Madrid – An international team of astronomers has discovered the ‘Neptunian Ridge’, a new structure in the distribution of planets outside the solar systema discovery that provides valuable information on the dynamics and atmospherics of exoplanets.
The study, which is a collaboration between the Spanish Centre for Astrobiology (CAB), INTA-CSIC and the universities of Geneva, Warwick, Coimbra and Paris, was published this Tuesday in Astronomy & Astrophysics.
To study the vast population of exoplanetary systems, the researchers analyzed the distribution of known planets based on parameters such as their radius and orbital period.
As the number of detections increases, this distribution reveals new patterns and peculiarities that astronomers are trying to understand and whose origin is closely related to the processes of planetary formation and evolution.
One of the most puzzling regions is the “Neptunian desert,” where there are almost no Neptune-sized planets in close orbits around other stars. This scarcity of hot exo-Neptunes is thought to be the result of intense stellar radiation, which would erode their atmospheres to the point of completely eliminating them, turning these planets into planetary-sized spheres of iron and rock.
Beyond this inhospitable desert lies the “Neptunian savannah,” a region far from the intense stellar radiation where Neptunian planets are most frequently found and where environmental conditions are more favorable, allowing the planets to maintain their original gaseous envelopes for millions of years.
One of the most relevant questions in exoplanetary research is to discover how and when these exo-Neptunes reached the close orbits in which they are found today, since planetary formation theories suggest that these giant planets formed at much greater distances, beyond the desert and savannah, in orbits similar to those of Jupiter and Saturn with respect to the Sun.
Understanding how the desert and savannah were populated has become a key question in exoplanetary research.
The exo-Neptunian mountain range
The new study focuses on the transition between the desert and savanna of Neptunes.
Researchers found an unexpected concentration of planets at the edge of the desert, which forms a sharp dividing line between the two regimes, a feature they have dubbed the “Neptunian Ridge.”
“We found that a large number of Neptunian planets orbit their stars with orbital periods between 3.2 and 5.7 days. We estimate that the probability of finding a planet in this region is about 8 times higher than that of finding it at shorter distances (in the desert), and about 3 times higher than that of finding it at longer distances (in the savannah), suggesting that these planets have been subject to specific processes that led them to this very particular orbital region,” explained Amadeo Castro-González, INTA predoctoral researcher at the CAB and lead author of the study.
The discovery was made possible by analyzing data from NASA’s Kepler space mission, corrected for observational biases using advanced statistical techniques.
The researchers meticulously mapped the relationship between radius and period of these exoplanets, an exhaustive mapping that showed the complex processes involved in the migration and atmospheric evaporation of these planets.
“Observational evidence suggests that a substantial fraction of the planets in the ridge may have arrived from their birthplaces through a mechanism called high-eccentricity tidal migration, which is capable of bringing planets closer to their stars at any stage of their life,” explained Vincent Bourrier of the University of Geneva and co-author of the study.
Instead, “the planets in the savannah might have been brought there primarily through another type of migration, called disk-driven migration, which occurs just after planet formation,” he added.
“These migration processes, together with the evaporation of planetary atmospheres, likely shape the distinct features observed in the Neptunian landscape,” Bourrier concluded.