In less than an hour, the decision was made. It was 2 December and Avi Loeb, an astronomy professor at Harvard, was with Yuri Milner, the Russian billionaire and founder of Breakthrough Listen, a $100m alien-hunting venture. Milner had invited Loeb, an adviser on the project, to his Palo Alto home to discuss the bizarre features of the interstellar object, ‘Oumuamua.

The first known visitor from another solar system, the monolithic lump appeared long and slender, a curious shape for a space rock. The two agreed there was the slimmest chance ‘Oumuamua was not what it seemed. Eleven days later, Breakthrough Listen swung the world’s largest steerable telescope, at Green Bank in West Virginia, into position and scanned the 400-metre-long body for signs that it was a passing spacecraft.

It was a long shot. After 10 hours of observations the telescope, which can detect a mobile phone signal at twice the distance of the sun, found no evidence that ‘Oumuamua was the work of an alien civilisation. By all accounts, it is a dark, skyscraper-sized lump of carbon, ice and dust that simply tumbled into our solar system from, well, somewhere beyond. But even as the search came up empty, it proved a point: the riskier, more audacious hunts for life elsewhere are driven by private money, not governments and national space agencies.



It is not the only example. Earlier this week, the US Department of Defense confirmed that from 2007 it ran a programme to investigate unidentified flying object (UFO) sightings, but dropped funding five years later in favour of more pressing concerns. Luis Elizondo, the military intelligence official who ran the Advanced Aerospace Threat Identification Program, as the effort was named, resigned in October. He has since joined a private venture to continue his work in the field.

Of all the methods brought to bear on the question of life elsewhere, UFO sightings are at the farthest end of the credibility scale. When Monica Grady oversaw Britain’s meteorite collection at the Natural History Museum in London, she received plenty of letters about alien spacecraft over Britain. Most ended up in the bin, but one exemplified the problem many scientists have with such reports. It came from a man who had photographed what he described as an alien vessel at the bottom of his garden. No-one else noticed it, he noted ruefully, but he was adamant the spacecraft was responsible for road rage, cot death and measles. “For me, it’s the difference between astrology and astronomy,” said Grady, now professor of planetary science at the Open University. “We don’t have any credible sightings that are more likely to be an alien spacecraft than not.”

Few scientists would bet on finding anything more exotic than extraterrestrial microbes, in our solar system at least. “We are not expecting to get to the bottom of Enceladus’s ocean and find a whole load of gnomes smelting metal,” said Grady, referring to the tantalising subterranean water on the moon of Saturn, which occasionally sends geysers into space.

Andrew Siemion, who leads the Breakthrough Listen effort at the Berkeley Search for Extraterrestrial Intelligence Research Center in California, is sceptical that UFOs have ever visited Earth. “Astronomers of all types spend their lives looking up at the sky with every conceivable instrument we can build, and we do so independently, and we have never taken a picture of a spaceship. And I can tell you every single graduate student and postdoc would love nothing more than to be the one to take that photo.”



No wonder, then, that governments leave the more speculative searches for alien life to others. The programmes they do fund tend to avoid all mention UFOs – the term has lost its intentional ambiguity since it was coined in the 1950s and become synonymous with alien spacecraft. Instead, programmes emphasise the potential threats posed by unexplained aerial objects that might be the secretive work of hostile nations. When Nick Pope worked on UFO sightings at the UK Ministry of Defence in the 1990s, all mention of UFOs was scrubbed in favour of the more fundable UAP, or Unidentified Aerial Phenomena. “The term UFO has a lot of pop culture baggage,” Pope told the Guardian. “It will never really lose its fringe tag.”



While ufology struggles for credibility, the search for alien transmissions is serious science. Much of the sky has been swept for alien signals in the form of optical, infrared and radio waves. But the searches are far from extensive. Future scans could tune into directed energy beams used to propel craft, x-ray and gamma-ray broadcasts, or even gravitational radiation. When the massive radio telescope known as the Square Kilometre Array comes online in 2020 or thereabouts, it will be the first facility that is sensitive enough to detect the equivalent of TV broadcasts on planets around Alpha Centauri, the closest star system to Earth. Pope believes that it is searches like these that will finally answer the question “are we alone?”

Discoveries from land and space telescopes, and robotic planetary missions, strongly suggest that life should exist elsewhere. Water and organic molecules needed for life as we know it are ubiquitous in space. And from Nasa’s Kepler mission, astronomers now believe that almost every star in the galaxy has at least one orbiting planet. “Everything necessary for life to arise and thrive on this planet exists in abundance throughout the universe,” said Siemion.

The question, then, is where has life gained a foothold? “It should be on Europa, a moon of Jupiter; it should be at the bottom of Enceladus’s ocean, and it should be on Mars,” said Grady. “If the processes that got life going on Earth are universal, then some form of life should have got going.”

“The reason people are so incredibly interested and excited is that it’s such a profound human question. We have a basic desire to know what is beyond, what is out there,” said Siemion. “What we have done so far is very minimal. We have far more work to do.”