Diamond Planets

In our search for planets beyond our solar system, a wide variety of worlds have been revealed to us in a relatively short time—giants twice the size of Jupiter, small frozen ice balls, lava-spewing worlds, and even planets orbiting multiple stars. But planets made of diamonds?

In the past year, astronomers have announced a few diamond-candidate planets, including 55 Cancri e and a planet orbiting pulsar PSR J1719-1438, but how could this happen? Diamonds after all, are just properly compressed carbon. How do we get from pressurized carbon to a planet composed of diamonds? Partially, media hype, but mostly science. Some of it depends on the chemical composition of the planet when it forms.

Planets form along with their stars in protoplanetary disks—disks of dense gas and dust orbiting a young star. Carbon-rich, oxygen-poor protoplanetary disks are chemically the best candidates for creating diamond worlds; this means the planet, and the star it orbits, must have a high concentration of carbon.

Theoretically, on the surfaces of these rocky worlds, volcanoes could erupt, sending diamonds to the surface, creating literal mountains of diamonds and silicon carbides.

“This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth,” says Yale researcher Nikku Madhusudhan, describing 55 Cancri e. He adds that the discovery of the carbon-rich planet also means rocky exoplanets can no longer be assumed to have chemical constituents, interiors, atmospheres—let alone biologies—similar to Earth.

Hype aside, some of these sparkling worlds are already being called into question. Further analysis of the planet 55 Cancri e indicate the concentration of oxygen to carbon might be higher than previously indicated, based on measurements taken from its star. Since the star and planet would form from the same material, a lower carbon star means the planet, too, might not have a diamond-friendly composition.

This is, of course, all based on theory and modeling. No spacecraft has actually detected diamond worlds, photographed them, or collected them. But the idea remains tantalizing.

And much closer to home, within our own solar system, another surprise!

Scientists have long speculated that gems might be created in the atmospheres of Neptune and Uranus, but recent analysis of the chemistry of Jupiter and Saturn indicate a different process—lightning in the atmosphere turns methane into soot, which is then pressurized as it falls, first turning into graphite and, then, pressed into a nonreactive diamond. Heat near the cores of Jupiter and Saturn would liquify the diamonds as they continue to fall, but on Uranus and Neptune, with their ice-cores, the crystals would last much, much longer.

Will we be collecting the precious gems from other worlds? Not so much (although one can imagine what a space diamond might fetch as celebrity bling). Instead, studying the compositions of these planets further informs our understanding of the Universe around us, how it forms and how different these far-off worlds can be.

Elise Ricard is the Senior Presenter at the Morrison Planetarium and holds a master’s degree in museum education.

Image: NASA