As the Sun ages, Earth's fate hinges on two invisible forces: how much the Sun inflates and how tightly it grips the planets with its gravity. New calculations show Earth might live to tell the tale—like a boat weathering a storm—if the 'wind' of the solar wind stays mild. But just how solid is this hope?
In 5 billion years, the Sun, having exhausted its fuel, will start to swell, turning into a red giant. Like a balloon losing air, the star will weaken its gravitational grip, and Earth — a grain of sand on an invisible thread — will drift to a more distant orbit.
Previously, it was thought that tidal forces, which cause ocean tides on Earth, would slow the planet down and drop it into the Sun. But new models show that this effect is negligible. It all comes down to the rate at which the star sheds mass. Slowly — Earth will end up in a dangerously close orbit and could be swallowed. Quickly — it will slip outward, like a speck of sand caught in a stream of air. In the most optimistic scenario, it could end up at the orbit of Mars.
Observations of the star L2 Pup, which is currently at the same stage, give hope: the mass-loss rate there is high. Similar processes are tracked through cosmic dust and the fates of exoplanets around other stars. Moreover, the outer layers of a red giant are so rarefied that Earth won't burn up, but merely sink into a cloud of gas.
🎯 The density of the outer layers of a red giant is comparable to the best laboratory vacuum. If Earth ended up inside, it wouldn’t burn up — it would float in the tenuous cloud, like in a fog.