An atmosphere composed of 99.9% pure oxygen

And it's a star - not a planet:

A newly discovered star is unlike any ever found. With an outermost layer of 99.9 percent pure oxygen, its atmosphere is the most oxygen-rich in the known universe... The strange stellar oddity is a radically new type of white dwarf star, and was discovered by a team of Brazilian astronomers led by Kepler de Souza Oliveira at the Federal University of Rio Grande do Sul in Brazil. The star is unique in the known pool of 32,000 white dwarf stars, and is the only known star of any kind with an almost pure oxygen atmosphere... The discovery was reported today in a paper in the journal Science.