The fate of exomoons
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...