The discovery of exoplanets has revealed a diversity of worlds far exceeding the variety found in our own cosmic neighborhood. The possibility of a world rich in carbon, potentially forming vast reserves of diamond, captured the public imagination. This concept has moved from science fiction to scientific possibility through the study of a particular super-Earth, representing a fundamental shift in our understanding of planetary chemistry.
Identifying the Diamond World
The exoplanet most famously associated with the term “diamond planet” is 55 Cancri e, which also carries the official name Janssen. This world is classified as a Super-Earth, meaning it is significantly larger than Earth but smaller than the ice giants like Neptune. Located about 41 light-years away in the constellation Cancer, the planet orbits its host star, 55 Cancri A, in an extremely tight, rapid path.
Janssen is nearly twice the diameter of Earth and possesses about eight times its mass, giving it a high density. Its orbit is so close that a year on the planet lasts less than 18 hours, subjecting it to an intense flux of radiation from its star. This proximity creates an environment of extreme heat, which is a major factor in the planet’s unique composition.
The Exotic Composition
The chemical basis for the diamond planet theory rests on the planet’s elemental makeup, specifically its carbon-to-oxygen (C/O) ratio. On Earth, the C/O ratio is low, resulting in an interior dominated by oxygen-rich silicate minerals, like those found in our mantle. The initial hypothesis for 55 Cancri e was that the planet formed from a protoplanetary disk with a significantly higher C/O ratio than our solar system’s.
A high carbon concentration means oxygen-rich compounds would be replaced by carbon-rich alternatives, such as silicon carbide or pure carbon. This abundance suggests the planet’s interior could be composed of materials like graphite and diamond, rather than familiar silicates and water ice. Measurements of the host star’s composition, used to infer the planet’s initial building blocks, suggested a carbon-rich environment.
Later studies, however, refined the host star’s C/O ratio, suggesting it might be lower than initially thought. This debate means the extent of the carbon-rich interior is still a subject of active research and modeling. While the planet may not be entirely made of diamond, models incorporating a high carbon content remain plausible explanations for its observed mass and radius.
Conditions for Diamond Formation
The immense pressures and temperatures within 55 Cancri e drive the transformation of carbon into its crystalline, diamond form. The planet’s large mass and density create crushing pressures deep beneath its surface. For carbon to crystallize into diamond, it requires a high-pressure, high-temperature (HPHT) environment, the same method used to create synthetic diamonds on Earth.
The extreme proximity to its star results in searing surface temperatures that can reach up to 2,700 degrees Celsius on the day side. While this heat is too high for stability on the surface, the heat conducted into the interior, combined with the planet’s internal pressure, provides the conditions for carbon atoms to rearrange into the tightly bonded lattice structure of diamond. This process would occur in the deep mantle, many kilometers beneath the molten surface.
The interior pressure is far greater than what is found deep within Earth, where natural diamonds form. This pressure, likely reaching millions of times that of Earth’s surface atmosphere, acts on the abundant carbon compounds to facilitate the phase change into diamond. The diamond material is predicted to exist not as a single giant gem, but as a substantial layer within the planet’s mantle structure.
Other Carbon Worlds and Nuance
The idea of a diamond world is sometimes confused with the atmospheric phenomenon known as “diamond rain,” theorized to occur on ice giant planets like Uranus and Neptune. On these giants, methane in the atmosphere is broken down by intense pressure and heat, causing carbon atoms to compress into solid diamonds that then sink toward the core. This is an atmospheric process of precipitation, distinct from the proposed diamond-rich solid mantle of 55 Cancri e.
The “diamond planet” label for 55 Cancri e is theoretical, based on models interpreting its mass, radius, and stellar composition. Astronomers have not directly observed a diamond mantle, and the planet’s actual interior composition remains a subject of ongoing scientific investigation. Current evidence suggests the planet is a rocky world with a magma ocean; while it may contain diamond, it is not a literal, solid, polished diamond sphere.