HD 23472 d is a remarkable exoplanet for one simple reason: it is small, extremely dense, and located in a compact planetary system that gives astronomers an unusual laboratory for studying how rocky worlds form and evolve. Orbiting a K-type star roughly every four days, the planet has a radius of about 75% that of Earth but an estimated mass of only around half an Earth mass. Its measured density is therefore surprisingly high, leading researchers to identify it as a strong candidate for a so-called super-Mercury—a small, metal-rich planet with an unusually large iron core relative to its overall size.
HD 23472 d belongs to the five-planet system surrounding the star HD 23472, also known as TOI-174. The system is particularly interesting because its planets occupy tightly packed orbits, with orbital periods ranging from approximately 3.98 days for planet d to nearly 30 days for the outermost known planet, HD 23472 c. The system therefore contains several worlds that formed and evolved under different levels of stellar irradiation, providing astronomers with an opportunity to investigate how a planet's environment can influence its composition.
The discovery and characterization of HD 23472 d were part of a 2022 study led by astronomers using observations from the European Southern Observatory's ESPRESSO spectrograph. The planet was identified through a combination of transit observations and precise radial-velocity measurements. The transit method detects the tiny dip in a star's brightness that occurs when a planet passes across the stellar disk, allowing researchers to estimate the planet's radius. Radial-velocity observations measure the subtle gravitational tug that the planet exerts on its host star, providing information about its mass. Together, these techniques make it possible to estimate a planet's bulk density—one of the most valuable clues to its internal composition.
According to the NASA Exoplanet Archive, HD 23472 d has a radius of approximately 0.75 Earth radii and an estimated mass of about 0.55 Earth masses, although the uncertainties are substantial. It completes one orbit in about 3.9766 days and travels at an average orbital distance of roughly 0.043 astronomical units from its star. For comparison, Mercury orbits the Sun at an average distance of about 0.39 astronomical units. HD 23472 d is therefore extraordinarily close to its host star by Solar System standards.
That proximity has major consequences. The planet receives an estimated 117 times as much stellar energy as Earth receives from the Sun, and its calculated equilibrium temperature is approximately 909 kelvin, or about 636 degrees Celsius. An equilibrium temperature is not the same as a measured surface temperature—it is a theoretical temperature that depends on assumptions about how the planet absorbs and redistributes incoming energy—but it provides an important indication of the extreme environment in which HD 23472 d exists.
The most intriguing feature of HD 23472 d is its density. The current values listed by the NASA Exoplanet Archive give an estimated bulk density of approximately 7.5 grams per cubic centimeter, with large uncertainties. Earth's mean density is about 5.5 grams per cubic centimeter, while Mercury's is about 5.4 grams per cubic centimeter. HD 23472 d's central estimate is therefore unusually high for a planet of its size, although the uncertainty range is broad enough that its exact composition remains unresolved.
This is where the term "super-Mercury" becomes relevant. In exoplanet science, a super-Mercury is generally understood as a small rocky planet with an unusually high proportion of iron and other heavy elements, giving it a disproportionately large metallic core. The term does not mean that the planet is simply a larger version of Mercury. Rather, it describes a class of planets whose bulk composition appears to be more metal-rich than that of typical rocky planets. The 2022 study of HD 23472 proposed that planets d and e could be super-Mercuries based on their high inferred densities and internal-structure modelling.
The comparison with Mercury is especially interesting because both worlds are close to their respective stars, but the resemblance should not be overstated. Mercury is the innermost planet of our Solar System and has a large iron core relative to its size, while HD 23472 d is much smaller than Earth and orbits a different type of star under a radically different set of planetary-system conditions. The evidence for a super-Mercury interpretation comes primarily from the planet's inferred bulk density and models of its interior, not from direct observations of its core. Astronomers therefore cannot yet say with certainty exactly how much iron lies beneath its surface.
HD 23472 d's small size is itself significant. With a radius of only about three-quarters that of Earth, HD 23472 d lies firmly in the realm of small terrestrial-size exoplanets. Measuring the masses of such low-mass planets is technically difficult because their gravitational influence on their host stars is extremely weak. The 2022 research highlighted the ability of ESPRESSO to obtain radial-velocity measurements precise enough to characterize planets among the lowest-mass worlds for which such measurements had been obtained at the time.
