HD 23472 e is a small, intriguing exoplanet orbiting the K-type star HD 23472, also known as TOI-174. Located roughly 127 light-years from Earth, the planet is part of an unusually compact system containing five confirmed worlds. Its discovery and subsequent characterization have attracted particular interest because HD 23472 e is approximately Earth-sized yet appears to have a remarkably high density, raising the possibility that it belongs to the rare class of planets sometimes described as "super-Mercuries."
The planet was identified in 2022 as part of a major study of the HD 23472 planetary system. Researchers combined transit observations with extremely precise radial-velocity measurements obtained with ESPRESSO, the high-resolution spectrograph installed on the European Southern Observatory's Very Large Telescope. This combination was crucial: the transit method revealed the planet's size, while the star's subtle motion under the planet's gravitational influence provided an estimate of its mass. The result was one of the more technically demanding detections of a small exoplanet, demonstrating the ability of modern instruments to measure planetary masses well below that of Earth.
According to the NASA Exoplanet Archive, HD 23472 e has a radius of approximately 0.82 times that of Earth and a mass of about 0.72 Earth masses, although the mass uncertainty is substantial. Its orbital period is just 7.91 days, meaning that a year on HD 23472 e lasts less than eight Earth days. The planet orbits its host star at an average distance of approximately 0.068 astronomical units, far closer to its star than Earth is to the Sun. The measured orbital eccentricity is modest, around 0.07, although the uncertainty is large enough that the precise shape of its orbit remains relatively poorly constrained.
Its physical properties make HD 23472 e particularly interesting. Combining its estimated mass and radius gives an average density of roughly 7.5 grams per cubic centimeter, with large uncertainties. For comparison, Earth's mean density is about 5.5 grams per cubic centimeter, while Mercury's is approximately 5.4 grams per cubic centimeter. The central estimate for HD 23472 e is therefore surprisingly high for a planet of its size. The original research suggested that the two innermost planets in the system, HD 23472 d and HD 23472 e, could be "super-Mercuries"—small planets with unusually large iron cores relative to their overall size. This interpretation is not yet definitive, however, because the uncertainties in their masses and densities remain significant.
The term "super-Mercury" is useful as a description of a possible composition rather than a formally established planetary category. In the case of HD 23472 e, the hypothesis is that a substantial fraction of the planet's interior could consist of iron and other dense materials, leaving a relatively small rocky mantle. Such a structure could have important implications for planetary formation. One possibility is that intense irradiation from the host star removed part of an originally more volatile-rich planet. Another is that the planet formed from material unusually enriched in refractory elements, or that giant impacts stripped away a significant portion of its mantle. At present, the available observations do not uniquely distinguish between these possibilities.
The location of HD 23472 e within its planetary system makes the story even more compelling. The planet is not an isolated world but one member of a tightly packed sequence of five planets. Moving outward from the star, the system contains HD 23472 d, e, f, b, and c, with orbital periods of approximately 3.98, 7.91, 12.16, 17.67, and 29.80 days, respectively. Their estimated radii range from less than Earth's size for the two innermost planets to roughly twice Earth's radius for the two outermost planets. This architecture provides astronomers with a natural laboratory for investigating how planets with different compositions can emerge from the same protoplanetary environment.
HD 23472 e was also challenging to detect because of its proximity to the orbit of HD 23472 d. The two planets have orbital periods of approximately 3.98 and 7.91 days, placing them close to a 2:1 period relationship. The original study noted that HD 23472 e's small size, its proximity to the transit signal of planet d, and its near-resonant orbital relationship contributed to the difficulty of identifying it. The planet's eventual characterization highlights how signals from multiple worlds can overlap or complicate the interpretation of observations, particularly when astronomers are searching for planets only slightly smaller than Earth.
The system's compact architecture may also provide clues about how planetary systems evolve under intense stellar irradiation. The research team found a broad trend in which the estimated gas and water fractions of the planets increase with distance from the host star. The innermost worlds appear to be denser and more likely to be dominated by rocky and metallic material, while the outer planets may retain larger amounts of volatile material. This pattern is consistent with a scenario in which irradiation has played a major role in shaping the planets over billions of years, although the exact evolutionary history remains uncertain.
This makes HD 23472 e scientifically valuable beyond its own characteristics. Comparing it with its neighboring planets allows researchers to study planetary diversity while controlling for some of the variables that normally complicate exoplanet research. All five planets orbit the same star and therefore formed within the same broad protoplanetary disk. Their contrasting sizes, masses, and inferred compositions offer an opportunity to investigate why planets born in the same system can evolve into such different worlds.
The planet's estimated equilibrium temperature also emphasizes how different its environment is from Earth's. NASA's catalog lists an equilibrium temperature of approximately 723 K based on the adopted planetary parameters. This is not a direct measurement of the planet's actual surface temperature and should not be interpreted as a weather forecast or atmospheric temperature. Equilibrium temperature is a model-dependent quantity that generally assumes a simplified energy balance and does not capture the full effects of atmospheric circulation, greenhouse warming, or heat redistribution. Nevertheless, the value indicates that HD 23472 e receives substantially more stellar energy than Earth does.
Because HD 23472 e is relatively small and likely has a rocky composition, it is not an obvious target for atmospheric characterization in the same way as a large, inflated gas giant. Its small radius produces a comparatively weak transit signal, while its proximity to the host star places it in a challenging observational environment. Even so, the system could become increasingly important as observational techniques improve. Better measurements of the planet's mass and radius would directly refine its density and help determine whether the super-Mercury interpretation is robust.
One of the most important limitations in our current understanding of HD 23472 e is therefore the uncertainty surrounding its interior. The published estimate of approximately 0.76 Earth masses, later represented in the NASA Exoplanet Archive at around 0.72 Earth masses, carries an uncertainty of roughly 0.3 Earth masses. Its radius estimate of about 0.82 Earth radii also has significant uncertainty. Because density depends strongly on both mass and radius, these uncertainties propagate into the inferred interior composition. The evidence for a dense, Mercury-like interior is intriguing, but it should be regarded as a scientific hypothesis supported by current measurements rather than a definitive description of the planet's internal structure.
The discovery of HD 23472 e also illustrates the importance of combining different exoplanet detection techniques. Transit observations are particularly effective at measuring planetary radii, but they do not directly reveal mass. Radial-velocity observations provide the complementary information needed to estimate how strongly a planet gravitationally tugs on its star. For a small planet such as HD 23472 e, that stellar motion is extremely subtle. The ability of ESPRESSO to extract such a signal demonstrates the extraordinary precision now possible in exoplanet astronomy and represents an important step toward characterizing increasingly Earth-like worlds.
HD 23472 e is not considered a second Earth, despite its broadly Earth-sized dimensions. Its short orbital period, intense stellar irradiation, and potentially iron-rich interior make it a radically different world from our planet. Its scientific importance lies precisely in that contrast. By studying a planet that is similar to Earth in size but potentially very different in composition and environment, astronomers can test theories about how rocky planets form, lose volatile material, acquire their internal structures, and respond to the radiation of their host stars.
Ultimately, HD 23472 e is best understood as one piece of a much larger planetary puzzle. Its small size, high estimated density, close orbit, and position within a five-planet system make it an exceptional target for comparative exoplanet science. The possibility that it is a super-Mercury is particularly fascinating, but the most valuable feature of the system may be the opportunity to compare several planets that formed around the same star yet appear to have followed very different evolutionary paths. As future observations refine the masses and compositions of HD 23472 e and its neighboring worlds, this compact system could become an important benchmark for understanding why planetary systems produce such a remarkable diversity of worlds.
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