The recent announcement that bursts of radio waves have been detected coming from deep space has rekindled one of the biggest controversies in science: are we alone in the universe?
Detected by a team of astronomers working at a radio telescope in British Columbia, Canada, the radio waves seem to come from a galaxy 1.5 billion light-years away. Their source is a complete mystery, and for now it has simply been labelled an FRB (Fast Radio Source).
But whatever it is, it’s joined the growing list of bizarre phenomena adding fuel to the debate over the existence of aliens.
In August 2017, astronomers detected a flurry of radio bursts from another galaxy, again of unknown origin.
Meanwhile, the strange behaviour of a star in our own galaxy - code-named KIC 8462852 - has led to suggestions it may be surrounded by an alien structure.
And then there’s the oddest event of them all: the cosmic fly-by of an object called ‘Oumuamua, which zipped through the solar system just over a year ago.
Its trajectory and incredible speed of over 300,000kph showed that ‘Oumuamua is the first known object to enter the solar system from another part of our galaxy. Within days it had vanished back into deep space. But exactly what it is remains a mystery.
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Glimpses of it through the world's most powerful telescopes hinted at an unusual shape. Rather than the usual boulder-like profile of a comet or small asterioid, 'Oumuamua was up to 1,000 metres long but only a few hundred metres wide.
At first, artists’ impressions portrayed it as being a giant slab of rock. But then another idea began to do the rounds: perhaps it was an alien spacecraft.
The suspicions were fuelled by the discovery that ‘Oumuamua was not following the path predicted using the law of gravity. Instead, some other force seemed to be at work, nudging it in a different direction.
Initially, astronomers thought the object must therefore be a comet. These “dirty snowballs” are known to give off vapour jets as they approach the sun and melt.
But there was no sign of the familiar comet “tail” pointing away from the sun.
There was one explanation most astronomers were quick to reject, however: that ‘Oumuamua is an alien spacecraft.
Yet some scientists are refusing to toe the party line, and are taking the possibility seriously.
Professor Avi Loeb is a hugely respected astrophysicist, and chair of Harvard University’s astronomy department.
Even so, he has risked his reputation by publicly stating that ‘Oumuamua “may be a fully operational probe sent intentionally to Earth vicinity by an alien civilization".
For Prof Loeb, a big clue to the real nature of the object comes from its speed relative to our solar system.
In an interview with The New Yorker earlier this month, he explained that if ‘Oumuamua was just an inert chunk of rock from another star, it should be travelling with roughly the same speed as the star.
Studies suggest, however, that only around 1 in 500 stars in our part of the galaxy are moving at a speed consistent with that of ‘Oumuamua.
That suggests either it has either come from a very unusual star – or its speed is dictated by some power source.
Calculations by Prof Loeb suggest that one possibility is that the object may not be slab-like, but more like a vast sail. That would allow it to be pushed through space using the delicate pressure of starlight.
Prof Loeb and fellow Harvard astrophysicist Dr Shmuel Bialy, published their calculations late last year in the highly respected Astrophysical Journal Letters.
But while their calculations may be right, the implication that ‘Oumuamua must be the work of aliens has yet to convince many other scientists.
The counter-argument is that while aliens may well exist somewhere in the vastness of space, chances are they’re nowhere near us.
Given that low probability, the evidence needed to make the case for aliens being nearby has to be extremely strong – and estimates based on glimpses of a fast-moving dot in space don’t cut it.
It’s a line of argument that has saved many scientists from embarrassment in the past.
In 1967, radio astronomers at Cambridge University picked up incredibly regular signals from deep space.
At the time, it seemed unlikely the signals could be produced by some natural phenomenon, prompting the astronomers to nickname the source LGM-1 – Little Green Man 1.
But shortly afterwards, its true identity was discovered: a rapidly-spinning – and entirely natural – collapsed star known as a pulsar.
A decade later, an astronomer at Ohio State University was scanning data from a radio telescope set up to find alien signals when he spotted a very unusual pattern.
It was strong, narrow, and in a frequency band ideal for interstellar communication. This led the astronomer to scribble "Wow!" on the print-out – which became the event’s moniker in scientific circles.
Not until 2017 was an explanation found for the Wow Signal: it now appears to have been a comet emitting natural radio signals as it orbited the sun.
Prof Loeb and Dr Bialy’s ideas will probably never be debunked – not because they’re correct, but because ‘Oumuamua is unlikely to be seen again.
They think the best hope of discovering its true nature is by looking for similar objects passing through our solar system.
We may not have to wait long to find them. A colossal telescope ideal for the purpose is currently being built high in the Chilean Andes.
The Large Synoptic Survey Telescope will take images of the entire night sky every few days, and should be completed next year.
If more ‘Oumuamua-like objects are found, Prof Loeb thinks we should try communicating with them – just in case they really are alien spacecraft.
“If these beings are peaceful, we could learn a lot from them”, he told Der Spiegel.
Anyone fretting that this might not be a good idea should stop worrying: the Earth has been beaming TV and radio signals into space for almost a century.
Perhaps this explains why ‘Oumuamua zoomed back into deep space so quickly.
Robert Matthews is Visiting Professor of Science at Aston University, Birmingham, UK
