If you put a steamy cup of coffee in the refrigerator, it wouldn’t immediately turn cold. Likewise, if the sun simply “turned off” (which is actually physically impossible), the Earth would stay warm—at least compared with the space surrounding it—for a few million years. But we surface dwellers would feel the chill much sooner than that.
Within a week, the average global surface temperature would drop below 0°F. In a year, it would dip to –100°. The top layers of the oceans would freeze over, but in an apocalyptic irony, that ice would insulate the deep water below and prevent the oceans from freezing solid for hundreds of thousands of years. Millions of years after that, our planet would reach a stable –400°, the temperature at which the heat radiating from the planet’s core would equal the heat that the Earth radiates into space, explains David Stevenson, a professor of planetary science at the California Institute of Technology.
Although some microorganisms living in the Earth’s crust would survive, the majority of life would enjoy only a brief post-sun existence.