Packing as much mass as the Sun into a volume as small as the Earth, white dwarfs are very compact objects with intensely strong gravities. This causes heavier elements to sink and lighter elements to stay on top. Elements heavier than hydrogen and helium tend to quickly sink out of the outermost layer of the white dwarf. As a result, heavy elements detected on the surface of a white dwarf must have come recently from an external source, such as a planet that has accreted onto the white dwarf.
Additionally, the abundance of oxygen appears too high to have just come from the accretion of rocky material. Since water is comprised of hydrogen and oxygen, the oxygen excess implies SDSS J1242 accreted a dwarf planet with water content ~38 percent by mass. The abundance of heavy elements in the outermost layer of the white dwarf is expected to decrease exponentially with time. Based on what is observed, the accreted water-rich dwarf planet is estimated to have at least 7 times the mass of Ceres (1/10th the mass of Moon) if the accretion took place ~3 million years ago and 1.5 times the mass of Pluto (1/4th the mass of Moon) if the accretion took place ~5 million years ago. Such discoveries can offer insight into the ubiquity of water-rich worlds.
R. Raddi et al. (2015), “Likely detection of water-rich asteroid debris in a metal-polluted white dwarf”, arXiv:1503.07864 [astro-ph.SR]a