"Spreading out may be the only thing that saves us from ourselves," he said this week.
Hawking also shared some ideas on how to get people off this planet. He is famous for being smart, after all.
And now he's the latest name on a long list of celebrities - deep thinkers, billionaires, heads of state and Kris Jenner - who want to put a human back on a space rock, ASAP.
Let's look at their ideas.
We might as well start with the presidents.
President Barack Obama
Where: Mars (but definitely NOT the moon)
Shortly before he stopped being president, Obama wrote an op-ed for CNN about NASA's plan to send someone to Mars and back sometime within the next few presidencies.
It would be hard, Obama said, requiring cooperation between the government and business. But so worth it.
"Someday, I hope to hoist my own grandchildren onto my shoulders," he wrote.
"We'll still look to the stars in wonder, as humans have since the beginning of time. But instead of eagerly awaiting the return of our intrepid explorers, we'll know that because of the choices we make now, they've gone to space not just to visit, but to stay - and in doing so, to make our lives better here on Earth."
Obama did not mention in his essay that he had killed a George W. Bush-era plan to return to the moon that might have realised his grandpa-and-grandchild-stargazing dream long before the 2030s.
When: Much sooner than Obama
When Trump became president, he decided Obama's plan to land on Mars before 2040 was way too slow.
So he signed a bill in March that funded NASA with nearly US$20 billion. The next month, he called the International Space Station from the Oval Office and said, verbatim: "Who's ready to go to Mars?"
An astronaut, in space at the time, told Trump they'd be ready in the 2030s. Trump replied: "We want to try to do it during my first term, or at worst during my second term, so we'll have to speed that up a little bit, okay?"
When: "Maybe I'll wait till the last 10 years of my life, and then maybe go, if my wife will let me."
Branson is still more plane-famous than he is space-famous, though his Virgin business empire dips into pretty much everything, and the final frontier isn't that far down the list.
He wants to create the first commercial space line, for instance. And in 2012, he told the New York Post that it would also be a good idea to populate Mars.
"Obviously, you are going to want scientists initially," Branson said.
"You're going to want physicians, you're going to want comedians, you're going to want fun people, beautiful people, ugly people - a good cross-section of what happens on Earth - on Mars."
If he went into much detail about how he'd accomplish this, the tabloid didn't say.
At the same event, a reporter heard celebrity Susan Sarandon tell Branson that she wanted to go to space, too, "and open a ping-pong bar".
Jeffrey P. Bezos
Where: The moon
When: Order now and get it by the mid-2020s
Disclaimer: Bezos, most famous for owning Amazon, also owns The Washington Post and thus indirectly paid for this article. And now we will tell you about his exciting pitch to set up an inter-orbital delivery service to build a base on the moon.
"It is time for America to return to the Moon - this time to stay," Bezos told The Post in March, after the paper got hold of a confidential proposal that his space company, Blue Origin, had sent to NASA.
Basically, as Christian Davenport wrote, Bezos wanted to run a series of deliveries to a crater near the moon's south pole - cargo for future human habitats.
"I'm excited about this and am ready to invest my own money alongside NASA to make it happen," Bezos wrote to space officials, though he also urged NASA to provide "incentives to the private sector" to help make his lunar cargo delivery dream come true.
Where: "The solar system"
Must have been space fever going around in 2012. A few months before Branson imagined a Red Planet full of people, HuffPost (called the Huffington Post in those days) decided to ask a slew of celebrities if they'd be willing to go to Mars.
Lucy Lawless was a no.
But Kris Jenner of Kardashian fame said: "Absolutely - adventure, seeing the solar system, great episode for the family, opportunity to share Zestra with life on other planets."
It's not totally clear she understood the question, or that Mars is a dead planet, but good enthusiasm.
Where: A self-sustaining city on Mars
When: Sometime between one and 100 years from now
Maybe it's a cheat to include Musk in the list of celebrities, as he's partly famous because he likes space so much, naming his company SpaceX after it, and occasionally launching rockets into it.
But this list badly needs some substance after Branson and Jenner, and Musk has just published a detailed plan to colonise Mars and "become a space-bearing civilisation and a multi-planetary species."
Musk's plan is actually based on a talk he gave last year, which The Post's Davenport wrote up.
The entrepreneur pitched an "incredibly ambitious timeline," Davenport wrote, with the first launch in 2018, and many more for decades to come, until the city is up and running.
Musk showed the crowd a video of a rocket with 100 people taking off from Florida, fuelling up in orbit and plopping them down on the Red Planet. Imagine those on the regular.
Eventually, Scientific American reported, the plan could put 1 million people on Mars within a century.
Where: Alpha Centauri
When: An incredible 20 years after launch but, sorry, no room for people
Though the Facebook titan's interstellar travel plan involves no humans, we're including it because, honestly, there's too much Mars in this list. Celebrities just don't seem that interested in exploring the broader range of rocks and planets space provides.
Zuckerberg wants to explore a whole different star system, Alpha Centauri, which is so far away it takes light - the fastest thing in the universe - more than four years to get there.
He's teamed up with Stephen Hawking and Russian celebrity Yuri Milner, The Atlantic reported, who announced from the top of a skyscraper a US$100 million research program they've dubbed 'Starshot'.
They'd take a tiny little probe, put a tiny little camera on it, and then use photons like wind to propel it at a decent fraction of the speed of light across 25 trillion miles of void, to the sun's nearest neighbor, where hopefully there's a habitable planet or two to take pictures of.
But even in the best-case scenario, and even if the probe launched today (which it won't), the first grainy images of Alpha Centauri wouldn't beam back to Earth until the mid-21st Century.
So, back to Mars.
Where: Earth. Bill Gates has no time for your space dreams
Spurning the fad among other billionaires, the Microsoft-founder-turned-global-philanthropist seems to care only about one planet: the one he is stuck on.
A Fox News host tried to get Gates excited about Musk's plan to colonise Mars last year.
"I'm not big on this whole Mars thing," Gates said.
The host tried again: Say someone offered him a trip. Would he go?
"No," Gates said. "Mars is pretty far away, and has no deep fascination for me."
Why's he on this list then? Just in case the trip is not up to him in the end.
With his dreams firmly grounded on Earth, Gates has invested hundreds of millions of dollars in potentially planet-saving technologies, from energy to agriculture.
In January, a writer for the Conversation was watching a TV show about space and "had an epiphany": All those technologies would be perfect for colonising a planet.
So the writer typed out his pitch. It was called 'Why we should send Bill Gates to Mars'.
2017 © The Washington Post
This article was originally published by The Washington Post.