Detection of atmospheric haze on an extrasolar planet: the 0.55-1.05 μm transmission spectrum of HD 189733b with the Hubble Space Telescope

Abstract

The nearby transiting planet HD 189733b was observed during three transits with the Advanced Camera for Surveys of the Hubble Space Telescope in spectroscopic mode. The resulting time‐series of 675 spectra covers the 550–1050 nm range, with a resolution element of ∼8 nm, at extremely high accuracy (signal‐to‐noise ratio up to 10 000 in 50‐nm intervals in each individual spectrum). Using these data, we disentangle the effects of limb darkening, measurement systematics and spots on the surface of the host star, to calculate the wavelength dependence of the effective transit radius to an accuracy of ∼50 km. This constitutes the ‘transmission spectrum’ of the planetary atmosphere. It indicates at each wavelength at what height the planetary atmosphere becomes opaque to the grazing stellar light during the transit. In this wavelength range, strong features due to sodium, potassium and water are predicted by atmosphere models for a planet like HD 189733b, but they can be hidden by broad absorption from clouds or hazes higher up in the atmosphere. We observed an almost featureless transmission spectrum between 550 and 1050 nm, with no indication of the expected sodium or potassium atomic absorption features. Comparison of our results with the transit radius observed in the near and mid‐infrared (2–8 μm), and the slope of the spectrum, suggest the presence of a haze of submicrometre particles in the upper atmosphere of the planet.

A Gemini ground-based transmission spectrum of WASP-29b: a featureless spectrum from 515 to 720 nm

“…Burrows, Marley & Sharp 2000), where atomic Na can be lost in compounds such as disodium monosulphide (Na 2 S) or ansite (NaAlSi 3 O 8 ), depending on the presence of other species in the atmosphere. Similar observations at shorter wavelengths could distinguish between a flat featureless spectrum and one dominated by a Rayleigh scattering haze, such as the prototypical HD 189733b (Pont et al 2008;Sing et al 2012), and higher resolution (at similar S/N) is required to rule out the presence of a Na core if there are clouds or haze present.…”

“…Until recently, transmission spectroscopy has only been feasible using space-based telescopes, which have been tremendously successful, particularly at optical wavelengths (e.g. Charbonneau et al 2002;Pont et al 2008;Sing et al 2008Sing et al , 2011Huitson et al 2012). Measurements in the near-infrared (NIR) have proved more controversial, with different groups reporting conflicting conclusions from the same data set (e.g.…”

Variability in the super-Earth 55 Cnc e

“…Second, clouds are less likely to form in the atmospheres of extremely hot exoplanets compared to cooler objects (Madhusudhan & Redfield 2015), which is particularly relevant in the light of the numerous non-detections of spectral features in exoplanets (e.g. Pont et al 2008;Sing et al 2011; Knutson et al 2014; Kreidberg et al 2014). The combination of the high photometric precision possible for transit/occultation obser-vations and the high planetary temperature means that 55 Cnc e currently represents one of the most promising super-Earth for infrared spectroscopic observations of its atmosphere.…”

NICMOS spectroscopy of HD 189733b

“…The alkali element sodium has also been detected in exoplanet atmospheres using Hubble, (Charbonneau et al 2002) and ground-based measurements have also been made (Redfield et al 2008;Snellen et al 2008). Spectroscopic measurements reporting an absence of atomic or molecular features have also been reported (Pont et al 2008(Pont et al , 2009, with useful constraints being placed by the measurements. At the present time, numerous teams have exoplanet spectroscopy programs in place, and at least one purpose-built instrument is being constructed (Jurgenson et al 2010).…”