Objects: carbon planets
Rarity: one in a thousand
The Milky Way was supposed to be a prospector鈥檚 paradise, studded with planets made of diamond. But it seems these carbon-rich worlds are actually much rarer than imagined 鈥� and that may be good news for life.
Material left over from a star鈥檚 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鈥檚 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.
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Stars born with more carbon than oxygen, on the other hand, should give rise to carbon planets. Such a world鈥檚 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 might be carbon worlds.
鈥淚t鈥檚 a fun idea,鈥� says of the University of California, Santa Cruz, 鈥渂ut I do think it has been kind of one-sided.鈥� 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 are cool enough to see strong signs of carbon in their atmospheres 鈥� 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.
Space oddity
Now Poul Nissen of Aarhus University in Denmark has scrutinised 33 sun-like stars known to host planets. Based on earlier observations, some appeared to have more carbon than oxygen, hinting that they might host diamond worlds.
鈥淚 got very suspicious about those high carbon-to-oxygen ratios,鈥� Nissen says. So he did a new, more detailed analysis of the stars鈥� spectra and compared them with that of the sun itself. He found that all 33 stars have more oxygen than carbon, supporting the notion that diamond worlds are much rarer than had been thought.
鈥淭hey鈥檙e more likely to be the oddity rather than the general rule,鈥� agrees Carter-Bond. Nissen鈥檚 observations have also convinced another , of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland. He now estimates there鈥檚 only one carbon planet for every 1000 silicate worlds, at least in our part of the galaxy. 鈥淭hat鈥檚 down quite a bit from before,鈥� he says.
It鈥檚 bad news for galactic bling, but could be a boon for treasure hunters of a different sort: those seeking extraterrestrials. Carbon planets would probably be dry and lifeless, but silicate worlds like ours will have plenty of oxygen and therefore a better chance of hosting water 鈥� a key ingredient for life as we know it.
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