Astronomers Discover New Feature in Exoplanet Distribution Between Neptunian Desert and Savanna

Astronomers have discovered the “Neptunian Ridge,” a newly identified feature in the distribution of exoplanets. The discovery, led by an international team including members from the University of Geneva, NCCR PlanetS, and the Centro de Astrobiología (CAB), highlights the complex dynamics within the Neptunian Desert, a region where hot Neptunes are rare, and the Neptunian Savanna, where these planets are more common.

By understanding these critical areas, researchers gain valuable insights into the dynamic processes that influence the formation and evolution of nearby exoplanets. The discovery of the Neptunian Ridge is published in the journal Astronomy and astrophysics.

To visualize the extent of exoplanetary systems, researchers often plot the distribution of planets based on their radius and orbital period. This reveals patterns and regions of exoplanets with similar properties that astronomers are trying to understand.

One of the most puzzling regions is the “Neptunian Desert,” where Neptune-sized planets are surprisingly rare. This dearth of Neptunes in the distribution of planets orbiting close to their host stars is thought to be the result of intense stellar radiation stripping away the planets’ atmospheres, eroding them, and transforming them into smaller planets.

Beyond this harsh desert lies the “Neptunian savanna,” a less severe region where Neptune-sized planets are more commonly found. Conditions here allow these planets to retain their gaseous envelopes, resulting in a region more populated with Neptunes that have migrated to orbits where they can survive the star’s radiation. Understanding the shape of the Neptunian desert and savanna has become a key question in exoplanet research.

Presentation of the Neptunian Ridge

The new study focuses on the transition between Neptune’s desert and savanna. Astronomers discovered an unexpected concentration of Neptune-sized planets at the edge of Neptune’s desert, a feature now called the “Neptune Ridge.”

“We found an overdensity of planets in this region, indicating a clear transition between the arid Neptunian desert and the more populated Neptunian savannah,” explains Dr. Vincent Bourrier, assistant professor in the Department of Astronomy of the Faculty of Science at UNIGE and co-author of the study. This newly identified ridge marks a critical area where planets have managed to migrate inward while resisting the intense radiation close to their star.

Discovering the ridge: methods and tools

The discovery was made possible by analyzing data from NASA’s Kepler mission, corrected for observational bias using advanced statistical techniques. The researchers meticulously mapped the period-radius space of these exoplanets, revealing distinct regions that define the Neptunian landscape.

The team’s analysis identified the Neptunian Ridge at orbital periods between 3.2 and 5.7 days, nestled between the Neptunian desert and 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 savannah. It provides us with a key to understanding the physical mechanisms that shape the desert,” explains Vincent Bourrier. Most Neptunes can be distributed over the savannah and desert at the beginning of their life by migrating within the disk in which they formed.

The existence of the ridge suggests that some Neptune-sized planets are brought to this region by a type of migration called high-eccentricity migration, which occurs later in their lives and allows them to survive the erosion of stars.

These migration processes, coupled with photoevaporation, likely shape the distinct features observed in the Neptunian landscape. The similarities between the Neptunian Ridge and another feature of exoplanet distribution, Jupiter’s hot stack, suggest that similar evolutionary processes may influence both groups of planets.

An ambitious observation program

To further unravel the mysteries of the Neptunian desert and savannah, a team of researchers led by UNIGE has set up a large-scale observation programme using the high-resolution ESPRESSO spectrograph mounted on ESO’s Very Large Telescope.

This program aims to perform a complete census of the orientation of planetary orbits within a sample of nearby Neptunes. This orientation depends on the migration process and will thus provide crucial data on the formation and evolution of these planets, providing essential clues to understand the particularities of Neptune’s distribution.

“The Neptunian Ridge is just the beginning,” concludes Amadeo Castro-González, a PhD student at the Madrid Astrobiology Center and first author of the study. “With the upcoming results of this observational program, we will be able to test our hypotheses about the origins and evolution of these fascinating worlds, providing a more complete view of the nearby Neptunian landscape.”