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Earth-sized white dwarfs show oxygen atmosphere
Posted: November 12, 2009

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Astronomers have discovered two Earth-sized bodies with oxygen-rich atmospheres, but there is no chance of finding life on these worlds for they are two unusual breeds of white dwarf star.

The white dwarfs are roughly the same size as the Earth. Image: ESA/NASA.

These two white dwarfs are the remnants of massive stars that reached the end of their existence after having consumed all their hydrogen, helium and carbon fuel. Models suggest that stars around seven to ten times the mass of our own Sun will either exhaust their fuel supply and end up with very oxygen-rich cores, or explode in a spectacular nuclear fireworks display, collapsing into a neutron star. Finding such oxygen-rich white dwarfs, like SDSS 0922+2928 and SDSS 1102+2054 are an important confirmation of the model, since almost all white dwarfs have hydrogen and/or helium envelopes that shield the core from direct view.

“These surface abundances of oxygen imply that these are white dwarfs displaying their bare oxygen-neon cores, and that they may have descended from the most massive progenitors stars in that class,” says Boris Gänsicke from the University of Warwick. Gänsicke and colleagues found the pair within a data set of the Sloan Digital Sky Survey.

Current models predict that white dwarfs with such oxygen and neon cores host a thick thick carbon-layer that would prevent upward diffusion of large amounts of oxygen. But calculations also show that the thickness of this layer decreases the closer the progenitor star is to upper mass limit for stars ending their lives as white dwarfs.

The implication for the formation of SDSS 0922+2928 and SDSS 1102+2054 is that they descended from the most massive stars avoiding core-collapse. In this case they would be expected to be very massive themselves, although current data is insufficient to confirm the mass of these two bodies.

SDSS 0922+2928 and SDSS 1102+2054 are 400 and 220 light years from Earth, respectively. Full details of the discovery are reported in the current issue of Science.

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