With at least 100 billion planets in the Milky Way, you’d think at least one would have produced a race of intelligent alien beings. So how come we’ve never seen signs of space aliens even after decades of SETI (search for extraterrestrial intelligence) research?

Scientists have long puzzled over that simple question. Now, one of England’s best-known physicists, Professor Brian Cox of Manchester University, has weighed in with an answer.

And it’s rather bleak.

As reported by The Times, Cox maintains that alien civilizations might snuff themselves out not long after they come to be ― done in by nuclear war, runaway climate change or other threats that rise as their technological sophistication advances.

“It may be that the growth of science and engineering inevitably outstrips the development of political expertise, leading to disaster,” he said.

In other words, we haven’t seen signs of extraterrestrial life because alien civilizations that did exist are now gone ― and, presumably, alien civilizations that might exist now are too primitive technologically to have made their presence known to us.

Bummer.

But this isn’t the first time this argument has been made, and one prominent SETI scientist thinks it’s overstated.

Dr. Seth Shostak, senior astronomer with the SETI Institute in Mountain View, California, told The Huffington Post in an email that it’s certainly possible that some advanced alien civilizations have inadvertently prompted their own destruction with the technology they created. But “it’s really difficult to imagine they ALL do,” he said. “And if 10 percent or 1 percent don’t, then we should still have hope of finding plenty of survivors.”

Even if our own individual lives end before humanity makes contact with space aliens, there may be reason to believe such contact will come eventually. Because when it comes to life in the universe, a study published earlier this year in the Journal of Cosmology and Astroparticle Physics suggests that human civilization might simply be a bit premature.

“If you ask, ‘When is life most likely to emerge?’ you might naively say, ‘Now,’” Dr. Avi Loeb, a theoretical physicist at Harvard University, said in a news release issued last August in conjunction with the completion of the study. “But we find that the chance of life grows much higher in the distant future.”

Life as we know it first became possible some 30 million years after the Big Bang, which occurred about 14 billion years ago, according to the release. Before then, the raw materials for life, including carbon and oxygen, simply didn’t exist. And life should be possible in the universe for another 10 trillion years, when astronomers predict the last stars will grow cold.

That leaves a lot of time for alien life to evolve.

Maybe that’s cold comfort, especially given another of Cox’s contentions: that our own nuke-owning, climate-changing civilization may be headed for extinction.

”We could be approaching that position,” he said, according to the Times.

But Shostak is sanguine on that point too.

“It’s trendy these days, given such existential threats as climate change, to suggest that science and technology will ultimately do us in,” he told HuffPost Science. “As a scientist, I can’t say that I’m particularly fond of being part of the problem, but of course science is also part of the solution.”

Fingers crossed!