What is the last place on Earth? The very last place on Earth, ever? Here鈥檚 a clue: wherever on the surface of the planet you are now sitting, it鈥檚 around 6400 kilometres and 5 billion years away.
That date, more or less, is when the sun becomes a red giant. It will do so twice. First it will swell to about the size of Earth鈥檚 orbit today. We are likely to escape being swallowed at that point, because the sun will have shed some of its mass, weakening its gravitational grip, so we will be orbiting somewhat farther out than today. Then after a few hundred million years, during which the sun will have briefly shrunk to a more sensible size, it will become a giant again, this time growing larger still.
Its outer envelope of hot gas probably won鈥檛 quite reach the Earth at first, but we are likely to be dragged in eventually. 鈥淚f the Earth is not engulfed the first time, it will almost certainly be engulfed the second,鈥� says astrophysicist Arnold Boothroyd of the University of Toronto in Canada. The sun might even play cat and mouse with us, flaring up from time to time to toast Earth in 3000 掳C plasma, then contracting, only to finally swallow us millennia later.
Advertisement
That is only the beginning of the end. The outer layers of a red giant are remarkably tenuous, so Earth will plough on, continuing to orbit the sun鈥檚 core for thousands of years. The crust will melt into an ocean of magma, but for a while not much of the planet will evaporate.
As Earth gradually slows down and spirals inwards, the temperature will climb. Eventually, when we are a few million kilometres from the core, it will reach a critical level of about half a million degrees, according to astrophysicist Noam Soker of the Israel Institute of Technology in Haifa. At that temperature, the oxygen and silicon atoms boiling out of the magma ocean will be travelling fast enough to escape Earth鈥檚 gravity, and the evaporation will suddenly accelerate. Soker estimates that it will only take a year for the whole planet to go. 鈥淭he hot solar envelope will peel the Earth layer by layer,鈥� he says.
There is in fact a chance that the red giant will never grow quite large enough to swallow Earth. If it escapes this fiery end, our dry and lifeless planet will continue orbiting for aeons as the sun settles down to become a white dwarf. Eventually, Earth should spiral in and hit the sun鈥檚 cinder 鈥� though there is an outside chance that it could endure until the end of the universe, squashed into the final singularity of a big crunch, or torn apart in a big rip.
Overall, though, the odds are that the end will come as our sometime home is swallowed up by the sun. Then the last place on Earth 鈥� the very last fragment to go 鈥� will be a piece of iron from near the centre of today鈥檚 inner core: a white-hot droplet of liquid metal rapidly boiling away to nothing.
