How long can the earth be livable?

All living things on earth are carbon-based life. Carbon-based life refers to organic matter with carbon as the core. The two necessary points are oxygen and liquid water, and an appropriate temperature.

To make the environment on the earth meet the above points, the earth needs to keep a proper distance from the sun. At this distance, the temperature on the earth is suitable for small temperature difference and can maintain stable liquid water. This distance range is called the livable zone of the sun.

As can be seen from the above figure, with the increase of the star mass, the position of the livable zone moves away from the star. From red dwarfs to yellow dwarfs (the sun is one of them) to superstars, the greater the mass, the more intense the nuclear fusion of the star core, and the stronger the ability to release light and heat.

With the uninterrupted nuclear fusion of the sun, the sun will get hotter and bigger, and eventually it will become a red giant. At that time, the outer boundary of the sun will reach the orbital position of Mars, and the three rocky planets near the inner side-Mercury, Venus and Earth will be swallowed up by the sun.

The livable zone of the sun will gradually move outward over time. At least after 2 billion years, the liquid water on the earth will gradually evaporate and the temperature will rise sharply, making it uninhabitable. This is also the reason why Liu's science fiction novel The Wandering Earth escaped from the solar system. Helium flashes before the sun becomes a red giant, releasing huge energy instantly.

If the human species has been lucky enough to exist for billions of years, then the level of science and technology at that time should no longer have to worry about habitability. There are many large satellites around Jupiter and Saturn, such as Europa and Titan. These satellites are highly anticipated by scientists and can be used for interstellar migration.

But to be honest, according to the "care" of human beings for the earth, it may not be necessary to wait until the sun becomes a red giant before the earth will be "hurt". I don't know if this answer is right!