Is our planet actually inhabitable to humans? Most of us would answer yes, but the answer's a lot more complicated, writes Charles Stross. And those complications have dire implications for our hopes of colonizing other worlds.

Stross' blog entry, which really must be read in its entirety, runs through a thought experiment: suppose you dispatch a robot probe with mindless human-like "meat machines" to Earth, to see if humans could survive. Most of the planet's surface will kill those meat-machines instantly, because it's covered with ocean or too hot or too cold. Only about 15 percent of the surface won't kill them right away. But also, points out Stross, if the probe arrives too early or too late in Earth's history, it'll find a planet with an atmosphere and water content vastly different to its current make-up — we couldn't even breathe the air for the vast majority of the planet's history.

Concludes Stross:

So here's the upshot: of the 4.6 Gy of Earth's known history, there's only been enough oxygen in the atmosphere for us to survive for about 0.5 Gy. For roughly 90% of the Earth's history we couldn't even breathe the air. And about 10-25% of the time, there have been ice ages so savagely fierce that the glaciers reached the tropics: odds are good that any meat probe landing on solid ground during these periods would rapidly die of exposure. So historically, Earth has only been inhabitable about 8% of the time - assuming you are lucky enough to find some solid ground. Once you factor in the random surface distribution, we're down to about 2% survivability.

And the future is likely just as dire — so what are the chances of finding another planet that matches the minority of our own planet's surface, with the exact same atmosphere as our own brief era? [Charlie's Diary]