Towards the end of his life, the cosmologist Stephen Hawking was asked about the odds of finding intelligent alien life in the next two decades. “The probability is low,” he declared in 2016, and took a lengthy pause before adding: “Probably.”
This week, other scientists from the University of Cambridge reported tentative evidence for two compounds in the atmosphere of a planet, K2-18b, that sits in the constellation of Leo 124 light years away.
On Earth, dimethyl sulphide (DMS) and dimethyl disulphide (DMDS) are hallmarks of life, emanating only from microscopic organisms. And while marine phytoplankton might not rank as particularly intelligent, the claim unleashed a wave of excitement: the answer to the question “Are we alone?” has never seemed closer.
“This is the strongest evidence to date for a biological activity beyond the solar system,” Prof Nikku Madhusudhan, an astrophysicist in the team, told the Guardian before the announcement.
“Decades from now, we may look back at this point in time and recognise it was when the living universe came within reach.”
Two more years of observations with the James Webb Space Telescope (JWST) will nail down whether or not the compounds really exist in the planet’s atmosphere, Madhusudhan believes. The telescope – located about a million miles from Earth – is uniquely placed to make such measurements.
When a planet wanders across the face of its star, JWST can detect subtle shifts in the light it gathers, caused by compounds in the atmosphere absorbing starlight. Make enough observations and the measurements can confirm which compounds are abundant in the alien air.
But even if K2-18b’s atmosphere contains the two compounds, the question of life will not be solved: scientists cannot rule out that the chemicals form in other ways on other worlds, without calling on a cast of tiny aliens.
Conclusions, then, are hard to draw from the work, but the broader message is clear. Over the past decade or so, a swathe of technological advances and ambitious projects have transformed humanity’s chances of finding evidence for life elsewhere, says Monica Grady, emeritus professor of planetary science at the Open University.
“It’s partly because of new equipment,” she said. “JWST is a more powerful telescope than its predecessors because it can look at distant objects in greater detail. Some of the advances, though, result from new technologies applied to older equipment, or new methodologies applied to experimental techniques.”
Take the curious case of Venus. In 2020, a team led by Jane Greaves, an astronomer at Cardiff University, stunned the scientific community by publishing evidence for a pungent gas, phosphine, high up in the Venusian clouds.
They could find no other explanation for the gas beyond the presence of life. The work sparked an intense debate, but last year astrophysicists at Imperial College London found further evidence for phosphine in the planet’s atmosphere. To do so, they used a new detector fitted to the nearly 40-year-old James Clerk Maxwell telescope in Hawaii.
Newly devised analytical approaches are also boosting the search for alien life. Nasa’s Curiosity rover has been trundling around Gale crater on Mars for more than a decade.
In that time, it has drilled into rocks and studied the constituents of sediments that once lay at the bottom of an ancient lake bed. But it was a new analytical procedure that revealed the presence last month of long-chain alkanes on the planet, the largest organics found yet.
The compounds are not a smoking gun for life, but they are precisely what researchers find in rocks when biological cells degrade.
Software advances are coming, too. Scientists have not brought the full power of artificial intelligence to bear on vast quantities of historical observations that could be hiding signs of life, said .
Nor have major new telescopes, such as the Square Kilometre Array (SKA) in South Africa and Australia, or the Extremely Large Telescope (ELT) in Chile’s Atacama Desert, swung into action.
“The next decade promises to be even more exciting than the past one, in terms of moving closer to determining whether there is life beyond Earth,” Grady said.
If humans do share the cosmos with other life, much of it may be microbial. On Mars, scientists are looking for signs of microbes past or present, through biological remnants or waste gases such as methane emanating from bugs still eking out an existence deep in the Martian soil.
Intelligent life is considered a rarer prospect, but that hasn’t deterred efforts to find it.
The most extensive search, the Breakthrough Listen project, launched in 2016 with major telescopes around the world hoping to hear telltale signals or “technosignatures” from distant worlds.
To date, project scientists have detected interference from mobile phones and overflying satellites, and got excited about one intriguing signal, but have heard nothing from clever aliens.
Future missions will soon join the search for life. Nasa’s Europa Clipper should arrive at Jupiter’s moon in 2030 and determine whether an ocean thought to lie beneath the icy crust can support life.
Soon after, another Nasa mission, the Habitable Worlds Observatory, is due to launch. It is the first telescope specifically designed to hunt for signs of life on planets around distant stars.
When might we have an answer? What constitutes proof? Caroline Freissinet, an analytical chemist who discovered the largest organics on Mars, says definitive evidence may forever be elusive, at least on Mars.
“The search for life on Mars is a question of probability,” she said. “The question is, at what probability can we start to claim that it’s life?
“Is it when we are 60% sure, or 90% sure? There is no answer to that. But we will never be 100% sure. Except if there is a marmot coming out of the ground and saying ‘hi’ to the camera.”
