On average, the distance between Earth and the sun is slowly increasing because the sun is losing mass. As the thermonuclear fusion reactions continue inside the Sun, there is a decrease in the Sun's mass. The continued, steady loss of mass of the Sun over millions of years results in a reduced gravitational pull of the Sun and so an expansion of the orbits of the planets.
As a red giant, our Sun will expand and heat up, forcing its current habitable zone, which now encompasses Earth, outward. Credit; Astronomy: Roen Kelly
Over the course of the sun's remaining lifetime — i.e., another 5 billion years or so – it is estimated that the sun will lose about 0.1% of its total mass when it reaches the red giant stage. 0.1% doesn’t sound like much, but it is a helluva lot of mass – equivalent to Jupiter’s mass. When the sun loses mass, its pull on Earth weakens, leading it drift away from the Sun about 6 centimeters per year.
It is estimated that the Sun-Earth distance may increase to about 150% of what it is now.
