, including DMS, which could potentially indicate the presence of biological activity on the distant exoplanet.