Astronomers discover new feature in distribution of exoplanets between Neptune’s deserts and savannas

Voyager 2 took this photo of Neptune in 1989. Credit: NASA
Astronomers have discovered a new feature in the distribution of exoplanets: the “Neptunian Ridge.” The discovery, led by an international team including members from the University of Geneva, NCCR PlanetS, and the Centre for Astrobiology (CAB), highlights the complex dynamics within the Neptunian desert, where hot planets are almost entirely absent, and the Neptunian savanna, where these planets are more commonly found.
Understanding these key regions can provide researchers with valuable insight into the dynamic processes that influence the formation and evolution of nearby exoplanets. The discovery of Neptune’s ridges is published in the Journal of Astronomy and Astrophysics.
To visualize the vast range of exoplanetary systems, researchers often plot the distribution of planets as a function of radius and orbital period, which can reveal patterns and regions of exoplanets with similar properties that astronomers are trying to understand.
One of the most puzzling regions is the “Neptune desert,” which has a surprisingly low number of Neptune-sized planets in the distribution of planets orbiting close to the star. It is thought that this is because powerful stellar radiation strips the planets of their atmospheres and erodes them into smaller planets.
Beyond this harsh desert lies the “Neptune savannah”, a less harsh region where Neptune-sized planets are more commonly found. This region has better conditions for these planets to maintain their gaseous outer layer, and as a result is home to a greater number of Neptunian planets that have migrated into orbits where they can withstand the star’s radiation. Understanding how Neptune’s deserts and savannahs formed is a key question in exoplanet research.
Introducing Neptune’s Ridges
The new study focuses on the transition between Neptune’s deserts and savannas: the astronomers discovered an unexpected concentration of Neptune-sized planets at the edge of the Neptune desert, a feature they now call “Neptunian Ridge.”
“We find a high density of planets in this region, which marks an abrupt transition between the barren Neptune deserts and the more populous Neptune savannahs,” explains study co-author Dr Vincent Brier, Assistant Professor in the Department of Astronomy at the University of Geneva’s Faculty of Sciences. This newly identified ridge marks a key area where planets could migrate inwards while resisting the intense radiation close to the star.
Revealing ridges: methods and tools
The discovery was made possible by analyzing data from NASA’s Kepler mission and using advanced statistical methods to correct for observational biases. The researchers meticulously mapped the periodic radius space of these exoplanets, revealing distinct regions that define Neptune’s topography.
The team’s analysis determined that Neptune’s ocean ridges lie between the Neptune Desert and the Neptune Savannah, with orbital periods ranging from 3.2 to 5.7 days. This comprehensive map highlights the complex processes involved in the movement and survival of these planets so close to their stars.
Implications for planetary formation and evolution
“Neptune’s ridges rise above deserts and savannas, which give us the key to understanding the physical mechanisms that form deserts,” says Vincent Brier. Most of Neptune could have been distributed over savannas and deserts early in its life by moving through the disk in which it formed.
The presence of this ridge suggests that some of the Neptune-sized planets were delivered to this region by a type of migration called hypereccentric migration, which occurs later in the planet’s life and allows it to survive erosion from its star.
These migration processes, combined with photoevaporation, may be responsible for the distinctive features seen in Neptune’s topography. The similarity between Neptune’s ridges and another feature in the distribution of exoplanets, hot Jupiter pile-ups, suggests that similar evolutionary processes may be influencing both planetary groups.
An ambitious observation program
To further unravel the mysteries of Neptune’s deserts and savannahs, a research team led by UNIGE has secured a major observing programme with the high-resolution spectrograph ESPRESSO on ESO’s Very Large Telescope.
The aim of the program is to perform a comprehensive study of planetary orbital orientations within a sample of near-Neptunian planets, which, as they depend on migration processes, will provide important data on the formation and evolution of these planets and provide important clues for understanding the peculiarities of the Neptune distribution.
“Neptune’s mid-ocean ridges are just the beginning,” concludes Amadeo Castro González, a doctoral student at the Astrobiology Center in Madrid and lead author of the study. “Future results from this observing program will allow us to test hypotheses about the origin and evolution of these intriguing worlds and provide a more comprehensive view of the topography of Neptune’s neighborhood.”
Further information: A. Castro-González et al., “Mapping outer Neptune terrain: Ridges between deserts and savannas,” Astronomy & Astrophysics (2024). DOI: 10.1051/0004-6361/202450957
Provided by University of Geneva
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