Stephen Hawking is best known for thinking about time, space, and those teratoid trash mashers known as black holes. But in a recent talk in Hong Kong, the famous physicist digressed from his usual subject matter to tell the audience that they’d better get off the island, and he didn’t mean Kowloon. Instead, the Cambridge don was urging the crowd to get off the whole, gosh-darn planet. Hawking was hawking space colonization.

“Life on Earth is at the ever-increasing risk of being wiped out by a disaster, such as sudden global warming, nuclear war, a genetically engineered virus or other dangers,” Hawking disclosed.

Well, if you’re a space buff, your reaction to this pronouncement is probably half-closed eyelids and a big “duh.” Moving out into the solar system to save our precious, hominid skins is hardly a new idea. What makes these statements print-worthy is that they come from a luminous source.

But news or not, Hawking’s observation is correct, primarily and fundamentally because our planetary home is the wrong shape. A sphere has less surface area than any other form of the same volume. Less than a cube, an ellipsoid, or any polyhedron you can still recall from seventh grade geometry. What this means is that there’s precious little surface acreage to live on, despite Earth’s goodly heft.

Consider the Tonnage-to-Terran ratio, an index of how efficiently we use the planet: With the human population at six billion, the T-to-T ratio works out to about a trillion. In other words, for every lout strutting Earth’s stage, there are about a trillion tons of Earth!

That’s right: each of us is backed up by a trillion tons of natural infrastructure, a mostly useless collection of crushed iron core and creamy, hot magma, topped by a delicate continental crust. This, as you can easily surmise, is wasteful. It’s possible to live the good life with 100 tons of housing (including some office space and a car or two), which is 10 billion times more efficient.

So while moving some of our descendants to Mars or the moons of Jupiter, as Hawking suggests, will help—it won’t help much. The problem is that these worlds are also spheres. The amount of increased real estate they offer for human settlement is unimpressive.

For example, let’s ignore for a moment the environmental problems, the nukes and the pandemics, and consider population growth alone. The current doubling time for the number of humanoids is 50 years (that’s a growth rate of 1.4 percent per year). In other words, even if we began colonizing the Red Planet this week, we’d have it littered with critters by mid-century. This is only marginally helpful. Taking Malthus to the max, if we naively assume that the present doubling rate will continue well into the future, the mass of ever-reproducing humans will reach the moon in the year 4810 a.d. That’s not individual astronauts—it’s the expanding glob of protoplasm. (As a stupefying aside, this ball of beings would reach the nearest star, Alpha Centauri, relatively shortly thereafter: by 8825 a.d.)

Such fanciful extrapolations are unrealistic, but so is the opposite extreme: to assume that, after 300 thousand years of increase, the number of humans will stabilize and stagnate, not just for a while, but forever. More room is surely needed, unless you can picture our progeny endlessly stuck on a single planet, fighting for space and hustling for the dwindling natural resources. That scenario seems so fanciful, so airy-fairy, we have no choice but to heed the siren call of other solar system habitats. However, even the best of these (Mars) will be difficult to terraform, and offers only a short-term solution to a long-term problem.

Another well-known physicist, Freeman Dyson, weighed in on this issue years ago, and suggested that the small bodies of our solar system—asteroids and perhaps the diminutive worlds of the Kuiper Belt—could be choice future real estate. These objects, like the planets, are approximately round, but being small they enjoy a higher ratio of surface area to volume. All the asteroids together weigh only as much as Earth, but these hunky chunks of junk still sport ten thousand times as much square footage as our world. Dyson reckons that our descendants will migrate to where land is abundant.

There’s merit in his suggestion, but once again this nearby acreage—like Mars—will require major improvement. An asteroid is not a cheap fixer-upper.

Fortunately, there’s yet another approach for securing easy elbowroom: building artificial habitats in space. The idea of constructing mammoth, rotating aluminum cylinders in orbit (either around Earth, around the moon, or simply around the Sun) dates back to the 1970s, when Gerald O’Neill and Tom Heppenheimer wrote speculative books on how it might be done. It was their optimistic view that by the start of the 21st century, tens of millions of us would be spinning in space. That hasn’t happened yet, but the concept still makes sense. O’Neill and Heppenheimer even worked out in some detail how the raw materials for construction could be catapulted off the moon and smelted and assembled in orbit.

You might think that living on Mars would beat living in a colossal can, but that’s hardly obvious. As fabricated housing, a space colony can be easily tuned to our needs: Imagine a large environment boasting first-rate views, one-G’s worth of gravity, a breathable atmosphere, salubrious temperature, and all your favorite plants and animals (i.e., no mosquitoes). In addition, you can build as many of these as you want. You won’t run out of lebensraum in fifty years.

Indeed, that’s the important bottom line. Artificial habitats can have a very low Tonnage-to-Terran ratio. Rather than crowding a few billion people onto the moon, for example, where residents will have to contend with such domestic inconveniences as no air, no water, and no going outside without #5,000,000 sun-block, we can take the moon apart and redevelop it as a dream suburbia. Call it simple geometry—because it is—but rebuilding the moon to construct flotillas of space habitats is possible because the latter aren’t round balls of stuff. Doing so is imperative because, as Hawking says, our future depends on it. Self-destruction aside, we’ll be like cats in a cage if we don’t eventually spread out.

And besides, what’s all that hot magma doing for you anyway?