It’s been a few weeks since we had a good exoplanet discovery announcement, and the last one involved baby exoplanets. This week, in a new paper published in Astronomy & Astrophysics with lead author Susana Barros, a team using the ESPRESSO spectrograph at the Very Large Telescope has announced the discovery of a five-planet system around star HD 23472.

What’s really exciting about this particular discovery is that two of those newly found exoplanets are what are called ‘super-mercuries’. To date, we have only found eight of this type of planet, including these latest two, and as Barros explains: [The team set out to] study the transition between having or not having an atmosphere, which could be related to the evaporation of the atmosphere due to the irradiation of the star.

Mercury is a strange little planet in our solar system. The core of the innermost world is relatively large and the mantle is relatively small compared to the other planets orbiting our Sun. And we don’t understand the reason. Perhaps there was a massive impact that removed bits of the planet or perhaps that nearness to the Sun evaporated some of the mantle. Now we’re finding similar worlds in other systems, and Barros notes: For the first time we have discovered a system with two super-mercuries. This allows us to obtain clues about how these planets were formed, which…appears to be connected to the composition of the parent star. This new system can help us find out.

The masses of the planets were measured with the radial velocity method, and from the measurements of HD 23472, the team found that three of the five planets have masses smaller than that of Earth, making these some of the lightest exoplanets discovered. The two super-mercuries are both about 0.8 Earth radii. For comparison, Mercury’s radius is about one-third that of Earth’s.

Unfortunately, further characterization of these two exoplanets will have to wait until the Extremely Large Telescope is built, as the current instruments available do not have the sensitivity to gain information about the planets’ compositions or possible atmospheres.

More Information

IA press release

“HD 23472: a multi-planetary system with three super-Earths and two potential super-Mercuries,” S. C. C. Barros et al., 2022 September 27, Astronomy & Astrophysics