Dearth of short-period Neptunian exoplanets - a desert in period-mass and period-radius planes (1602.07843v2)

Abstract: A few studies have reported a significant dearth of exoplanets with Neptune mass and radius with orbital periods below $2$--$4$ d. This cannot be explained by observational biases because many Neptunian planets with longer orbital periods have been detected. The existence of this desert is similar to the appearance of the so-called brown-dwarf desert that suggests different formation mechanisms of planets and stellar companions with short orbital periods. Similarly, the Neptunian desert might indicate different mechanisms of formation and evolution for hot Jupiters and short-period super-Earths. We here follow a previous study and examine the location and shape of the desert in both the period-mass and period-radius planes, using the currently available large samples of planets. The desert in the period-mass plane has a relatively sharp upper edge, with a planetary mass that is inversely proportional to the planetary orbital period, while the lower, somewhat blurred, boundary is located along masses that are apparently linearly proportional to the period. The desert in the period-radius plane of the transiting planets is less clear. It seems as if the radius along the upper boundary is inversely proportional to the period to the power of one-third, while the lower boundary shows a radius that is proportional to the period to the power of two-thirds. The combination of the two upper bounds of the desert, in the period-mass and period-radius planes, yields a planetary mass-radius relation of $R_{\rm p}/R_{\rm Jup}\simeq (1.2\pm0.3)(M_{\rm p}/M_{\rm Jup})^{0.27\pm0.11}\,$ for $ 0.1\lesssim M_{\rm p}/M_{\rm Jup}\lesssim 1$. The derived shape of the desert, which might extend up to periods of $5$--$10$ d, could shed some light on the formation and evolution of close-in planets.

• The paper reveals a distinct deficit of Neptune-like exoplanets with orbital periods below 2-4 days, confirming the existence of a Neptunian desert.
• It employs both radial velocity and transit data to construct period-mass and period-radius diagrams, effectively reducing observational biases.
• The analysis defines clear mass and radius thresholds, offering insights into planetary formation and migration mechanisms.

Analysis of the Short-Period Neptunian Exoplanet Desert

The paper "Dearth of short-period Neptunian exoplanets—a desert in period-mass and period-radius planes" by Mazeh et al. presents a detailed examination of the apparent scarcity of Neptune-like exoplanets with short orbital periods. Leveraging the data from existing planetary surveys, the authors investigate the contours of this "Neptunian desert" within the period-mass and period-radius planes. This research spans the utilization of different detection techniques, namely radial velocity (RV) and transit surveys, to ensure a comprehensive analysis that mitigates observational biases.

Core Findings

The paper identifies a significant deficiency of Neptune-mass and radius planets with orbital periods below 2-4 days, a fact that remains consistent despite the improved sensitivity and breadth of exoplanet detections in these surveys. This scarcity is not mirrored in planets with longer orbital periods, indicating that observational biases are not responsible for the desert, unlike with some other phenomena in exoplanet demographics.

The analysis breaks down into two comparative planes:

• Period-Mass Plane: The investigation reveals a well-defined upper boundary in which planetary mass inversely correlates with orbital period—a pattern that suggests a distinct mass threshold beyond which short-period planets are rarely observed.
• Period-Radius Plane: The boundaries are less precise, yet they imply a radius that scales with the period with exponents of one-third and two-thirds for the upper and lower boundaries, respectively.

The paper quantifies these relationships with a derived mass–radius correlation, expressed as:

$$R_{\rm p}/R_{\rm Jup} \approx (1.2 \pm 0.3)(M_{\rm p}/M_{\rm Jup})^{0.27 \pm 0.11}$$

for planets within the mass range $0.1 \lesssim M_{\rm p}/M_{\rm Jup} \lesssim 1$. This result contributes to the understanding of planetary structure and composition, particularly in closer orbits.

Implications

The observation of this desert raises questions regarding the formation and evolutionary paths of these exoplanets. It mirrors the "brown dwarf desert," positing possibly different formation and migration mechanisms for hot Jovians and super-Earths. Potential interpretations could include:

• Migration Barriers: The "death line" hypothesis suggests a critical radius or mass threshold beyond which planets lose significant mass, inhibiting their proximity to the host star.
• In Situ Formation: Differences in accretion rates and available building material might hinder the formation of Neptune-class planets in very close orbits.
• Disk Interaction Cessation: Outward sloping boundaries might suggest a cessation of disk-driven migration due to density alterations or other dynamics in the protoplanetary disk.

Future Directions

These findings prompt further investigations into the mechanisms that govern these boundaries. High-precision observational campaigns focusing on intermediate-mass planets at varied orbital distances will be crucial. Additionally, simulations that accurately model the dissipative effects of close stellar proximity on planetary atmospheres and structures might elucidate the processes reinforcing this desert.

The paper adds to a growing body of evidence indicating that giant planet occurrence, composition, and architecture cannot be singly interpreted by existing models. It underscores the need for robust theories that incorporate multifaceted influences such as disk dynamics, planetary atmosphere evolution, and the impact of stellar radiation on nascent planetary systems. Future work could also explore potential correlations with stellar types and system metallicity, building an extensive matrix of conditions leading to planetary desertification.