We can use luminosity projections of the red giant Sun to calculate the approximate surface temperature of planets around the Sun (or another star) knowing only the distance of the planet from its star and the reflectivity of the planet. It’s important to know this reflectivity, or albedo, because it tells us how much energy the planet can absorb and what its approximate temperature will be. The higher the reflectivity, the lower the amount of energy the planet is able to absorb, and the cooler it will remain.
The actual temperature of a given planet will also depend on other factors, such as the degree of internal activity and the degree to which its atmosphere produces a greenhouse effect (i.e., whether miniscule like Mars’s, mild like Earth’s, or severe like Venus’s). There are other complications, too. For example, during the red giant phase, a star’s great luminosity and large size cause it to lose much of its mass. This causes planets to be bound less tightly to their stars and to move away to greater distances as the star loses mass.
For a common star like the Sun, the room temperature distance moves outward late in life from the 1 astronomical unit (AU) distance we know and love here on Earth to as far as 50 AU. Similar, but more extreme, behavior is exhibited in stars with higher masses. But the basic message remains unchanged: The cozy, warm kind of “habitable zone” around each star will move outward as the star gets older, moving from the inner to the outer solar system. The lesson for astrobiologists is clear: Which planetary abodes in a given system are habitable depends on what phase of evolution the system’s star is in.