A nearby planet may be made largely of diamond and graphite

About 40 light-years from Earth, a scorched world circles its star so tightly that a full year there lasts less than 18 hours. The planet is roughly twice the diameter of Earth and around eight times its mass, a category of world known as a super-Earth. What made it famous was a proposal that its interior might be built largely from carbon, with a substantial portion locked up as diamond and graphite rather than the silicate rock that forms Earth.

The idea captured attention because it implied a planet fundamentally unlike anything in the solar system, one whose bulk chemistry followed a carbon-rich recipe instead of the oxygen-rich one that produced the rocky inner planets here. Later observations have complicated and partly challenged that picture, turning the world into a case study in how difficult it is to determine what a distant planet is actually made of.

A super-Earth around a sun-like star

The planet orbits a Sun-like star and was detected in data gathered during 2003 and 2004, with the discovery announced in 2004. According to the planetary data on record, its mass is roughly eight times that of Earth and its radius is about twice Earth’s. It was among the first examples of a super-Earth found orbiting a normal, Sun-like star, and it carries the formal name Janssen. Its host lies close enough to Earth, at around 40 light-years, that the system is relatively accessible to detailed study by the standards of exoplanet science.

A year shorter than a day

The planet hugs its star at a distance of only about 0.016 astronomical units, a tiny fraction of the gap between Mercury and the Sun. As a result it completes an orbit in roughly 0.74 days, less than 18 hours. At that range it is almost certainly tidally locked, meaning one hemisphere permanently faces the star while the other faces away. The dayside is ferociously hot, with temperature estimates well into the thousands of kelvin, hot enough to melt rock, while the nightside is dramatically cooler. Such extreme heating is central to every question about the planet’s surface and interior. Because the same face is always turned toward the star, the contrast between the two hemispheres is severe, and how heat is carried from the dayside to the nightside, whether by a thick atmosphere, flowing lava, or not at all, is one of the properties observers have tried hardest to measure.

The diamond hypothesis

The proposal that drew headlines held that the planet formed from material richer in carbon than in oxygen, unlike the silicate-dominated bodies of the inner solar system. Under that scenario, roughly a third of the planet’s mass could be carbon, much of which might exist as diamond, given the enormous temperatures and pressures expected in the interior of a body this massive. The reasoning rested on estimates of the planet’s mass and radius, which together imply a density, and on assumptions about the chemical makeup of the disk from which it formed. It was always presented as a possibility requiring confirmation rather than an established fact. The appeal of the idea was partly that it connected the planet’s bulk properties to the chemistry of its birth: a disk with more carbon than oxygen would tend to build worlds dominated by carbides and graphite rather than the silicate minerals of Earth, and at depth that carbon could be squeezed into diamond.

Evidence for a lava world

Separate lines of study pointed toward a molten surface. In 2017, infrared observations were interpreted as evidence of a global lava ocean, possibly veiled by an atmosphere with a pressure somewhat greater than Earth’s. A planet with a permanent day side baking under intense stellar radiation is a natural candidate for surface melting, and the notion of a lava world became a competing description alongside the carbon-rich interior. The two ideas are not mutually exclusive, but they emphasize different aspects of a planet that observers can only probe indirectly.

Later observations push back

More recent measurements have challenged some earlier conclusions. Thermal emission spectra gathered across mid-infrared wavelengths were used to rule out one version of the lava-world model, the idea that the planet is covered by only a thin atmosphere of vaporized rock. Instead, the newer data pointed toward a genuine, more substantial atmosphere, possibly rich in carbon dioxide or carbon monoxide. Refined estimates of the planet’s mass and radius have also shifted the constraints on its composition over the years, which in turn affects how much room remains for a diamond-heavy interior.

Why certainty remains elusive

The planet illustrates a fundamental limitation of exoplanet research: mass and radius alone do not uniquely fix what a world is made of, because different mixtures of materials can produce the same overall density. Determining whether the interior is carbon-rich, silicate, or something else requires modeling combined with hard-won atmospheric measurements, and those measurements continue to evolve as instruments improve. The diamond-planet description remains an intriguing hypothesis that observations have neither fully confirmed nor entirely dismissed. For now the world stands as a vivid reminder that even a well-studied planet just 40 light-years away can keep its basic composition a matter of open scientific debate.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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