The Fate of Exomoonssu201712 | www.cfa.harvard.edu/
https://www.cfa.harvard.edu/news/su201712
When a star like our sun gets to be very old, after another seven billion years or so, it will shrink to a fraction of its radius and become a white dwarf star,
no longer able to sustain nuclear burning. Studying the older planetary systems around white dwarfs provides clues to the long-term fate of our Sun and its planetary
system. The atmosphere of a white dwarf star is expected to break up any material that accretes onto it into the constituent chemical elements and then to stratify
them according to their atomic weights. The result is that the visible, uppermost layers of the atmosphere of a white dwarf should contain only a combination of
hydrogen, helium (and some carbon). About one thousand white dwarf stars, however, show evidence in their spectra of pollution by some form of rocky material. This
suggests that there is frequent, ongoing accretion onto these white dwarf stars of fragmentary material coming from somewhere - the precise origins are not clear.
CfA astronomers Matt Payne and Matt Holman, with two colleagues, have completed a series of simulations of the late evolution of planetary systems to try to understand
where this material might be coming from. It was already known that the moons of planets can be easily knocked out of their orbits during planet-planet interactions in
white dwarf systems. The question was whether these freed moons might themselves accrete onto the star to provide the polluting elements, or whether they might act to
scatter asteroids towards the star. The difficulty has been the computational limits of simulating a complex evolving system that included the moons around planets.
