Objects: carbon planets
Rarity: one in a thousand
The Milky Way was supposed to be a prospector's paradise, studded with planets made of diamond. But it seems these carbon-rich worlds are actually much rarer than imagined – and that may be good news for life.
Material left over from a star's birth becomes a swirling disc of debris that can coalesce into planets. So stars born with more oxygen than carbon have rocky planets that are carbon-poor. That's what happened in our solar system, where oxygen atoms outnumber carbon two to one, and it explains why Earthly rock is mostly oxygen-bearing silicates.
Stars born with more carbon than oxygen, on the other hand, should give rise to carbon planets. Such a world's internal pressure would squeeze some of the element into a thick layer of diamond. In 2010, based on existing observations of sun-like stars, Jade Carter-Bond of the University of New South Wales in Sydney, Australia, estimated that a third of all rocky planets might be carbon worlds.
"It's a fun idea," says Jonathan Fortney of the University of California, Santa Cruz, "but I do think it has been kind of one-sided." The problem is that it is hard to make out carbon-to-oxygen ratios in the spectra of warmer stars like the sun, leaving room for uncertainty in existing measurements. Last year Fortney noted that red dwarf stars are cool enough to see strong signs of carbon in their atmospheres – and no more than 1 in 1000 is carbon-rich, even though red dwarfs are the most common stars in the galaxy. The prospects for planet-scale diamond mining suddenly seemed much remoter.
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