Testing the Origin of Hot Jupiters with Atmospheric Surveys

TL;DR

This work tackles the unresolved origin of hot Jupiters by proposing population-level atmospheric signatures, focusing on post-formation pollution from solid accretion and disk-driven migration. It develops a theoretical framework for three pollution pathways, identifies a critical dependence on disk metallicity and pebble properties, and predicts how atmospheric metallicity and especially water abundance should vary with orbital period. Using simulated Ariel Tier 2 capabilities, the study shows that only pebble accretion in metal-rich disks can yield observable supersolar metallicities, and that hot Jupiters formed by high-eccentricity migration should be measurably more water-rich than warm Jupiters, with detections at roughly 3–4σ under plausible target samples. The findings guide Ariel observing strategies and propose cross-population comparisons with cold Jupiters to constrain disk substructure and dust-trap efficacy, offering a concrete path to adjudicate hot Jupiter formation scenarios.

Abstract

In spite of their long detection history, the origin of hot Jupiters remains to be resolved. While multiple dynamical evidence suggests high-eccentricity migration is most likely, conflicts remain when considering hot Jupiters as a population in the context of warm and cold Jupiters. Here, we turn to atmospheric signatures as an alternative mean to test the origin theory of hot Jupiters, focusing on population level trends that arise from post-formation pollution, motivated by the upcoming Ariel space mission whose goal is to deliver a uniform sample of exoplanet atmospheric constraints. We experiment with post-formation pollution by planetesimal accretion, pebble accretion, and disk-induced migration and find that an observable signature of post-formation pollution is only possible under pebble accretion in metal-heavy disks. If most hot Jupiters arrive at their present orbit by high-eccentricity migration while warm Jupiters emerge largely in situ, we expect the atmospheric water abundance of hot Jupiters to be significantly elevated compared to warm Jupiters. We report on the detectability of such signatures and further provide suggestions for future comparative atmospheric characterization between hot Jupiters and wide-orbit directly imaged planets to elucidate the properties of the dust substructures in protoplanetary disks.

Figures (9)

• Figure 1: Cumulative distribution function of solid disk densities at 1 AU ($\Sigma_0$) calculated from protoplanetary disks in PPVII around stars of mass $\geq$1$M_\odot$.
• Figure 2: Predicted trend of metallicity with orbital period under local planetesimal accretion. The blue lines corresponds to highest $\Sigma_0$ (99th percentile) while the red lines illustrate $\Sigma_0$ of Dai2020. Solid lines show 1 Myr evolution while dotted lines show 10 Myr evolution. The gray lines signify the total available mass budget within the feeding zone.
• Figure 3: Same as Figure \ref{['fig:planetesimal-results']} but for pebble accretion. The top panel corresponds to no dust trapping while the bottom panel corresponds to perfect dust traps (i.e., local accretion) where different colors correspond to varying Stokes number.
• Figure 4: Predicted atmospheric metallicity under pollution by disk-induced migration with respect to the final orbital period of planets. Blue lines correspond to massive disks with $\Sigma_0$ at the 99th percentile while red lines correspond to that of typical MMEN Dai2020. Varying levels of transparency correspond to $f_{\rm dep}$ with 1 being no gas depletion. Planets are halted either after 10 Myr or once they reach 10 days which we set to be the inner edge of the protoplanetary disk. Even the highest metallicity falls below the assumed stellar value 0.02 by roughly two orders of magnitude, much more than the order unity numerical coefficient we ignored in the torque calculation.
• Figure 5: Predicted trend of total water abundance (number ratio) vs. orbital period. The blue line corresponds to pebble accretion with perfect dust trap with Stokes number = 0.01 and the highest $\Sigma_0$ whereas the yellow line corresponds to pebble accretion with no dust trap at the highest $\Sigma_0$. In both cases, a discontinuity appears at our adopted iceline since inside the iceline, solid pollution does not bring additional water and the total atmospheric water content is that of stellar value.