When we contemplate how long life has left on Earth, it’s natural to think about the ultimate end of the celestial body that sustains us — the Sun. It will be another 5 billion years before it begins to run out of fuel and expands into a red giant that will likely engulf and destroy our planet. That seems like plenty of time, right?
But in fact, time is going to run out for us on Earth a good deal sooner than that. While our star still has plenty of nuclear fuel to sustain itself, it is nevertheless steadily evolving, as all stars do during their life. That’s because at the Sun’s core, its nuclear reactions fuse hydrogen atoms into helium — and there’s nowhere for the helium to go. These ashes build up, the core becomes denser, its pressure becomes higher, and our star’s nuclear furnace starts to runs a little bit hotter.
As a result, over time, the Sun becomes brighter, currently by around 10 percent every billion years. And in about another billion years, that increase in energy will inevitably warm the planet and cause the oceans to evaporate and disappear. Our solar system’s habitable zone — what scientists call the region where liquid water can exist on a planet’s surface — will have migrated outward and left Earth behind.
So, it turns out that the story of life on Earth is already 80 percent finished, and moving into its final act.
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