With over 5,500 exoplanets discovered and counting, exoplanets stretch far and wide across the universe. But how are these planets arranged in their own star systems? And what can that tell us about our own solar system? In today’s astrobito, we focus on a curious pattern found in many of these extra solar systems—the lack of Neptune-sized planets orbiting close to their stars. Why does this gap exist and what are its boundaries? Let's explore!
Paper details:
Title: Mapping the exo-Neptunian landscape. A ridge between the desert and savanna
Authors: A. Castro Gonzalez, V. Bourrier, J. Lillo-Box, J. B. Delise, D. J. Armstrong, D. Barrado, A.C. Correia
First-author Institution: Centro de Astrobiología, CSIC-INTA, Madrid, España
Status: Published in Astronomy & Astrophysics
One of the most exciting pursuits in modern astronomy is understanding how planets form and evolve. In particular, exoplanet research has focused on studying planets that orbit their stars closely—within 30 days or less. Among these so-called "close-in" planets, astronomers have discovered an intriguing pattern: a mysterious absence of Neptune-sized planets in orbits near their host stars, a phenomenon known as the "Neptunian desert." This discovery poses a tantalising question: why are Neptunian planets scarce in these regions, and what processes shape their existence?
Two processes, one desert
The main suspect behind this puzzle is atmospheric escape. Many exoplanets, especially Neptunian-sized worlds, are thought to lose their atmospheres when they orbit too close to their stars. As the intense stellar radiation heats the planet's atmosphere, it gradually escapes into space, eroding the planet's gaseous envelope. This process is believed to shrink Neptune-like planets into smaller, rocky worlds over time, leading to the scarcity of Neptune-sized planets near their stars (Figure 1). But atmospheric escape isn't the only player in this story. Planet migration—the movement of planets from their original formation spots—also plays a significant role. The core accretion theory of planet formation predicts that large planets like gas giants (e.g., Jupiter-like planets) form far from their stars, where cooler temperatures allow gases to condense. However, after forming, planets can migrate inward because of strong planet-star interactions that change the orbit of the planet, making it smaller and more circular overtime (Figure 1). This process is known as high-eccentricity tidal migration (HEM).
Figure 1. Schematic representation of atmospheric escape and high-eccentricity tidal migration. Both are thought to play a major role in shaping the Neptunian desert.
HEM is thought to be a key driver for bringing Jupiter-sized planets close to their stars. Yet, tracing the migration history of Neptune-sized planets has proven more challenging. Understanding the migration and evolution of these planets requires more than just theory—it demands extensive observation.
Exoplanets can be categorised into three main groups based on their sizes: small planets (sub-Neptunes), Neptune-sized planets, and gas giants (Jupiter-sized planets). By studying the occurrence of these planets across various orbital distances, using large-scale astronomy surveys like Kepler, researchers have uncovered some intriguing trends. For Neptune-sized planets, the "desert" lies at periods shorter than 3.2 days, followed by a steep increase in their occurrence beyond 3.2 days. This sharp transition marks the boundary between the Neptunian desert and savanna (a region where Neptune-sized planets are more abundant) and even reveals a new feature: the "Neptunian ridge," where Neptune-sized planets are slightly more common (Figure 2).
Figure 2. exo-Neptunes distribution across multiple orbital periods
Interestingly, a similar pattern exists for gas giants. Gas giants exhibit what is known as the "hot Jupiter pileup," where many Jupiter-sized planets are found in close orbits. However, while the hot Jupiter pileup is a prominent feature, it doesn't drop off as sharply into a desert like the Neptunian ridge does (Figure 3). In contrast, smaller sub-Neptune planets are distributed more evenly across different orbital periods and do not exhibit the same overdensities. This difference highlights the unique evolutionary paths of these different planetary populations.
Figure 3. Planet radius as a function of orbital period for all exoplanets discovered to date.
The existence of the Neptunian ridge and savanna offers clues about the forces shaping planetary orbits. Migration processes, particularly HEM, are thought to bring Neptune-sized planets closer to their stars, where they enter the ridge. These planets, newly arrived in the ridge, may still have moderately oval orbits, suggesting that their migration is recent. Meanwhile, gas giants like hot Jupiters, which are more resistant to atmospheric erosion, often end up in circular orbits due to tidal forces over time.
For Neptunes, however, the story is more complex. Many of the Neptune-sized planets observed in the ridge are still undergoing significant atmospheric escape. This means that while some Neptunes survive their migration, others may lose their atmospheres and transform into smaller, rocky planets, which contributes to the desert-like scarcity of Neptune-sized planets near their stars.
As researchers continue to observe more exoplanets and refine their models, the boundaries of the Neptunian desert and savanna will become clearer. The Neptunian landscape, with its deserts, ridges, and savannas, holds key insights into the nature of planetary systems across the universe. Understanding how planets evolve in these extreme environments will not only deepen our knowledge of exoplanets but also provide new clues about the history and future of our own solar system.