A transition between the hot and the ultra-hot Jupiter atmospheres

Abstract

<jats:p>A key hypothesis in the field of exoplanet atmospheres is the trend of atmospheric thermal structure with planetary equilibrium temperature. We explore this trend and report here the first statistical detection of a transition in the near-infrared atmospheric emission between hot and ultra-hot Jupiters. We measure this transition using secondary eclipse observations and interpret this phenomenon as changes in atmospheric properties, and more specifically in terms of transition from non-inverted to inverted thermal profiles. We examine a sample of 78 hot Jupiters with secondary eclipse measurements at 3.6 and 4.5 <jats:italic>μ</jats:italic>m measured with <jats:italic>Spitzer</jats:italic> Infrared Array Camera. We calculate the planetary brightness temperatures using PHOENIX models to correct for the stellar flux. We measure the deviation of the data from the blackbody, which we define as the difference between the observed 4.5 <jats:italic>μ</jats:italic>m eclipse depth and that expected at this wavelength based on the brightness temperature measured at 3.6 <jats:italic>μ</jats:italic>m. We study how the deviation between 3.6 and 4.5 <jats:italic>μ</jats:italic>m changes with theoretical predictions with equilibrium temperature and incoming stellar irradiation. We reveal a clear transition in the observed emission spectra of the hot Jupiter population at 1660 ± 100 K in the zero albedo, full redistribution equilibrium temperature. We find the hotter exoplanets have even hotter daysides at 4.5 <jats:italic>μ</jats:italic>m compared to 3.6 <jats:italic>μ</jats:italic>m, which manifests as an exponential increase in the emitted power of the planets with stellar insolation. We propose that the measured transition is a result of seeing carbon monoxide in emission due to the formation of temperature inversions in the atmospheres of the hottest planets. These thermal inversions could be caused by the presence of atomic and molecular species with high opacities in the optical and/or the lack of cooling species. Our findings are in remarkable agreement with a new grid of 1D radiative and convective models varying metallicity, carbon to oxygen ratio (C/O), surface gravity, and stellar effective temperature. We find that the population of hot Jupiters statistically disfavors high C/O planets (C/O ≥ 0.85).</jats:p>

Spitzer Reveals Evidence of Molecular Absorption in the Atmosphere of the Hot Neptune LTT 9779b

“…Most ultrahot Jupiters with observed secondary eclipse spectra show a temperature inversion (e.g., Christiansen et al 2010;Haynes et al 2015;Evans et al 2017;Sheppard et al 2017;Kreidberg et al 2018), generally between 10 and 100 mbar, most often identified by a positive deviation of the 4.5 μm flux from a blackbody. Baxter et al (2020) have empirically found that this transition between highly irradiated gas giants with and without thermal inversions likely occurs at 1660±100 K, well below the temperature of LTT 9779b. To occur, these inversions generally require higher opacities in the optical than in the infrared, believed to be caused by the presence of optical absorbers such as TiO and VO (Hubeny et al 2003).…”

“…For example, hot Jupiters with T eq 2200 K are consistent with zero Bond albedo and inefficient global heat redistribution, indicating bright, hot daysides and relatively cold, dark nightsides (Garhart et al 2020). Furthermore, hot Jupiters with T eq 1900 K have consistent dayside brightness temperatures in the 3.6μm and 4.5μm Spitzer InfraRed Array Camera (IRAC) bandpasses (Garhart et al 2020;Baxter et al 2020). In addition, the hottest planets, such as the so-called "ultrahot Jupiters," also show qualitatively different atmospheres in which opacity from H − , hydrides, and other nonoxides begin to play a much larger role than in the (relatively) cooler population (Arcangeli et al 2018;Lothringer et al 2018;Mikal-Evans et al 2019).…”

Trends in Spitzer Secondary Eclipses

“…We note that a recent study by Baxter et al (2020) was unable to reproduce this trend in their compilation of Spitzer secondary eclipse data when using a PHOENIX stellar model and integrating across the Spitzer bandpass (although when using a blackbody as in Schwartz & Cowan (2015), they were able to retrieve the trend). However, this study used the observed brightness temperature at 3.6 µm as opposed to the error weighted average of the brightness temperatures in the 3.6 and 4.5 µm bandpasses (C. Baxter, email communication).…”

“…Garhart et al (2020) found evidence for such a correlation in their study, but concluded that the trends they saw were not well-matched by commonly used model atmosphere grids. Baxter et al (2020) used an expanded Spitzer data set to reproduce the trend found by Garhart et al (2020) and concluded that models including temperature inversions were better able to capture the qualitative shifts in spectral shape as a function of temperature. As shown in the left-hand panel of Figure 5, we also find that the most highly irradiated planets tend to lie above the line (meaning their 4.5 µm brightness temperature is higher than their 3.6 µm brightness temperature).…”

“…To date, most published population-level studies of transiting gas giant planet emission spectra have focused on searching for correlations with the incident flux (e.g., Cowan & Agol 2011, Schwartz & Cowan 2015, Schwartz et al 2017, Garhart et al 2020, Baxter et al 2020. However, we ex-arXiv:2103.15833v1 [astro-ph.EP] 29 Mar 2021 pect that planets with the same incident flux levels might nonetheless possess distinct thermal spectra if they have different atmospheric metallicities and/or surface gravities, both of which can alter their atmospheric chemistries, circulation patterns, and cloud properties.…”