Multiplanetary systems are especially valuable because they shared the same host star and were formed by the same accretion disk. The properties of multi-planetary systems have been used to constrain planet formation and evolution models.

A new study suggests multi-planetary systems appear less similar in mass than in radius. However, a larger sample of multiplanetary systems with well-characterized mass and radius should confirm this result and reveal additional correlations.

While observing the galaxy HD 23472, a team of scientists led by the Instituto de Astrofísica e Ciências do Espaço (IA) discovered three super-Earths and two super-mercury. This last type of exoplanet is still very rare – if we count these two, there are only eight known supermercury. The discovery offers clues about how planets like Mercury are formed.

For the study, scientists used the ESPRESSO1 spectrograph (ESO). Scientists particularly wanted to characterize the composition of minor planets and understand how it changes with the planet’s position, temperature and stellar properties. They wanted to study the transition between having or not having an atmosphere, which could be related to the evaporation of the atmosphere due to the star’s irradiation.

Susana Barros (IA & Dep. de Física e Astronomia – Faculdade de Ciências da Universidade do Porto) said: “The team found that this system consists of three super-Earths with significant atmospheres and, surprisingly, two super-mercuries, the closest planets to the star.”

Three of the five planets in the HD 23472 system are lighter in mass than Earth. These exoplanets are among the lightest, with their masses determined using the radial velocity method. This was only made possible by the extremely high precision of ESPRESSO, housed in the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile. And the crew was inspired to do more by discovering not one but two supermercury.

Barros adds: “For the first time, we have discovered a system with two supermercury. This allows us to get clues about how these planets formed, which could help us rule out some possibilities. For example, if an impact large enough to create a Super-Mercury is already very unlikely, then two giant impacts in the same system seem very unlikely. We still don’t know how these planets form, but it seems to be related to the composition of the parent star. With this new system we can find out.”

Team member Olivier Demangeon (IA & DFA-FCUP), commented: “To understand how these two super-Mercuries formed, further characterization of the composition of these planets is needed. Since these planets have radii smaller than Earth, current instrumentation does not have the sensitivity to investigate the composition of their surfaces or the existence and composition of a potential atmosphere. The future Extremely Large Telescope (ELT) and its first-generation high-resolution spectrograph ANDES7 will for the first time provide both the sensitivity and precision required to make such observations.”

Barros said, “The existence of the atmosphere gives us insight into the formation and evolution of the system and also has implications for the habitability of planets. I would like to extend this study to planets with a longer period of more pleasant temperatures.”

Magazine reference:

  1. SC C Barros, ODS Demangeon et al. HD 23472: A multi-planetary system with three super-Earths and two potential super-Mercuries★, ★★. Astronomy and Astrophysics, DOI: 10.1051/0004-6361/202244293