New avenues for thermal inversions in atmospheres of hot Jupiters

Abstract

Thermal emission spectra of hot Jupiters have led to key constraints on thermal inversions (or 'stratospheres') in their atmospheres with important implications for their atmospheric processes. Canonically, thermal inversions in hot Jupiters have been suggested to be caused by species such as TiO and VO which have strong visible opacity to absorb incident starlight. We explore two new avenues for thermal inversions in hot Jupiters, exploring both the visible and infrared opacities in their atmospheres. Firstly, by exploring a range of metal-rich species we find that four species (AlO, CaO, NaH and MgH) provide visible opacities comparable to TiO/VO and can cause strong inversions with reasonable abundances. Secondly, we show that a low infrared opacity caused by a low H 2 O abundance, e.g. through a C/O∼ 1, can also lead to strong thermal inversions even with low abundances of the visible absorbers mentioned above. We find that increasing the C/O ratio towards unity requires almost 2 orders of magnitude lower abundances for the visible absorbers in order for an inversion to form. Finally, we explore the thermal inversion in WASP-121b and find that it can be explained by all the visible absorbers listed above for different C/O ratios. Our study demonstrates the importance of both the refractory and volatile species in governing the physicochemical processes in hot Jupiter atmospheres. Spectroscopic observations in the visible have the potential to detect the newly proposed refractory species that can cause thermal inversions in addition to TiO and VO.

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

“…Instead, it could be that these optical absorbers are cold trapped on the planet's nightside (Parmentier et al 2013), or that they exist in subsolar abundances for other reasons. In the latter case, the lack of an inversion combined with the planet's high temperatures could potentially indicate that C/O < 1, because a C/O around 1 favors the occurrence of inversions even in the absence of absorbers like TiO and VO (Mollière et al 2015;Gandhi & Madhusudhan 2019). Observations at higher spectral resolution (attainable with the James Webb Space Telescope) and models including aerosols (which have been shown to impact the secondary eclipse spectra of a hot Jupiter; Parmentier et al 2016;Taylor et al 2020) should be able to verify and refine the structure of the spectrum in this wavelength region, thus addressing at least some of these questions.…”

Optical phase curve of the ultra-hot Jupiter WASP-121b

“…We also performed tests with additional chemical species, such as NH 3 , or the alkali metals K and Na, all of which were not favoured by the Bayesian framework. We expect that Al, Ca, and Mg (Gandhi & Madhusudhan 2019) will mostly be locked up in condensates in the deeper atmosphere. We also do not expect Fe to be present in atomic form at the high pressure / low temperature levels probed by the emission spectrum (Lothringer et al 2018).…”

“…(e-mail: vincent.bourrier@unige.ch) optical and infrared wavelengths further showed the signature of water in absorption at the atmospheric limb, and the possible presence of vanadium oxide and iron hydride, with no titanium oxide (Evans et al 2016(Evans et al , 2018. Alternative species could, however, explain features in the emission and transmission spectra (e.g., , Gandhi & Madhusudhan 2019.…”

Neutral Iron Emission Lines from the Dayside of KELT-9b: The GAPS Program with HARPS-N at TNG XX

“…A second important factor that contributes to the formation of thermal inversions is the lack of molecules with nearinfrared opacities, able to radiatively cool the atmosphere. This can be caused by high C/O atmospheres (Mollière et al 2015;Gandhi & Madhusudhan 2019) and/or by thermal dissociation (Arcangeli et al 2018;Lothringer et al 2018;Parmentier et al 2018), with the latter scenario predicted to be important in ultra-hot Jupiters (Lothringer & Barman 2019;Malik et al 2019).…”