An international team has launched an ambitious program aimed at mapping exoplanets located around the "hot Neptune desert," a zone around the star where Neptune-type planets are largely underrepresented.
The goal: to better understand the mechanisms of evolution and formation of planetary systems. This collaboration, named ATREIDES, is delivering its first results with the observation of the planetary system TOI-421. Analysis of this system reveals a surprisingly inclined orbital architecture, offering new insight into the chaotic history of these distant worlds. This inaugural study is published in the journal
Astronomy & Astrophysics.
Artist's illustration. © Elsa Bersier - CFPArts / ESBDi Genève
What are the physical mechanisms that govern the formation and evolution of planetary systems? To answer this broad question, a group of scientists led by the Department of Astronomy at UNIGE decided to focus on a specific type of exoplanet: exo-Neptunes, planets outside our solar system about twenty times more massive than Earth.
Understanding the mechanisms that shape the hot Neptune desert, the savanna, and the ridge will help us better grasp planetary formation as a whole.
Over the last decade, scientists have made important discoveries regarding the distribution of exoplanets. They have observed that in regions very close to stars, exo-Neptunes are absent. In contrast, recent studies, in which UNIGE participated, show that in zones slightly farther from stars—a more temperate region in the exoplanet distribution called the "savanna"—this type of planet is more present. And between this savanna and the desert lies a region called the "Neptunian ridge," where they might even be overrepresented compared to the other two regions.
"The complexity of the exo-Neptunian landscape is a true window into the processes of formation and evolution of planetary systems. This is what motivated the ambitious scientific cooperation ATREIDES, which relies in particular on a large observation program we are conducting on the largest European telescopes—the ESO's VLT—with the world's most precise spectrograph, ESPRESSO," explains Vincent Bourrier, a teaching and research fellow in the Department of Astronomy at the Faculty of Sciences of UNIGE, principal investigator of the ATREIDES program, and first author of the inaugural study.
This chart places exoplanets according to their size and distance from their star. Each dot represents an exoplanet. Jupiter-sized planets (located at the top of the chart) and Earth-sized and super-Earth planets (at the bottom) are found both close to and far from their star. But Neptune-sized planets (in the middle), close to their star, are rare.
This so-called hot Neptune desert shows that such alien worlds are rare, or that they were abundant at one time but have since disappeared. Highlighted in red is GJ3470b, a hot Neptune at the edge of the desert. Conquering the "desert"
The ATREIDES program thus focuses on exo-Neptunes to identify the processes responsible for the ridge, the savanna, and the desert and to derive more general information about the formation and evolution of planets. Scientists plan, on one hand, to observe a large number of Neptunes with ESPRESSO and, on the other hand, to analyze and model data from all planets within a homogeneous and consistent framework. This systematic approach should enable a real comparison between different planetary systems and a better understanding of the mechanisms that shape this complex Neptunian landscape.
Designed as an open and international community initiative, the ATREIDES collaboration invites all interested astronomers to join this scientific effort, as exemplified by the University of Warwick. "We use the NGTS telescopes, an exoplanet observation program using the transit method, to observe the transit of these Neptunes and thus optimize our use of ESPRESSO/VLT. We can then obtain much more precise measurements or even identify processes, such as stellar flares, that could influence ESPRESSO data," 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 as part of ATREIDES is named TOI-421. It has two planets: a hot Neptune TOI-421 c located in the savanna and a smaller planet closer to the star, TOI-421 b. Astronomers were able to trace the chaotic history of this system.
One hypothesis of the ATREIDES program states that the Neptunian landscape was shaped by how these planets migrated from their birthplace to their current orbits. Some planets would migrate gently and early through the gas disk in which they formed, a process that should produce aligned orbits. Others would be violently thrust onto their orbits much later, through a chaotic process called "high-eccentricity migration," which results in strongly misaligned orbits.
One key variable in this hypothesis is therefore the alignment between the star's equatorial plane and the orbital plane of each planet. By measuring this alignment for TOI-421, scientists were able to show that the two planets in the system are strongly misaligned, which is very different from our solar system where the planets are aligned and thus orbit almost in the equatorial plane of our Sun. This points to a turbulent history during the evolution of the TOI-421 system after its formation.
The analysis of TOI-421 is only a preview of the harvest to come. It provides valuable information to scientists but also, and most importantly, helps refine the analysis and modeling tools developed within the ATREIDES collaboration. However, a large number of planetary systems hosting exo-Neptunes will need to be observed and analyzed with the same rigor before we can outline the broad principles enabling the understanding of the evolution and formation of planetary systems.
"Truly understanding the mechanisms that shape the hot Neptune desert, the savanna, and the ridge will allow us to better grasp planetary formation as a whole...but it's a safe bet that the Universe has other surprises in store for us, which will force us to develop new theories," concludes Vincent Bourrier.