Stephen Hawking documentary Expedition New Earth
Professor Stephen Hawking has once again come clean about his fears that aliens could invade the Earth Getty Images/Jemal Countess

Science guru Stephen Hawking has once again confessed his fears that aliens could invade the Earth.

The professor said planet Gliese 832c had the potential to be home to alien life but humans needed to be very wary of getting in touch, news.com.au reports.

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"One day, we might receive a signal from a planet like this, but we should be wary of answering back," he said in the documentary Stephen Hawking's Favourite Places.

"Meeting an advanced civilisation could be like Native Americans encountering Columbus. That didn't turn out so well."

The Theoretical physicist claimed alien lifeforms could be "rapacious marauders roaming the cosmos in search of resources to plunder, and planets to conquer and colonise".

Hawking originally voiced his fears when he first spoke about aliens taking over the Earth on the Discovery Channel in 2010.

The bestselling author of A Brief History of Time said as he grew older he became more convinced humans were not alone.

Planet Gliese 832c has five times the mass of Earth and a similar temperature. It's been described as an inhabitable super Earth, that's 16 light-years from our planet – roughly 9.5 trillion kilometres.

Over the brink

Hawking has also warned Earth could become as hot as Venus as a result of climate change.

He told the BBC that climate change could turn the planet into a hothouse, and President Donald Trump's decision to pull out of the Paris Climate agreement to reduce CO2 levels could accelerate the threat.

"We are close to the tipping point, where global warming becomes irreversible," he said.

"Trump's action could push the earth over the brink, to become like Venus, with a temperature of 250C, and raining sulfuric acid."

Hawking also said humans may have to consider living on a different planet elsewhere if climate change ravages our own world.

But many experts don't believe Earth could hit those extreme temperatures because it is further away from the sun than Venus and doesn't have a carbon dioxide atmosphere as thick.

Pennsylvania State University climate scientist Michael Man said: "Hawking is taking some rhetorical license here".

Man added: "Earth is further away form the sun than Venus and likely cannot experience a runaway greenhouse effect in the same sense as Venus — such as a literal boiling away of the oceans.

"However Hawking's larger point — that we could render the planet largely inhabitable for human civilisation if we do not act to avert dangerous climate change — is certainly valid."