HD 23472 d is also important because it is not an isolated planet. It is part of a tightly packed system containing at least five confirmed planets. The innermost three—HD 23472 d, e, and f—are approximately Earth-sized, while planets b and c are larger, with radii around two Earth radii and 1.85 to 1.9 Earth radii, respectively, according to the 2022 characterization and current NASA Exoplanet Archive data. Their orbital periods increase outward from about four days to nearly 30 days.
The architecture of this system may provide clues about how its planets formed. The 2022 study found a pattern in which the estimated gas and water fractions generally increase with distance from the host star. The researchers argued that this trend is consistent with a system shaped by irradiation: planets closer to the star would have experienced stronger heating, potentially losing volatile material more readily, while planets farther away could retain greater amounts of water or gaseous material. In that picture, HD 23472 d represents an especially extreme inner member of a planetary system whose composition changes systematically with distance from its star.
This interpretation also helps explain why HD 23472 d and its neighboring planet HD 23472 e attracted particular attention. Both are small, dense planets on very short-period orbits. The study estimated similar bulk densities for the two worlds and suggested that they could represent a rare pair of super-Mercuries. If that interpretation is confirmed, the system would be an exceptional test case for competing theories of how metal-rich rocky planets form.
One possible explanation for super-Mercury formation is that intense stellar irradiation and atmospheric loss stripped away lighter materials, leaving behind a relatively metal-rich remnant. Another possibility is that the planets formed with unusually large iron cores from the beginning, perhaps because of the conditions in the protoplanetary disk or through giant impacts that removed part of their rocky mantles. Determining which scenario is correct requires more than measuring density alone. Astronomers need better mass and radius measurements, improved models of planetary interiors, and a larger population of comparable planets for statistical comparison.
HD 23472 d is also a reminder of how much information astronomers can extract from tiny changes in starlight and stellar motion. The planet cannot be photographed directly with current instruments, and its physical surface has not been observed. Instead, researchers infer its properties from the way it affects its star. Its radius comes primarily from transit geometry, while its mass is inferred from the star's motion. From these measurements, scientists can estimate density and then test possible interior structures. It is an indirect process, but one that has transformed the study of planets beyond the Solar System.
There is, however, an important scientific caution when discussing HD 23472 d. Calling it a "super-Mercury" is a compelling interpretation, not a direct observation of its core. The mass and density measurements carry significant uncertainties, and different assumptions about a planet's interior can produce different compositional models. The NASA Exoplanet Archive currently lists a mass estimate of about 0.55 Earth masses with an uncertainty of roughly 0.2 Earth masses and a radius of about 0.75 Earth radii with an uncertainty of approximately 0.06 Earth radii. Those uncertainties propagate into the density estimate, which is itself correspondingly uncertain.
Even so, HD 23472 d remains scientifically valuable. Its combination of small size, short orbital period, high irradiation, and apparently high density makes it a particularly useful target for studying the boundary between ordinary rocky planets and metal-rich worlds. Its position within a five-planet system adds another layer of significance: instead of studying one unusual planet in isolation, astronomers can compare multiple worlds that formed around the same star but now experience very different environments.
For now, HD 23472 d is best described as a small, hot, high-density exoplanet and a strong candidate for a super-Mercury composition. Its estimated radius is around 75% that of Earth, its orbital period is just under four days, and its inferred density is high enough to make a metal-rich interior a plausible explanation. Yet the most important story may not be the planet itself, but the planetary system around it. By comparing HD 23472 d with its four known companions, astronomers can investigate how stellar irradiation, orbital architecture, volatile loss, and planetary composition interact over billions of years.
HD 23472 d therefore occupies an intriguing place in modern exoplanet science. It is neither an Earth twin nor a conventional super-Earth, and its extreme environment makes it an unlikely candidate for habitability. Its real importance lies elsewhere: it may preserve evidence of a radically different planetary formation history, encoded in an unusually dense interior. As observations improve and the population of known small exoplanets grows, worlds like HD 23472 d will help scientists determine whether super-Mercuries are rare cosmic oddities or a common outcome of planet formation around other stars.
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