Question: I’ve been watching a very interesting series of television programmes in which Professor Brian Cox has been delving into Astronomy. One programme focused on the life cycle of stars. I understand from the programme that as our Sun enters its “giant red” stage the inner planets will be engulfed and destroyed but what will happen to the outer planets? As the Sun goes through its death throes and shrink down in size will it have enough “gravitational pull” to hold what is left of the solar system in their respective orbits or will the outer planets wander off into interstellar space? — David
Answer: There have been a couple of recent studies of just these questions. When the Sun exhausts it hydrogen fuel and enters its Red Giant phase it will expand to roughly 100 times its present size. This will make the distance from the Sun to Jupiter shrink from 765 million to roughly 500 million kilometers. At this shortened distance Jupiter’s surface will be heated beyond 1000 K, but it will apparently survive. Once the Red Giant phase is complete the Sun will evolve into a White Dwarf, during which it will lose about half of its mass. As the Sun loses mass the radii of the orbits of the remaining planets, from Jupiter outward, will increase, but still remain in orbit around a now lighter Sun.