A group of experts have weighed-in on the existence of aliens ahead of the Pentagon releasing a groundbreaking report this month detailing decades of military intelligence about UFO encounters.
The US Congress passed legislation in December mandating the Department of Defence and the National Intelligence Director to produce a report about "Unidentified Aerial Phenomena" (UAP) within six months.
The report is expected to be released on June 25 and, according to The New York Times, details more than 120 "UAP", more commonly referred to as UFO, incidents over the past two decades that couldn't be linked to the US military or other advanced government technology.
Senior US government officials who have seen the report told the publication it doesn't provide evidence of extraterrestrial activity but also doesn't rule it out as a possibility.
Anticipation over the report's release has unsurprisingly rekindled the age old debate over the existence of aliens.
The Conversation asked five experts their opinion on whether aliens exist, with four of those agreeing there is life beyond Earth.
When asked if aliens exist, University of Southern Queensland astrobiologist, Professor Jonti Horner, said the answer was a "definite yes".
However, Professor Horner said these other life forms may be too far away for us to ever make contact with, making finding proof "astonishingly hard".
"Space is unbelievably big. In the last few decades, we've learned almost every star in the cosmos has planets. Our galaxy, the Milky Way, is estimated to have up to 400 billion stars. If each of those has five planets, we'd have two trillion planets in our galaxy alone. And we know there are more galaxies in the cosmos than there are planets in the Milky Way," he said.
"In other words, there's a lot of real estate out there. And with so much variety, I find it impossible to believe Earth is the only planet that has life — including intelligent and technologically-advanced life."
Western Australia's Curtin Institute of Radio Astronomy (CIRA) executive director, Professor Steven Tingay, also agreed the existence of aliens was likely.
He said proof of life on other planets wouldn't necessarily mean the discovery of other advanced civilisations, claiming it could be as simple as finding bacteria on a planet other than Earth.
"It remains to be seen whether this life is like bacteria, or is an exciting 'technologically advanced civilisation' we can communicate with," Professor Tingay said, adding there was significant efforts underway to make contact any other life forms that may be out there.
Another of the experts, Dr Helen Maynard-Casely, told The Conversation it is "only a matter of time" before lifeforms are discovered somewhere other than Earth.
Dr Maynard-Casely, who is a senior instrument scientist at the Australian Nuclear Science and Technology Organisation, said officials are continually finding potential pockets within the solar system that may be hospitable to life.
"It really does feel more and more inevitable that we will find a niche for some active biology somewhere. Whether it can say hello to us? Well, that's a different question," she said.
Swinburne University of Technology Space Office project coordinator Dr Rebecca Allen agreed that alien life likely existed but said they probably don't look like how humans imagine.
"When we hear the word "alien", however, an image of a humanoid life form usually springs to mind. But even on Earth, the most predominant form of life is much older, smaller and more resilient. I'm talking about microorganisms, of course," she said.
"These organisms defy science by existing where life has no business existing, such as in the sludge around volcanic vents. I would bet alien life exists in the form of these 'extremophiles'."
Of the five experts, Australian Centre for Astrobiology director Professor Martin Van-Kranendonk was the only one to answer "no" when asked if aliens existed.
"If we use purely empirical data and assume the question refers to any type of life outside of Earth that is not related to human activity, then the answer — as far as we know — must be no," he said.
However, Professor Van-Kranendonk conceded that as humans haven't been able to investigate every corner of the universe for signs of life then it is impossible to know for sure whether we are alone.
"Perhaps one day we can know if we have nearby interplanetary neighbours, or if indeed we are alone. Or perhaps we never will," he said.