Last week, a team of astronomers led by the University of Cambridge professor Nikku Madhusudhan announced that they had found tentative evidence for a ‘biosignature’ embedded in the light from a distant planet. Scientists and non-scientists around the world tried to interpret the results. Was this it? Was this the moment when humanity could finally claim it had answered that ancient question: are we alone?
As an astrophysicist involved in the search for life beyond Earth, I can tell you that the results were not that moment. But that doesn’t make them any less exciting.
The simplest cell on Earth puts every other physical system in the universe – including black holes – to shame
To understand exactly what those James Webb Space Telescope findings mean we need to understand something about where the search for life stands today. First, let’s pull all the way back and recognise that humans have been asking about life in the universe for at least 2,500 years. The ancient Greeks argued over other inhabited worlds in the cosmos. It was also a controversial topic in the Renaissance. Giordano Bruno was burned at the stake in part for his belief in life on other planets. In the modern era, Frank Drake conducted the first astrobiological experiment in 1960when he used a radio telescope to look for signals from distant technological civilisations. He began the search for extraterrestrial intelligence, which captured the public’s imagination and inspired countless alien films.
While Drake’s experiment was a milestone, what people don’t realise is that the search for extraterrestrial life was always a marginalised field. It didn’t get much funding so there was never much actual searching.
Just as importantly, the search for intelligent life, as the name implies, involves looking for signals created by advanced beings. But unintelligent life is equally significant. The simplest cell on Earth puts every other physical system in the universe – including black holes – to shame. Finding even one example of life on another world would tell us that we’re not an accident. But unintelligent life doesn’t beam powerful radio signals into space. How, then, might we ever find evidence that it exists? Answering that question shows why last week’s results were so important, even if they prove to be wrong.
In 1995, astronomers discovered the first exoplanet (a planet outside our solar system). By 2010, they had discovered so many exoplanets that it was clear every star in the sky hosts a family of worlds. More importantly, they found that one in five stars had a planet in what’s called the habitable zone, which is the not-too-hot, not-too-cold region in a star’s orbit where a planet has the right conditions for water to pool on its surface. Since temperate oceans are believed to be essential for life, the discovery of so many habitable-zone planets meant the existence of extraterrestrial life became more plausible.
Astronomers had to learn how to probe the atmospheres of these distant worlds. As an exoplanet passes in front of its star (from our perspective), there is a brief window of time when starlight will pass through the planet’s veil of atmospheric gases – if it has an atmosphere. Every molecule in a planetary atmosphere emits or absorbs different wavelengths of light in a unique way. Molecules have specific spectral fingerprints. By looking at the light passing through a planet’s atmosphere and searching for those fingerprints, astronomers can detect the presence of, say, carbon dioxide.
The remarkable thing about life is its ability to hijack a planet’s evolution. At least, that’s what happened on Earth. Once microbes appeared on our world 3.5 billion years ago, they quickly began to reshape the planet. Oxygen exists in our atmosphere only because life put it there as a by-product of photosynthesis. In that way, Earth’s atmospheric oxygen is a signature that our planet has a biosphere – the sum total of life.
What Madhusudhan and his colleagues claimed to find was just that: a biosignature. That’s why their discovery made news. They were looking at K2-18b, an ocean planet about eight times the mass of Earth, which orbits a small red star 124 light years away. Using the James Webb Telescope, the team found spectral fingerprints in the planet’s atmosphere that could be attributed to the molecule dimethyl sulphide (DMS).
On Earth, DMS is released into the atmosphere by phytoplankton – tiny photosynthesising creatures in the oceans. As far as scientists know, there’s no way to make DMS except as a by-product of life. That’s why the compound has been on astronomers’ list of potential biosignatures for more than a decade. Last week’s results, which seemed to find evidence for DMS on a world expected to be nothing but ocean, made everyone sit up and take notice.
So were they evidence for life? The first thing to say about last week’s detection of DMS is that it wasn’t really a detection – not yet. Scientists have an exact way of describing their confidence in a result. The gold standard is what is known as ‘five-sigma’ detection, which indicates a very high degree of statistical certainty. If a signal – such as a biosignature – meets the five-sigma threshold, then there is only about a one-in-a-million chance that the findings are a fluke. But the DMS result announced last week was only at the three-sigma level (a 0.3 per cent probability that it occurred by chance). History is full of exciting three-sigma results that vanished when more data was gathered. So no, we can’t yet say we’ve found a signature of life on another world. Madhusudhan and collaborators were quite clear about this themselves in their paper and their news conference.
The day the news came out, I had many conversations with my astrobiologist colleagues. They were all deeply sceptical. Some argued the detection of DMS wasn’t even at the three-sigma level. Some questioned why other compounds expected in the atmosphere hadn’t been detected. Others critiqued the maths behind the data analysis. In general, they pushed back pretty hard.
Their scepticism is, ironically, why I’m so excited. You should be too. Remember those centuries of people arguing about life in the universe? All they had were hunches. Now we finally have data. For the first time, we can go beyond opinion and start doing real science. My colleagues’ objections were technical. For the first time, we’re debating a specific biosignature claim on a specific exoplanet using a cutting-edge instrument.
The other thing that struck me was how harsh the criticism was. That’s a good thing. We all want to know if life exists elsewhere. That’s why we will try to shred any claim when it first appears, even when it comes from colleagues we deeply respect. We want the truth; our colleagues expect us to be ruthless. That’s how, one day, we will find a biosignature that stands up to scrutiny. That’s how we’ll know if Earth and its creatures are unique – or part of a cosmic family.
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