The prevalence of dust on the exoplanet HD 189733b from Hubble and Spitzer observations

Abstract

The hot Jupiter HD 189733 b is the most extensively observed exoplanet. Its atmosphere has been detected and characterised in transmission and eclipse spectroscopy, and its phase curve measured at several wavelengths. This paper brings together the results of our campaign to obtain the complete transmission spectrum of the atmosphere of this planet from UV to infrared with the Hubble Space Telescope, using the STIS, ACS and WFC3 instruments. We provide a new tabulation of the transmission spectrum across the entire visible and infrared range. The radius ratio in each wavelength band was re-derived, where necessary, to ensure a consistent treatment of the bulk transit parameters and stellar limb-darkening. Special care was taken to correct for, and derive realistic estimates of the uncertainties due to, both occulted and unocculted star spots.The combined spectrum is very different from the predictions of cloud-free models for hot Jupiters: it is dominated by Rayleigh scattering over the whole visible and near infrared range, the only detected features being narrow sodium and potassium lines. We interpret this as the signature of a haze of condensate grains extending over at least five scale heights. We show that a dust-dominated atmosphere could also explain several puzzling features of the emission spectrum and phase curves, including the large amplitude of the phase curve at 3.6 µm, the small hot-spot longitude shift and the hot mid-infrared emission spectrum. We discuss possible compositions and derive some first-order estimates for the properties of the putative condensate haze/clouds. We finish by speculating that the dichotomy between the two observationally defined classes of hot Jupiter atmospheres, of which HD 189733 b and HD 209458 b are the prototypes, might not be whether they possess a temperature inversion, but whether they are clear or dusty. We also consider the possibility of a continuum of cloud properties between hot Jupiters, young Jupiters and L-type brown dwarfs.

Evidence for a spectroscopic direct detection of reflected light from 51 Pegasi b

“…Moreover, literature values suggest that hot Jupiters usually have albedos lower than ∼0.3 (Cowan & Agol 2011). However, several works (e.g., the case of HD 189733b, Evans et al 2013;Pont et al 2013) reported albedo values as high as 0.4-0.5. Such high values may be explained by the scattering of condensates in the high atmosphere (Sudarsky et al 2003).…”

Transmission spectral properties of clouds for hot Jupiter exoplanets

“…Gibson et al 2013;Pont et al 2013;Sing et al 2013;Nascimbeni et al 2013). At present, these observations focus on the optical and near-infrared regions of the spectrum, revealing information on the portion of transmission spectra for aerosols where only scattering features are seen (e.g.…”

“…At present, these observations focus on the optical and near-infrared regions of the spectrum, revealing information on the portion of transmission spectra for aerosols where only scattering features are seen (e.g. Pont et al 2013;Sing et al 2013; see Fig. 7).…”

“…We fit single grain size condensate cloud spectra from 0.001−10 µm calculated using Mie theory for each of the condensates in Table 1 to the HD 189733b data from 0.3−1.0 µm of Pont et al (2013) where significant Rayleigh scattering is observed. Figure 7 shows the observed HD 189733b transmission spectrum from 0.3 to 24 µm McCullough et al 2014) with a representative condensate spectrum for each of the major diatomic vibrational modes shown in Fig.…”

Atmospheric Electrification in Dusty, Reactive Gases in the Solar System and Beyond

“…Currently, models utilise molecular opacities, primarily due to species such as CO, H 2 O, and CH 4 . However, transit observations (Pont et al 2013;Sing et al 2014) taken for different wavelength bands suggest a cloud coverage throughout the atmosphere. While in hindsight based on observations of brown dwarfs or our gas giant planets this is not surprising, it makes the coupled problem significantly more complex.…”