The question of whether a planet composed entirely of diamond exists captures the imagination. While a world made of solid diamond from crust to core is unlikely, worlds with vast, diamond-rich layers are highly probable and have been identified. These exotic exoplanets form under conditions of extreme heat and pressure, resulting in geological compositions radically different from Earth’s silicate-dominated structure. The search focuses on these “carbon planets,” which form using chemical building blocks foreign to our solar system.
The Chemical Building Blocks of Carbon Planets
The fundamental difference between Earth-like worlds and diamond candidates lies in their initial material composition, specifically the ratio of carbon to oxygen (C/O ratio). Earth is a silicate world, rich in oxygen, silicon, and magnesium, with carbon making up less than 0.001% of its mass. This reflects the relatively low C/O ratio of the nebula from which our solar system formed.
A carbon planet, in contrast, forms in a nebula with a significantly elevated C/O ratio, where carbon is more abundant than oxygen. When this ratio is high, there is not enough oxygen available to bond with all the silicon to form common silicate rocks like quartz and olivine. Instead, the surplus carbon forms carbon-rich compounds like graphite, carbon monoxide, and various carbides, such as silicon carbide.
The composition of a star is a strong indicator of the planets that form around it, as both originate from the same material cloud. Exoplanets orbiting stars with a high C/O ratio are much more likely to be carbon-rich, fundamentally altering their internal geology.
The Internal Recipe for Creating Diamonds
Converting the abundant carbon into diamond requires a specific physical process involving immense internal pressure and temperature. Diamond is a crystalline form of carbon that only becomes stable under extreme conditions, similar to those found deep within Earth’s mantle.
The colossal mass of a planet generates gravitational pressure that can exceed millions of atmospheres, often measured in gigapascals (GPa) or megabars. For a large, rocky exoplanet, internal pressures can reach the necessary range of 10 to over 100 GPa, and temperatures must simultaneously exceed thousands of degrees Celsius. These conditions force the carbon atoms to rearrange their structure from less dense forms, like graphite or amorphous carbon, into the tightly packed, tetrahedral lattice of diamond.
In some theoretical models, especially for super-Earths, scientists hypothesize a planetary structure with a metallic core surrounded by a vast mantle layer composed of carbon compounds. If water is present, the combination of high pressure and temperature causes silicon carbide to react with the water, producing diamonds and silica. This process suggests that a significant fraction—potentially up to a third—of a carbon-rich planet’s mass could be converted into a compressed, crystalline diamond layer over time.
Confirmed Astronomical Candidates
Astronomers use indirect evidence to infer the composition of exoplanets, and two candidates stand out for their potential diamond-rich interiors. The super-Earth 55 Cancri e, orbiting a star about 40 light-years away, was one of the first planets theorized to have a large carbon content. Its mass is approximately eight times that of Earth, and its radius is twice as large, giving it an extremely high density. Initial studies of its host star, 55 Cancri, showed a higher C/O ratio than our Sun.
This supported the idea that carbon could crystallize under the planet’s intense heat (2,100 degrees Celsius) and crushing pressure. While later analyses suggested the star’s C/O ratio might not be as high as first thought, the planet’s characteristics still indicate a geological structure unlike Earth’s, with carbon compounds playing a major role.
A far more extreme example is PSR J1719−1438 b, a planet orbiting a millisecond pulsar (a rapidly spinning neutron star). This object is believed to be the remnant of a former white dwarf star whose outer layers were stripped away by the pulsar’s gravity, leaving behind a dense core. With a mass slightly greater than Jupiter but a size only about 40% of Jupiter’s, this planet possesses an extraordinary density. Scientists conclude its composition must be primarily crystalline carbon, or diamond, making it a stellar-mass diamond of immense proportions.