Hahn and Savin in Nautilus:
Let’s be optimistic and assume that we manage to avoid a self-inflicted nuclear holocaust, an extinction-sized asteroid, or deadly irradiation from a nearby supernova. That leaves about 6 billion years until the sun turns into a red giant, swelling to the orbit of Earth and melting our planet. Sounds like a lot of time. But don’t get too relaxed. Doomsday is coming a lot sooner than that. The Earth is, in some ways, in a precarious spot in the solar system. There’s a range of orbital distances inside which a planet can have both liquid surface water (which is believed to be necessary for life) and enough atmospheric CO2 to carry on photosynthesis. This range is called the photosynthesis habitable zone. The Earth orbits barely within the sun’s zone. Some scientists estimate that the inner edge lies just 7.5 million kilometers away, which is only 5 percent of the distance between the Earth and the sun.
…In a paltry 500 million years or so, no humans will remain on the surface of the Earth—at least, not outside of some hypothetical controlled environment. And things get worse from there. After the atmospheric CO2 is gone and no longer able to regulate Earth’s surface temperature, things will start to get very hot. In about a billion years, the average surface temperature will increase to above 45 degrees Celsius from the current 17 degrees Celsius. Important biochemical processes turn off at temperatures above 45 degrees Celsius, leaving most of the planetary surface uninhabitable. Animal life will need to migrate to the cooler poles to survive; but by 1.5 billion years from now, even the poles will be too hot. Not even cockroaches will survive. Now, there are a few things we can do to stay our execution. We could, for example, move the Earth’s orbit. If we fired a 100 km wide asteroid on an elliptical orbit that passed close to the Earth every 5,000 years, we could slowly gravitationally nudge the planet’s orbit farther away from the sun, provided that we don’t accidentally hit the Earth. As a less precarious alternative, we could build a giant solar sail behind the Earth with enough mass to drag the planet away from the sun. Such a sail acts like a kite, where the photons from the sun are the wind and the gravity between the solar sail and the Earth acts as the string. The sail would need to have a diameter 20 times that of the Earth but a mass only about 2 percent that of Mt. Everest, a mere trillion metric tons.