An international team led by the University of Geneva (UNIGE), including scientists from the National Centre for Competence in Research PlanetS, the University of Warwick, and the Instituto de Astrofísica de Canarias, has launched an ambitious project to map exoplanets around the Neptune desert. The goal is to better understand the formation and evolution of planetary systems.
This collaborative project, named ATREIDES, has already achieved initial results through observations of the TOI-421 planetary system. Analysis of the system has revealed a surprising tilted orbital structure, offering new insights into the chaotic history of these distant worlds.
The preliminary study has been published in the journal Astronomy & Astrophysics.
What are the physical mechanisms that control the formation and evolution of planetary systems? To address this major question, scientists from the Department of Astronomy at the University of Geneva decided to focus on a special class of exoplanets: exo-Neptunes, planets with masses around 20 times that of Earth outside our solar system.
Over the past decade, scientists have made major discoveries about the distribution of exoplanets. Exo-Neptunes are generally absent very close to their host stars. However, recent studies involving UNIGE show that in slightly more distant regions—known as the “tropical savanna,” a milder climatic zone in the exoplanet distribution—these planets are more common. Finally, between this savanna and the desert lies a region called the “Neptune ridge,” where exo-Neptunes are even more abundant than in the other two zones.
“The complexity of the exo-Neptune landscape provides a unique window into the processes of planetary system formation and evolution. This inspired us to launch the ambitious ATREIDES scientific collaboration project, which is primarily based on a large-scale observing program using ESPRESSO—the world's most precise spectrograph—on the Very Large Telescope (VLT) of the European Southern Observatory (ESO), the largest telescope in Europe,” explains Vincent Bourrier, Senior Lecturer and Researcher in the Department of Astronomy at the Faculty of Science of UNIGE, principal investigator of the ATREIDES project, and lead author of the first study from the consortium.
Conquering the “Desert”
The ATREIDES project focuses on studying exo-Neptunes to investigate the formation processes of the Neptune ridge, savanna, and desert, and to gain more general information about planet formation and evolution. Scientists plan to observe a large number of Neptunes with ESPRESSO and analyze and model all planetary data within a consistent and coherent framework. This systematic approach should enable genuine comparisons between different planetary systems and a better understanding of the mechanisms shaping the complex landscape of Neptunes.
The ATREIDES collaboration is an open international community initiative that invites all interested astronomers to join the scientific effort, following the example of the University of Warwick.
“We are using the NGTS telescope (a transit-based exoplanet survey) to observe transits of these Neptunes, thereby optimizing our use of ESPRESSO/VLT. This allows us to obtain more precise measurements and identify processes that could affect ESPRESSO data, such as stellar flares,” says Daniel Bayliss, Associate Professor in the Department of Physics at the University of Warwick.
TOI-421: A “Misaligned” Orbital Architecture
The first system observed and analyzed by the ATREIDES project is TOI-421. It contains two planets: a hot Neptune, TOI-421 c, located in the savanna, and a smaller planet, TOI-421 b, closer to the star. Astronomers have been able to trace the chaotic history of this system.
One of the hypotheses of the ATREIDES program is that the landscape of Neptunes is shaped by the way these planets migrated from their birthplaces to their current orbits. Some planets migrate slowly through the gas disk early in their formation, which should produce aligned orbits. Others are violently pushed onto their orbits much later, requiring a chaotic process known as “high-eccentricity migration,” resulting in highly misaligned orbits.
A key variable in this hypothesis is the alignment between the stellar equatorial plane and the orbital plane of each planet. By measuring this alignment in TOI-421, the scientists demonstrated that the two planets in the system exhibit severe misalignment—very different from our solar system, where planets are neatly arranged and orbit almost exactly in the Sun’s equatorial plane. This indicates that the TOI-421 system experienced a turbulent evolutionary phase after formation.
The analysis of TOI-421 is just a preview of future developments. It not only provides scientists with valuable information but, more importantly, helps refine the analysis and modeling tools developed within the ATREIDES collaboration. However, before we can sketch the evolution and formation of planetary systems, many more planetary systems containing exo-Neptunes must be observed and analyzed with the same rigor.
“Fully understanding the formation mechanisms of the Neptune desert, savanna, and ridge will help us better comprehend the formation of all planets… but one thing is certain: the universe still has other surprises in store for us, which will force us to develop new theories,” Bourrier concludes.
