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White Dwarf Accreted a Water-Rich Dwarf Planet

13 Apr 2015, 22:00 UTC
White Dwarf Accreted a Water-Rich Dwarf Planet
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White dwarfs are the dense remnant cores of low to intermediate mass stars. Planetary systems are ubiquitous around stars and when a star evolves into a white dwarf, its planetary system can become destabilised. Planets can get ejected; spiral into the star or even collide with one another. Nevertheless, a significant fraction of white dwarfs are expected to have retained at least parts of their planetary systems.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.SDSS J1242 is a cool white dwarf with a helium dominated atmosphere and an effective temperature of 13,000 K. Observations of SDSS J1242 show that its outermost layer contains a significant amount of rock-forming elements, almost all in the form of oxygen, magnesium, silicon and iron. ...

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