First Results from the Hubble Transmission Survey of Hot Jupiter Atmospheres

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An outline of the paper: An HST Optical to Near-IR Transmission Spectrum of the Hot Jupiter WASP-19b: Detection of Atmospheric Water and Likely Absence of TiO by Huitson. et al.

Last year, a team led by the University of Exeter was awarded nearly 200 hours on the Hubble Space Telescope to measure the atmospheric compositions of eight hot Jupiters. The goal was to obtain complete low-resolution transmission spectra over the entire optical range and into the near-infrared, attempting to compare planets across the hot Jupiter class, and to directly search for inversion-causing molecules.

The majority of the data has now been obtained and the first published results from the survey were announced last week at the Royal Astronomical Society National Astronomy Meeting (with an associated press release). These first results concern the spectrum of WASP-19b, a ~2000 K hot Jupiter orbiting an active G8 star. The paper published by our team reveals two potentially surprising results in the atmosphere of WASP-19b.

Firstly, the TiO molecule suggested to be responsible for thermal inversions is most likely absent from the atmosphere. The characteristic large bandhead in the optical transmission spectrum is a poorer fit to the data than a TiO-free atmosphere at the 2.7 sigma level. While not totally conclusive, this result suggests that there is some mechanism operating in the atmosphere which may break down TiO or trap it in the lower atmosphere, such as a night side or vertical cold trap and inefficient vertical transport (Spiegel et al. 2009, Knutson et al. 2010, Parmentier et al. 2013). The lack of observed TiO is, however, consistent with secondary eclipse observations, which rule out strong inversions (Madhusudhan 2012, Anderson et al. 2013).

optical_notio

Figure 1: Optical transmission spectrum of WASP-19b from Huitson et al. (2013) obtained using HST STIS compared with models based on the formalisms of Fortney et al. (2010) (blue) and Burrows et al. (2010) (green) with TiO opacities removed.

The other discovery is the presence of water in the quantities predicted by solar abundance models, in contrast to the muted features seen in other hot Jupiters such as HD 189733b and HD 209458b (Gibson et al. 2012, Deming et al. 2013).  The water feature is significant at the 4 sigma level compared to a featureless near-infrared spectrum, and the size of the feature suggests that the upper atmosphere of WASP-19b is free of clouds. The water detection also rules out a high C/O ratio as an explanation for the lack of TiO features.

near_infrared

Figure 2: Near-Infrared transmission spectrum of WASP-19b from Huitson et al. (2013) obtained using HST WFC3 (archive data from observing program by D. Deming) compared with models based on the formalisms of Fortney et al. (2010) (blue) and Burrows et al. (2010) (green) at solar abundance.

Overall, WASP-19b looks quite different from many of the planets already studied, possibly because of its high temperature, which is ~ 500 K above the other currently studied planets. One of the aims of the large Hubble programme was to sample a cross-section of the hot Jupiter class, spanning 1100-2600 K in equilibrium temperature and search for significant differences. The findings for WASP-19b are a good case to continue expanding transmission spectral surveys so that trends become clear and we can begin to classify these planets.

Further Reading:

WASP-19b: The Shortest Period Transiting Exoplanet Yet Discovered by Hebb  et al. (2010)

Hubble Space Telescope reveals variation between hot extrasolar planet atmospheres: Royal Astronomical Society press release summarising the WASP-19b results for a general audience

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About Author

I am undertaking an observational astronomy PhD focusing on the characterisation of planetary atmospheres, working with Dr. David Sing and Prof. Frédéric Pont. I am specialising in using transmission spectroscopy, applied to the hot Jupiter HD 189733b with HST. I am interested in obtaining a more complete picture for the few well-studied exoplanets, comparing observations in different regimes of planetary atmospheres, from secondary eclipse and transmission observations at different wavelengths, both from the ground and from space. (More)