The study of exoplanets—worlds orbiting distant stars—is still in its early days. Yet already researchers have found hundreds of worlds with no nearby analogue: giants that could steamroll Jupiter; tiny pebbles broiling under stellar furnaces; puffy oddballs with the density of peat moss. Still other exoplanets might look familiar in broad-brush, only to reveal a topsy-turvy realm where rare substances are ordinary, and vice versa.

Take carbon, for instance: the key constituent of organic matter accounts for some of humankind's most precious materials, from diamonds to oil. Despite its outsize importance, carbon is uncommon—it makes up less than 0.1 percent of Earth's bulk.

On other worlds, though, carbon might be as common as dirt. In fact, carbon and dirt might be one and the same. An exoplanet 40 light-years away was recently identified as a promising candidate for just such a place—where carbon dominates and where the pressures in the planet's interior crushes vast amounts of the element into diamond.

The planet, known as 55 Cancri e, might have a crust of graphite several hundred kilometers thick. “As you go beneath that, you see a thick layer of diamond,” says astrophysicist Nikku Madhusudhan, a postdoctoral fellow at Yale University. The crystalline diamond could account for a third of the planet's thickness.

Carbon-based worlds would owe their distinct makeup to a planet-formation process very different from our own. If the composition of the sun is any indication, the cloud of dust and gas that coalesced into the planets of our solar system ought to have contained about twice as much oxygen as carbon. Indeed, Earth's rocks are mostly based on oxygen-rich minerals called silicates. Astronomers have determined that 55 Cancri e's host star, however, contains slightly more carbon than oxygen, which may reflect a very different planet-forming environment. And Madhusudhan and his colleagues calculated that the planet's bulk properties—denser than a water world but less dense than a world made of Earth-like minerals—match those predicted for a carbon planet. The researchers published their findings in the November 10, 2012, Astrophysical Journal Letters.

Life-forms on a carbon planet—if they exist—would little resemble the oxygen-dependent organisms of Earth. Precious oxygen would prove valuable as a fuel in much the same way that humans covet hydrocarbon fuels on Earth, says Marc Kuchner of the NASA Goddard Space Flight Center. Even courtship customs would be worlds apart from ours. “You would not be impressed if someone gave you a diamond ring,” Kuchner muses. “If your suitor showed up with a glass of water, that would be really exciting.”