Close Orbiting Planet Atmospheres

The first spectroscopic probe of exoplanet atmospheres was provided by Charbon-neau et al.'s (2002) detection of the additional dimming of sodium absorption during transit due to absorption from sodium in the atmosphere of HD209458b. The observed dimming is reasonably well modelled by planetary atmosphere models that incorporate irradiation and allow for sodium to be out of thermal equilibrium (Barman et al. 2002). Vidal-Madjar et al. (2003, 2004) have also detected atomic hydrogen, carbon and oxygen during transits of HD209458b. The large implied physical radii exceeds the Roche limit, leading them to conclude that material is escaping from the planet. However, the minimum escape rate based on the data is low enough to reduce the planetary mass by only 0.1% over the age of the system (confirmed 'empirically' by Melo et al. 2006). Combining this with models for HD209458b, which put the upper atmosphere at a temperature of 10,000 K, this gives strong evidence for atmospheric evaporation. This evaporation confirms the conclusions by Hebrard et al. (2003) and may lead to new types of planets being discovered with hydrogen poor atmospheres or even with no atmospheres at all (Trilling et al. 1998).

Spitzer has detected radiation from several hot Jupiters, over six bandpasses from 3.6 to 24 ¡m and has detected phase-dependent flux implying significant day-night temperature contrasts on two hot Jupiters (Harrington et al. 2006, Knutson et al. 2007). Further announcements are expected. Such observations are now fuelling new research into the meterology of exoplanets (Cooper & Showman 2005, 2006; Fortney et al. 2006). Spectroscopic observations of stars during primary and secondary eclipse of close-orbiting planets have been particularly revealing. The detection of sodium in the atmosphere of HD209458b (Charbonneau et al. 2002) and detections of water in the far red optical regime have been made on HD209458b by Barman (2007) and on HD189733b by Tinetti et al. (2007). These detections can now be well modelled by a near isothermal vertical profile for the planet's atmo sphere. Furthermore, the lack of water vapour and possible silicate features observed by Richardson et al. (2007) and Grillmair et al. (2007) during secondary eclipse is consistent with the expected strong circulation on close-orbiting planets which can flatten the day side temperature gradient. Mid-infrared measurements have been made possible by the exquisite precision made possible by transit spectroscopy and space-borne instrumentation. Ground-based near-infrared spectroscopy has proved more difficult (Richardson et al. 2003) though is expected to soon enable access to near-infrared flux measurements (e.g. Barnes et al. 2007a and Fig. 6.11 from Barnes et al. 2007b), which will allow the resolution to resolve different atomic and molecular species.

Reflected light studies carried out by (Collier Cameron et al. 1999; Charbonneau et al. 1999; Collier Cameron et al. 2002; Leigh et al. 2003a,b) (as well as results from MOST photometry, e.g., Rowe et al. 2006) place albedo upper limits on the atmospheres of CEGPs. These upper limits to the planet/star contrast ratio have established the low reflectivity when compared with the solar system gas giants. High precision polarimeters (e.g. Hough et al. 2006) are now able to measure the polarisation of reflected light from extra-solar planets. Combining this with M sin i measurements from radial velocity data will provide the orbital inclination of the planet and hence the planet's mass. Detailed modelling will determine the nature of the scattering particles and the geometric albedo as a function of wavelength.

Fig. 6.11. Model planet/star flux ratio for the HD189733 system. Spitzer eclipse depth measurements are plotted for 8 ¡m (Knutson et al. 2007), 11.1 ¡m (Grillmair et al. 2007) and 16 ¡m (Deming et al. 2006). A horizontal bar with vertical down-pointing arrow indicates Barnes et al. (2007) 1 a limit; the width of the horizontal bar represents the wavelength range of the data. The single point plotted in the inset at log10e0 = -3.16 represents the model mean flux over the range of the non-detection. The upper limit is at a level of log10e0 = 0.24 lower than the model (i.e. e0(model)/e0(observed)=1.7).

Fig. 6.11. Model planet/star flux ratio for the HD189733 system. Spitzer eclipse depth measurements are plotted for 8 ¡m (Knutson et al. 2007), 11.1 ¡m (Grillmair et al. 2007) and 16 ¡m (Deming et al. 2006). A horizontal bar with vertical down-pointing arrow indicates Barnes et al. (2007) 1 a limit; the width of the horizontal bar represents the wavelength range of the data. The single point plotted in the inset at log10e0 = -3.16 represents the model mean flux over the range of the non-detection. The upper limit is at a level of log10e0 = 0.24 lower than the model (i.e. e0(model)/e0(observed)=1.7).

Was this article helpful?

0 0

Post a comment