Alien worlds made of diamonds

Ken Tapping's weekly astronomy column


Over recent weeks the hot news in astronomy is the discovery of a planet that is likely to be largely made out of diamond. Because of the value we place on diamonds, such a world sounds like the stuff of fantasy in a tale from the Arabian Nights. However, diamond, or more correctly, carbon planets are a distinct possibility.

The form of carbon with which we are most familiar is the soft, black stuff that is the main ingredient in soot, coal and charcoal. We call this form “graphite”. It is black, soft and greasy feeling, and is often used as a lubricant and the main ingredient in “pencil lead”. The name graphite comes from the Greek word for writing. However, when under very high temperature and pressure, the graphite crystals collapse into a more compact form, which when pure is colourless, transparent and very hard –  diamond.

On Earth, diamonds occur in rocks buried deep down, where the temperatures and pressures are enormous. Then when the rocks containing the diamonds come up to the surface in the feed pipes to volcanoes, they bring the diamonds with them.

The “diamond planet” that has been discovered is orbiting a pulsar, a rapidly rotating neutron star. However, the two bodies were born as a pair of large stars, orbiting each other at a fairly close distance. The more massive the star, the faster it ages, so eventually the star with the largest mass collapsed and exploded as a supernova. Its core was compressed until its atoms collapsed into a compact, rapidly spinning ball of neutrons – a neutron star. This object started pulling off the material of its partner, until only its core was left.

Stars make energy in their cores by fusing hydrogen into heavier elements, such as carbon, nitrogen and so on. So the stripped core of that unfortunate star was a large ball of mainly carbon, very hot and highly compressed. As the core cooled it solidified as a large lump containing a lot of highly compressed carbon – diamond.  Since no nuclear fusion was happening any more, this lump of diamond no longer shone with its own light. It had become a large planet.

The idea of a planet-sized lump of diamond is not new. Arthur C. Clark, the well-know science fiction writer, postulated their existence in his short stories, and then in his 2001 series of novels. He suggested that a diamond core could exist in the middle of Jupiter, the largest planet in the Solar System.

Jupiter contains a lot of carbon. Clark suggested that during the formation and subsequent history of the planet, a slow snowfall of carbon had been falling down through the planet’s deep atmosphere, accumulating around the core, getting hot and increasingly compressed, until it formed a huge and gradually growing diamond.

The cores of diamond planets are inaccessible to us and are likely to remain so. On the other hand, if a diamond planet were to be shattered by a collision, there could be some valuable debris out there. However, finding it will be a challenge, and it would probably cost more to get it than the prize is likely to be worth.

Saturn is getting lost in the sunset glow, and very hard to see.  Jupiter rises around 10 p.m., Mars comes up around 3am. The Moon will be full on the 12th.


Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton.