Model atmospheres for massive gas giants with thick clouds: Application to the HR 8799 planets and predictions for future detections

TL;DR

This study develops advanced atmospheric models for massive gas giants with thick clouds, fitting data for HR 8799 planets, constraining their properties, and predicting IR colors for future exoplanet detections.

Contribution

Introduces a comprehensive suite of cloud-inclusive atmosphere models that better fit observed data and provide new constraints on exoplanet properties and future detection predictions.

Findings

Thick cloud models fit HR 8799 data significantly better.

Estimated masses range from 2 to 13 Jupiter masses.

Predicted IR colors distinguish exoplanets from brown dwarfs.

Abstract

We have generated an extensive new suite of massive giant planet atmosphere models and used it to obtain fits to photometric data for the planets HR 8799b, c, and d. We consider a wide range of cloudy and cloud-free models. The cloudy models incorporate different geometrical and optical thicknesses, modal particle sizes, and metallicities. For each planet and set of cloud parameters, we explore grids in gravity and effective temperature, with which we determine constraints on the planet's mass and age. Our new models yield statistically significant fits to the data, and conclusively confirm that the HR 8799 planets have much thicker clouds than those required to explain data for typical L and T dwarfs. Both models with 1) physically thick forsterite clouds and a 60-micron modal particle size and 2) clouds made of 1 micron-sized pure iron droplets and 1% supersaturation fit the data. Current data are insufficient to accurately constrain the microscopic cloud properties, such as composition and particle size. The range of best-estimated masses for HR 8799b, HR 8799c, and HR 8799d conservatively span 2-12 M_J, 6-13 M_J, and 3-11 M_J, respectively and imply coeval ages between ~10 and ~150 Myr, consistent with previously reported stellar age. The best-fit temperatures and gravities are slightly lower than values obtained by Currie et al. (2011) using even thicker cloud models. Finally, we use these models to predict the near-to-mid IR colors of soon-to-be imaged planets. Our models predict that planet-mass objects follow a locus in some near-to-mid IR color-magnitude diagrams that is clearly separable from the standard L/T dwarf locus for field brown dwarfs.