Photoevaporation and High-Eccentricity Migration Created the Sub-Jovian Desert (1807.00012v1)

Abstract: The mass-period or radius-period distribution of close-in exoplanets shows a paucity of intermediate mass/size (sub-Jovian) planets with periods ~< 3 days. We show that this sub-Jovian desert can be explained by the photoevaporation of highly irradiated sub-Neptunes and the tidal disruption barrier for gas giants undergoing high-eccentricity migration. The distinctive triangular shape of the sub-Jovain desert result from the fact that photoevaporation is more effective closer to the host star, and that in order for a gas giant to tidally circularise closer to the star without tidal disruption it needs to be more massive. Our work indicates that super-Earths/mini-Neptunes and hot-Jupiters had distinctly separate formation channels and arrived at their present locations at different times.

An Analysis of the Genesis of the Sub-Jovian Desert via Photoevaporation and High-Eccentricity Migration

In the paper "Photoevaporation and High-Eccentricity Migration Created the Sub-Jovian Desert," James E. Owen and Dong Lai investigate the observed gap of sub-Jovian exoplanets with orbital periods of less than approximately three days—an area in parameter space referred to as the "sub-Jovian desert." They propose two primary mechanisms responsible for this phenomenon: photoevaporation and high-eccentricity migration.

Identification of the Sub-Jovian Desert

The authors confirm the existence of a dearth of intermediate-mass planets, spanning from super-Earths to sub-Jovian-sized planets, in the mass-period and radius-period distributions of close-in exoplanets. Historically identified by studies such as Szabo & Kiss (2011) and Mazeh et al. (2016), this feature reveals a triangular shape in which a lack of sub-Jovian planets is most prominent in the closest orbits to their stars.

Role of Photoevaporation

The paper explores the effects of photoevaporation, a phenomenon where intense stellar radiation strips lighter elements such as hydrogen and helium from an exoplanet's atmosphere. The authors present a modeled framework which indicates that this process can effectively explain the lower boundary of the sub-Jovian desert. Through numerical models, the results suggest that the maximum core mass of these planets, necessary to remain stable against total atmospheric loss due to photoevaporation, is slightly over 10 Earth masses. This aligns with empirical data showing these planets are, indeed, rare at very close distances to their host stars.

Evaluation of High-Eccentricity Migration

The upper boundary of the sub-Jovian desert, where more massive planets are absent at very short periods, cannot be adequately explained by photoevaporation alone. Instead, the authors consider high-eccentricity migration, a dynamic process whereby a planet is pushed into a highly eccentric orbit and then circularized to a short-period orbit via tidal interactions with its star. The condition requires a planet’s final circularized orbit to be greater than twice its tidal radius to avoid disruption. Using constraints derived from this model, the analysis suggests that the upper boundary concurs with the presence of more massive planets (those larger than sub-Jovian size) at these periods, except for a few outliers like WASP-52b, implying high-eccentricity migration as a viable mechanism.

Tidal Stripping Effects

In addressing the role of tidal interactions, Owen and Lai factor in the evolutionary effect of stellar tides. They estimate the stellar tidal quality factor $Q_*' \sim 10^7$, suggesting that planetary orbits may evolve inward after initial circularization. This post-synchronization decay especially affects planets heavier than approximately one Jupiter mass, aligning the theoretical predictions of their positions with observational data.

Implications and Conclusions

The findings of Owen and Lai indicate a bifurcated formation history for close-in planets dependent on their mass. Low-mass exoplanets likely formed nearer their current positions or migrated early, experiencing atmospheric erosion via photoevaporation. Conversely, the gap, or desert, at higher masses implies that these planets formed further afield and later migrated via mechanisms like high-eccentricity migration. The research posits that in-situ formation for massive planets is unlikely, while high-eccentricity migration offers a coherent explanation for the positioning of hot Jupiters, contingent on long-term tidal influences.

Future Considerations

While the paper advances a structured narrative regarding the formation and evolution scenarios of close-in exoplanets, further observational data are required to refine these models, particularly in constraining the dynamical histories and assessing any potential biases in current exoplanet catalogs. Comparative studies on planet incidence rates against stellar metallicity and age could elucidate the evolution timings and pathways more thoroughly.

This paper offers significant insights into the complex interplay of planetary atmospheres and orbital dynamics, using theoretical models consistent with observed data to sketch a plausible narrative explaining the sub-Jovian desert within the larger context of planetary formation and migration theories.