The speculation that some form of alien life exists beyond our planet goes back to ancient Greece and Rome, yet humans have still not made contact.

There have been various theories, including the lonely proposal that human-level intelligence is totally ‘unique’ in the universe, but one scientist has revealed a more likely scenario – alien civilizations were destroyed by gamma-ray bursts (GRBs).

GRBs are immensely energetic explosions that have been observed in distant galaxies, occurring when the core of a massive star runs out of nuclear fuel and collapses under its own weight, unleashing a massive ‘super nova’ of radiation.

‘It’s a tightly focused beam,’ astronomy professor Dr. Frederick Walter told DailyMail.com. ‘And, if it’s directed through the plane of the galaxy, it could basically sterilize about 10 percent of the planets in the galaxy.’

‘It’s just one of many possible explanations,’ he noted, ‘sort of morbid, I suppose.’

NASA calls gamma-ray bursts ‘the most powerful class of explosions in the universe.’

A typical burst delivers a bright beam of directed energy: one quintillion (a 1 followed by 19 zeros) times the brightness of our own sun, more than enough, in other words, to burn an extraterrestrial civilization clean off its perch in the galaxy.

Fortunately, based on decades of research by NASA’s orbital Compton Gamma-Ray Observatory, gamma-ray bursts are relatively rare in our home galaxy, the Milky Way.

When the Compton observatory satellite was launched in 1991, NASA researchers expected to see more ‘GRBs’ emanating from our pancake-shaped galactic disc.

But the reality proved to be that these explosive ‘super nova’ events were much more common in younger, distant galaxies, still hot in the process of forming new stars, or in the case of long-duration GRBs, transforming collapsing stars into black holes.

‘It’s estimated that there is a gamma-ray burst every 100 million years or so, in any galaxy,’ according to Dr. Walter, who has taught a course on the search for extraterrestrial life at Stony Brook University in New York.

‘Over a billion years, on average, you might expect a significant number of civilizations to be eradicated,’  he said, ‘should they exist.’

Other estimates have suggested that gamma-ray bursts could occur as frequently as once every 10 million years per galaxy — but either range would mean that many alien species could be wiped out before they get a chance to do much space exploration.

The Stony Brook astronomer pointed out that planet Earth is approximately 4.5 billion years old, and took nearly that long to evolve the only technologically advanced species humanity currently has proof of: humanity, itself.

That would mean, like a game of Russian Roulette, the Milky Way is estimated to have experienced anywhere from 45 to 450 local GRB events in Earth’s history.

Some geophysicists and other researchers, in fact, have argued that either Earth’s Ordovician and/or its Devonian mass extinctions approximately 359 to 445 million years ago may have been the result of a blast of gamma rays from just such a collapsing star or black hole-forming event.

But Dr. Walter emphasized that this is just one hypothesis for why humanity has yet to officially discover signs of an extraterrestrial civilization somewhere out there in the universe.

Although these super nova blasts can be apocalyptically deadly, Dr. Walter advised that any potential threat to humanity from a GRB is ‘not worth worrying about.’

‘They’re rare, and they’re directed,’ which makes them much less likely to hit Earth, the astronomer said. ‘And, it’s not something you can prepare for.’

Gamma-ray bursts were first recorded in 1967, via a pair of US Vela satellites in operation to detect evidence of any Soviet nuclear tests that might have been conducted in defiance of the 1963 Nuclear Test Ban Treaty.

Two years later, the odd bursts came to the notice of a team at Los Alamos National Laboratory, who later pioneered the lab’s gamma-ray astronomy group.

While the hypothesized ancient GRB bursts that hit Earth hundreds of millions of years ago would have annihilated and dissolved the DNA of any living thing on the planet, today the distant radiation from far away GRBs mostly leaves tell-tale evidence in the form of bright star-like flashes or gamma-ray particle evidence in our ionosphere.

In the 74 years since nuclear physicist Enrico Fermi first wondered why, in a universe theoretically teeming with life, humanity has yet to empirically detect any of it, possible solutions to his famous ‘Fermi paradox’ have proliferated.

One book, by University of Portsmouth physicist Stephen Webb, explored 75 viable explanations currently in play, from the terrifying (e.g. all aliens are hiding from a deadly predator civilization) to the mundane (e.g. space travel is too hard).

‘There are a lot of ways around this,’ Dr. Walter said.

One example, he noted, was that a highly advanced aquatic species on another world, like whales or dolphins here on Earth, would be unlikely to develop the technology needed for space travel, despite large brains and powerful social skills.

‘If you live underwater,’ as he told DailyMail.com, ‘you’re not going to play around with electricity.’

Alternatively, a wise, but risk-averse civilization, might simply decide that ‘it may not be worth it to travel between the stars and look for other aliens,’ Dr. Walter said, electing instead to ‘focus inwards’ and form their own ‘little utopia’ on their home world.

But one of the most common theories, among physicists and astronomers, according to Dr. Walter is that technologically advanced civilizations might be universally prone to killing themselves off — whether through mismanagement, greed, war or worse.

‘Just look around, you know?’ the astronomer said. ‘We’re polluting the atmosphere. We’re making it questionable as to whether we’re going to have a viable civilization in a century, unless we do something drastic.’

‘If civilizations tend to evolve in the same way that they have on our planet, then that’s going to hit everybody,’ Dr. Walter continued, opining, ‘our technology exceeds our wisdom, perhaps.’

‘I would guess that’s the leading theory,’ he said. ‘But, look, it’s all speculation.’

‘We have one example: That’s us.’

If one takes the 19th Century Industrial Revolution as humanity’s first leap into the ranks of technologically advanced species, Dr. Walter noted that we have only thus far ‘made it through about 150 years.’

‘There’s always a risk we’re not going to make it through another one or 150 years.’