Transmission spectral properties of clouds for hot Jupiter exoplanets

Abstract

Clouds play an important role in the atmospheres of planetary bodies. It is expected that, like all the planetary bodies in our solar system, exoplanet atmospheres will also have substantial cloud coverage, and evidence is mounting for clouds in a number of hot Jupiters. To better characterise planetary atmospheres, we need to consider the effects these clouds will have on the observed broadband transmission spectra. Here we examine the expected cloud condensate species for hot Jupiter exoplanets and the effects of various grain sizes and distributions on the resulting transmission spectra from the optical to infrared, which can be used as a broad framework when interpreting exoplanet spectra. We note that significant infrared absorption features appear in the computed transmission spectrum, the result of vibrational modes between the key species in each condensate, which can potentially be very constraining. While it may be hard to differentiate between individual condensates in the broad transmission spectra, it may be possible to discern different vibrational bonds, which can distinguish between cloud formation scenarios, such as condensate clouds or photochemically generated species. Vibrational mode features are shown to be prominent when the clouds are composed of small sub-micron sized particles and can be associated with an accompanying optical scattering slope. These infrared features have potential implications for future exoplanetary atmosphere studies conducted with JWST, where such vibrational modes distinguishing condensate species can be probed at longer wavelengths.

Observing transiting planets with JWST

“…For the self-consistent cloud with a small f sed = 0.01 value (model d) we find that the slope in the optical is already relatively steep, approaching a Rayleigh scattering slope. However, although the average particle size is well below 0.08 µm, the slope is less steep than in the mono-disperse particle model (a) because the largest particles within the distribution dominate the opacity (Wakeford & Sing 2015). We thus do not find a good fit for HAT-P-12b when using the Ackerman & Marley (2001) model.…”

“…In the example shown here we only consider crystalline Mg 2 SiO 4 for the silicates. But also different silicates, such as MgSiO 3 , iron-enriched olivines and pyroxenes, or species such as SiO 2 , FeSiO 3 and Fe 2 SiO 4 are possible (see Wakeford & Sing 2015). Another complication arises from the wavelength-dependent shape and position of the absorption features for particles larger than ∼1 µm, but note that a strong Rayleigh signal observed in the optical and NIR transmission spectrum would suggest particles that are smaller than 0.1 µm.…”

“…Observing exoplanet transit spectra in the MIR may be key (Wakeford & Sing 2015) to identifying cloud species which often weaken or even fully blanket the atomic and molecular features in the optical and NIR (see, e.g., GJ1214b (Kreidberg et al 2014), HD 189733b Sing et al 2011, WASP-6b Jordán et al 2013and WASP-12b Sing et al 2013).…”

“…Depending on the size of the cloud particles, their opacity in the optical and NIR transitions from a Rayleigh slope (small particles) to a flat, gray opacity (large particles). The resonance features of possible chemical equilibrium cloud species all lie in the MIR (Wakeford & Sing 2015) such that a distinction between cloud species may only be possible within the MIR region. The formation of clouds and hazes by non-equilibrium processes is another possibility, although hot Jupiters, especially, seem to be too hot for the "classical" case of hydrocarbon hazes (Liang et al 2004) as well as for newly suggested pathways such as photolytic sulfur clouds (Zahnle et al 2016).…”

Probing exoplanet clouds with optical phase curves

“…We identified four candidate cloud components that have both condensation temperatures of 1,000-2,000 K, relevant to the equilibrium temperature of Kepler-7b (47,48), and n i ≤ 0.003: two silicates (Mg 2 SiO 4 and MgSiO 3 ), perovskite (CaTiO 3 ), and silica (SiO 2 ). Without further information it is not possible to favor one candidate above another.…”