The growing sample of transiting planets is proving to be the key towards understanding the structure, composition and formation of exoplanets through anal- ysis of the mass-radius relation [e.g. Pollacco et al., 2008]. The extreme densities of WASP-17b and WASP-18b (see Section 1.4.1) represent examples of cases that are challenging the current models of planetary formation and bulk structure and it is clear that further observations are needed to understand the atmospheres of these planets.
Planet transits allow the unique opportunity to sample the absorption profile of the atmosphere of an exoplanet, i.e., essentially, the radius of the planet to be determined as a function of wavelength. In wavebands where the opacity of the atmosphere is enhanced due to the presence of a specific absorber, the planet will appear a little larger, and so by making very precise measurements of transits we
Figure 1.18: Pictorial representation of the method of transmission spectroscopy. As the light from the star passes through the exoplanetary atmosphere during transit, an absorption spectrum can be obtained. Original image credit ESA, additional illustrations by D. K. Sing.
can probe the atmospheric composition and chemistry of the planet (a technique known as transmission spectroscopy). This is demonstrated in Figure 1.18.
Transmission studies
The first detection of the wavelength dependence on the planet radius of an exoplanet was achieved in a narrow band containing the NaI doublet. This ob- servation of the transit of HD 209458b using the Hubble Space Telescope (HST) [Charbonneau et al., 2002] shows an enhanced planet radius in this band, implying a higher opacity of its atmosphere in this wavelength range due to the presence of atomic sodium. The detection of sodium had been predicted because alkali metals remain in the atomic state at low temperatures, when other abundant elements have formed molecules [Seager & Sasselov, 2000; Hubbard et al., 2001]. The atmosphere of HD 209458b has also been explored at low-spectral resolution across the optical waveband, also using HST [Sing et al., 2008; Lecavelier Des Etangs et al., 2008b]. The resulting transmission spectrum is dominated by a short wavelength broadband opacity source (around 300-500nm), interpreted as Rayleigh scattering by H2. More- over, the sharp NaI feature appears to be superimposed upon a broad Na absorption thought to be a Stark-broadened component of the NaI line. A low-resolution HST
transmission spectrum of the other very bright hot Jupiter, HD 189733b, also shows evidence for a broadband scattering continuum [Pont et al., 2008; Sing et al., 2011] (plotted in Fig. 1.19). However, in this case the scattering particles are larger, perhaps silicate condensates [Lecavelier Des Etangs et al., 2008a]. There is also no evidence for a broad component to the sodium line.
The di↵erence between the broadband transmission spectra of HD 209458b and HD 189733b demonstrates the need to study a sample of transiting exoplanets in order to probe atmospheric chemistry under a wide range of physical conditions. Detections from the ground are extremely challenging, and so far have been achieved only in the narrow bands around the cores of the NaI lines [e.g. Snellen et al., 2008; Redfield et al., 2008]. Other attempts at repeating the results from HST have so far been reported to be consistent with previous observations but dominated by systematic e↵ects [e.g. Narita et al., 2005]. In part, this is because spectrographs are not designed to be photometrically stable, and it is necessary to decorrelate data against a large number of parameters to remove these systematics [e.g. Pont et al., 2008; Snellen et al., 2008]. Fortunately, however, the main features revealed by HST spectroscopy are broadband components and so broadband observations from the ground, both in low-resolution spectroscopy and photometry, can provide further constraints for the composition of these bodies.
In addition to detections of these atmospheric features in absorption, plane- tary atmospheres have also been detected in emission using secondary eclipse mea- surements. An example of such measurement is shown in Figure 1.20. Observations in the IR using the Spitzer space telescope have been done for a large number of planets [e.g. Deming et al., 2005; Todorov et al., 2010; Nymeyer et al., 2011; Beerer et al., 2011; Deming et al., 2011]. Moreover, observations from the ground have now been performed of this phenomenon for several very hot exoplanets, mostly centred in the K-band [e.g. Sing & L´opez-Morales, 2009; de Mooij & Snellen, 2009; Croll et al., 2011; de Mooij et al., 2011; Burton et al., 2012]. A surprisingly wide range of brightness temperatures have been measured, and this is thought to result from temperature inversions in the atmospheres driving emission at low pressures [Fortney et al., 2008].
The presence of a temperature inversion is thought to be determined by the high irradiation of the upper atmosphere, where TiO and VO absorption starts to take place and is probed directly by transmission spectroscopy. Fortney et al. make a clear prediction that the transmission spectra of highly-irradiated planets should be dominated by TiO opacity, which can be distinguished from Rayleigh scatter- ing because the opacity decreases in the u band. Specifically, hot planets (class
Figure 1.19: STIS and ACS transmission spectra for HD189733b. (From Sing et al. [2011]). The wavelength bins are indicated by the X-axis error bars and the 1- error is indicated by the Y-axis error bars. The prediction from ACS Rayleigh scattering (solid and dashed lines) is also shown, as is a haze-free model atmosphere for HD 189733b from Fortney et al. [2010] which uses a planet-wide average T-P profile, and is normalized to the radii at infra-red wavelengths.
Figure 1.20: . Secondary eclipse observations of WASP-18b using the Spitzer space telescope. The panels show the raw Spitzer fluxes (left), the detrending applied based on known intra-pixel dependencies of the detectors (middle) and the resulting light curves with the corresponding fits (right) for all 4 bands available (3.6µm, 4.5µm, 5.8µmand 8.0µm). From Nymeyer et al. [2011]
pM) are predicted to have optical opacities dominated by TiO molecular bands, while in cool planets (class pL) the TiO should have condensed out of the atmo- spheres. The strong optical TiO opacity results in a temperature inversion in the upper atmosphere of the planet, driving infra-red molecular bands into emission and explaining the high brightness temperatures measured with Spitzer. They pre- dict this transition to occur for irradiation levels higher than⇡1⇥109ergs 1cm 2. However, recent measurements suggest that this threshold may not be accurate. Observations of the secondary eclipse of WASP-4b by Beerer et al. [2011] suggest this highly-irradiated planet has a weak temperature inversion or no inversion at all, contrary to expectations. Moreover, recent discussions on this topic seem to show a change in opinion with regard to the existence of any sharp features in the atmosphere profile of exoplanets.
Further measurements are required to determine the bulk characteristics of the atmospheres of exoplanets. Specifically, it is often the case that measurements in a small number of wavelengths are insufficient to distinguish between models with reasonably di↵erent parameters [Knutson et al., 2007; de Mooij et al., 2011] and therefore secondary eclipse depths at other wavelengths are required for a complete picture of the atmosphere of a given planet. Additionally, other high precision measurements are starting to reveal other features, such as the suggested presence
of water in the atmosphere of HD209458b [Swain et al., 2009; Beaulieu et al., 2010]. Observations of this nature mark the beginning of a new era in exoplanetary research, in which the properties of exoplanets are being measured and the emphasis is being placed on characterising planetary systems as much as possible with the current facilities.