Some stars in our galaxy could harbour planets with thick layers of diamond in their mantles. They may sound like prime real estate but new lab experiments suggest that these blingworlds, though carbon-rich, would be cold, devoid of most of the mechanisms that sustain life on Earth.
Our solar system is relatively carbon-poor. Consequently, Earth's core is made of iron and its mantle of silica-based minerals. Some stars have high carbon-to-oxygen ratios compared with the sun, however, and such stars might host planets with mineral compositions very different from that of Earth.
In 2005, astronomer Marc Kuchner, now at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and colleagues showed that planetary systems with high carbon-to-oxygen ratios would form planets with layers of graphite. High pressure would convert parts of this layer into layers of diamond many kilometres thick.
Now Wendy Panero and Cayman Unterborn of Ohio State University in Columbus have extended the study to better understand both the composition of such planets and their likely geological processes.
Black planets
In her lab, Panero subjected tiny samples of iron, carbon and oxygen to 65,000 gigapascals of pressure and temperatures of 2400?kelvin, mimicking conditions deep inside the Earth. She found that samples with high carbon-to-oxygen ratios formed diamonds.
Panero and Unterborn used these findings to estimate the composition of planets that might form in carbon-rich solar systems. They found that nearly 50 per cent of the atoms in such planets would turn into diamond, and that nearly a quarter of such planets by volume would be diamond.
The study also showed that planets with masses ranging from half the Earth's mass to about 10 Earth masses would all have a similar composition. A planet of 10 Earth masses, for example, would have core of liquid steel and a 3500-kilometre-thick mantle that was mostly diamond, topped with a crust mostly of graphite and rock.
As a result, these diamond planets won't be shiny. "They'll be very black planets," says Unterborn, who presented the results at the autumn meeting of the American Geophysical Union in San Francisco on Tuesday.
Convection sluggish
They'll also be very cold. Diamond conducts heat very well, so any heat from the core will quickly move to the surface and then radiate away into space. Diamond is also famously hard and so will resist any changes to its shape. This would hamper the convection currents in the planet's interior that would be responsible for the movement of heat.
"Taking the viscosity plus the thermal conductivity, you pretty much shut down all convection in a [carbon-rich] planet up to the mass of the Earth," says Panero. "Once you get to planets five to 10 times the mass of the Earth, you can get something that has incredibly sluggish convection."
The net result is that these diamond-rich planets will not have global carbon cycles nor plate tectonics ? which in turn may have implications for ocean formation. "I don't see any mechanism for maintaining an ocean on the surface," says Panero. On Earth, oceans are born because of the activity of tectonic plates.
Diamond explosion
Until now, astrophysicists have been wondering about the primary form of carbon in such planets: would it be some crystal structure, or diamond, or perhaps carbon mixed up with iron?
"This study is a boon to us astrophysicists who are interested in carbon planets ? which now seem to be popping up in a variety of different environments around the galaxy," says Kuchner. "[It] also reminds us that there could be huge deposits of carbon, even diamond, buried in the planets in our own solar system."
The finding follows the likely sighting of a dead star made of diamond
848 light years away earlier this year.
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