Astronomers have discovered a star system that is home to not one but three Super-Earths and two Super-Mercuries, a type of planet extremely rare and exotic. In fact, super-Mercuries are so rare that only eight such planets have been discovered to date.

A spectrograph operated by ESPRESSO has discovered two ‘super-Mercury’ worlds in the star system HD23472. As revealed by astronomers, these planets are extremely rare. This study, published in Astronomy & Astrophysics, examined how small planet composition varies with planet position, temperature, and stellar properties. As part of the study, Susana Barros, a researcher at the Institute of Astrophysics e Ciências do Espaço (IA), who led the project, explains the reason for observing this planetary system is to characterize the composition of small planets and to study the transition between having an atmosphere and not having an atmosphere. The evaporation of the atmosphere could be related to star irradiation. “Surprisingly, the team found that this system is composed of three super-Earths with a significant atmosphere and two Super-Mercuries, which are the closest planets to the star,” the researcher revealed.

HD 23472 is home to five exoplanets, three of which have masses smaller than the Earth. The five planets were measured by radial velocity method to be among the lightest exoplanets ever discovered. Using this technique, you can detect small variations in the velocity of a star caused by orbiting planets. ESPRESSO, a spectrograph mounted on the VLT at the European Southern Observatory (ESO) in Chile, provided the high precision that enabled the discovery. As far as composition is concerned, super-Earths and super-Mercuries are the higher mass counterparts to Earth and Mercury. Their main difference is that super-Mercuries contain a greater amount of iron. Exoplanets of this type are extremely rare.

In fact, only eight are known, counting in the two recently discovered ones. We don’t know why Mercury has a relatively larger and more massive core than the Earth and other planets in our Solar System, considering it is one of the densest planets in the Solar System. A giant impact could have removed parts of Mercury’s mantle or, because Mercury is the hottest planet in the solar system, its high temperatures may have evaporated part of its mantle. In order to understand the formation of such objects, it is important to find other dense, Mercury-like planets around other stars.

It is interesting to note that the discovery of two super-Mercuries in the same planetary system, instead of just one, provides a revealing picture for scientists. “For the first time, using the ESPRESSO spectrograph, we have discovered a system with two super-Mercuries. This helps us to understand how these planets were formed,” explains Alejandro Suárez, a researcher at the IAC and co-author of this study. “The possibility of a large impact to create a Super-Mercury is already very unlikely, two giant impacts in the same system seems very improbable.” This study’s co-author Jonay González, a researcher at the IAC, comments that further characterization of the planet’s composition will be necessary to understand how these two super-Mercuries formed.

Using the Extremely Large Telescope (ELT) and its first-generation high-resolution spectrograph ANDES, scientists will be able to probe the surface composition or the existence of a potential atmosphere for the first time. In the end, the team’s ultimate goal is to find another planet like Earth. As a result of the existence of an atmosphere, scientists can better understand the formation and evolution of planetary systems. Additionally, it can determine whether a planet is habitable. As Barros concludes, “we would like to extend this type of study to longer period planets with more amenable temperatures.”.