Exoplanets like Neptune accumulate in a particular region close to their stars

An international team of astronomers has discovered the “Neptunian ridge”, a new structure in the distribution of exoplanets. This finding highlights the complex processes that occur in the “Neptunian desert” (a region with a shortage of exoplanets similar to Neptune close to their stars) and the “Neptunian savannah” (a more distant region in which these planets are found more frequently). frequency). This research provides valuable information on the dynamical and atmospheric processes that dominate the evolution of Neptunian exoplanets in close orbits.

The study has been led by the Center for Astrobiology (CAB), a joint entity of the Higher Council for Scientific Research (CSIC) and the National Institute of Aerospace Technology (INTA), in Spain all of these institutions. The universities of Geneva in Switzerland, Warwick in the United Kingdom, Coimbra in Portugal and Paris in France have also collaborated.

To study the extensive population of exoplanetary systems, researchers analyze the distribution of known planets based on, for example, 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,” an almost complete absence of Neptune-sized planets in orbits close to other stars. This shortage of hot exoneptunes is believed to be the result of intense stellar radiation, which would erode their atmospheres to the point of completely eliminating them, turning these planets into iron spheres and planetary-sized rocks. Beyond this inhospitable desert lies the “Neptunian savanna,” a region located further away from the intense stellar radiation where Neptunian planets are most frequently found. In this region, environmental conditions are more favorable and allow the planets to maintain their original gaseous envelopes for many millions of years.

One of the most relevant questions in exoplanetary research is to discover how and when these exoneptunes reached the close orbits in which they are found today, since theories of planetary formation 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.

Therefore, understanding how the desert and savanna were populated has become a key question in exoplanetary research.

Neptune, photographed by the Voyager 2 space probe. (Photo: NASA JPL)

The exoneptunian mountain range

The new study focuses on the transition between the desert and the Neptune savannah. The authors of this work found an unexpected concentration of planets at the edge of the desert, which forms a sharp dividing line between both regimes, a feature they have called the “Neptunian mountain range.”

“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 greater than that of finding it at shorter distances (in the desert), and about 3 times greater than that of finding it at longer distances (in the savannah), which “which suggests that these planets have been subject to specific processes that led them to this very particular orbital region,” explains Amadeo Castro-González, INTA predoctoral fellow at the Center for Astrobiology (CAB), INTA-CSIC, and lead author of this study.

Discovering the Cordillera: Methods and tools

The discovery was made possible thanks to the analysis of 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, revealing distinct regions that define the new Neptunian landscape. This comprehensive mapping shows the complex processes involved in the migration and atmospheric evaporation of these planets.

“The quality and systematicity of the observations of the Kepler mission have been decisive in being able to carry out this study, as well as the efforts to monitor its planets from the ground, for which the Astrobiology Center has dedicated significant efforts in recent years,” comments Jorge Lillo-Box, co-author of the study.

Implications for planetary formation and evolution

The authors of the study have interpreted the results in the context of theories of planetary formation and evolution and conclude that the accumulation of planets in the Neptunian mountain range could be interpreted through the existence of two migration mechanisms that would be populating the mountain range and the savanna in a manner different.

“Current observational evidence suggests that a substantial fraction of the planets in the mountain range could have arrived from their birthplace through a mechanism called high-eccentricity tidal migration, which is capable of bringing planets closer to their stars at any stage. of his life. “On the contrary, the planets in the savanna could have been brought mainly through another type of migration, called disk-driven migration, which occurs just after the formation of the planets,” explains Vincent Bourrier, a researcher at the University of Geneva and co-author of the study.

“These migration processes, together with the evaporation of planetary atmospheres, probably shape the different features observed in the Neptunian landscape,” concludes Vincent Bourrier.

Ambitious Observation Programs

To further unravel the mysteries of the Neptunian landscape, different observation programs are being carried out. The HARPS-NOMADS collaboration, an observing program led by David J. Armstrong—a researcher at the University of Warwick and also a co-author of this study—is using the European Southern Observatory’s (ESO) HARPS high-resolution spectrophotographer to search for new planets in the desert, the mountain range and the Neptune savannah with the final objective of carrying out statistical studies of their properties. Some of these new planets will be monitored with the ESPRESSO high-resolution spectrograph, also from ESO, in the context of the ATREIDES collaboration, led by Vincent Bourrier. This program aims to carry out an exhaustive census of the orientation of the planets’ orbits, which depend on the migration process, and will therefore provide relevant information on the formation and evolution of the Neptunian planets as a whole.

“These observation programs will be completed in the future by new massive searches for planetary systems, such as those carried out by the PLATO space telescope of the European Space Agency, and in which Spain and the CAB have invested a large amount of resources,” explains David Barrado, also co-author of the study.

“The discovery of the Neptunian mountain range represents a paradigm shift in the understanding of one of the most relevant questions in current exoplanetary exploration,” comments Jorge Lillo-Box; and “opens a new door to our understanding of the origin and formation of planets like our Neptune,” concludes Amadeo Castro-González.

The study is titled “Mapping the exo-Neptunian landscape.” And it has been published in the academic journal Astronomy & Astrophysics. (Source: CAB)