An international team, including UNIGE, has unveiled the "Neptunian ridge," an overabundance of Neptunes nestled between the Neptunian desert and the Neptunian savannah. Explanations.
Astronomers have uncovered the "Neptunian ridge," a newly identified feature in the distribution of exoplanets.
This discovery, led by an international team including members from the University of Geneva (UNIGE), the PRN PlanetS, and the Centro de Astrobiología (CAB), highlights the complex dynamics between the Neptunian desert, a region where hot Neptunes are rare, and the Neptunian savannah, where these planets are more frequent. By understanding these critical zones, astronomers are gaining valuable insights into the dynamic processes that influence the formation and evolution of exoplanets close to their star. These findings are available in the journal Astronomy & Astrophysics.
To visualize the wide diversity of exoplanetary systems, scientists often plot the distribution of planets based on their radius and orbital period. This helps to highlight regions where exoplanets exhibit similar properties, which astronomers then attempt to understand. One of the most puzzling regions is the "Neptunian desert," where Neptune-sized planets are surprisingly rare.
It is thought that this scarcity of Neptunes in the distribution of planets orbiting near their star is due to intense stellar radiation, which strips away the planets' atmospheres, eroding them and turning them into smaller planets.
Beyond this barren desert lies the "Neptunian savannah," a less intense region—meaning cooler, specifically—where Neptune-sized planets are more commonly found. In this zone, conditions allow them to retain their gaseous envelopes, resulting in a region more densely populated with Neptunes. Understanding the formation of the Neptunian desert and savannah is a key question in exoplanet research.
Revealing the ridge: a meticulous analysis
The new study focuses on the transition between the Neptunian desert and the savannah. Astronomers have discovered a concentration of Neptunes at the edge of the desert, an unexpected feature now called the "Neptunian ridge."
"We found an overabundance of planets in this region, indicating a clear transition between the empty Neptunian desert and the more populated Neptunian savannah," explains Vincent Bourrier, assistant professor in the Department of Astronomy at the Faculty of Science of UNIGE and co-author of the study. This newly identified ridge marks a critical zone where planets succeed in migrating close to their stars while resisting their intense radiation.
This discovery was made possible by analyzing data from NASA's Kepler mission, corrected for observational biases using advanced statistical techniques. The scientists meticulously mapped the period-radius distribution of these exoplanets, revealing the distinct regions that define the Neptunian landscape. The team's analysis enabled them to identify the Neptunian ridge at orbital periods between 3.2 and 5.7 days, nestled between the Neptunian desert and the Neptunian savannah.
This comprehensive mapping highlights the complex processes involved in the migration and survival of these planets near their stars.
Implications for planet formation and evolution
"The Neptunian ridge dominates the desert and the savannah. It provides us with a key to understanding the physical mechanisms that shape the desert," emphasizes Vincent Bourrier. Most Neptunes can be distributed across the savannah and desert early in their life, migrating inside the disk in which they formed. The existence of the ridge suggests that some Neptune-sized planets are brought to this region through high-eccentricity migration, which occurs later in their life and allows them to survive erosion by their stars.
These migration processes, coupled with photo-evaporation, are likely responsible for the distinct features observed in the Neptunian landscape. The similarities between the Neptunian ridge and another characteristic in the distribution of exoplanets, the hot Jupiter cluster, suggest that similar evolutionary processes may influence both groups of planets.
An ambitious observation program
To better unravel the mysteries of the desert and savannah, the team led by UNIGE has launched a large-scale observation program using the high-resolution ESPRESSO spectrograph, mounted on ESO's Very Large Telescope. This program aims to conduct a comprehensive survey of the orbital orientations of planets within a sample of hot Neptunes. This orientation depends on the migration process and will therefore provide critical data about their formation and evolution, offering key clues to understanding the unique distribution of Neptunes.
"The Neptunian ridge is just the beginning," concludes Amadeo Castro-González, a PhD student at the Centro de Astrobiología (CAB) in Madrid, visiting UNIGE and lead author of the study. "Through the upcoming results from this observation program, we will be able to test our hypotheses about the origin and evolution of these intriguing worlds and gain a more complete view of the landscape of hot Neptunes."