Methane in the Atmosphere of the Transiting Hot Neptune Gj436b?

Abstract

We present an analysis of seven primary transit observations of the hot Neptune GJ436b at 3.6, 4.5 and 8 µm obtained with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. After correcting for systematic effects, we fitted the light curves using the Markov Chain Monte Carlo technique. Combining these new data with the EPOXI, HST and ground-based V, I, H and K s published observations, the range 0.5 − 10 µm can be covered. Due to the low level of activity of GJ436, the effect of starspots on the combination of transits at different epochs is negligible at the accuracy of the dataset. Representative climate models were calculated by using a three-dimensional, pseudo-spectral general circulation model with idealised thermal forcing. Simulated transit spectra of GJ436b were generated using line-by-line radiative transfer models including the opacities of the molecular species expected to be present in such a planetary atmosphere. A new, ab-initio calculated, linelist for hot ammonia has been used for the first time. The photometric data observed at multiple wavelengths can be interpreted with methane being the dominant absorption after molecular hydrogen, possibly with minor contributions from ammonia, water and other molecules. No clear evidence of carbon monoxide and dioxide is found from transit photometry. We discuss this result in the light of a recent paper where photochemical disequilibrium is hypothesised to interpret secondary transit photometric data. We show that the emission photometric data are not incompatible with the presence

Infrared spectroscopy of exoplanets: observational constraints

“…In the K filter, with a resolving power of about 150, one could expect to separate CH 4 , NH 3 and CO. CH 4 is the main absorber in the L band and CO dominates the M band. These results are consistent with the observations and modelling of GJ 436b [29].…”

The impact of atmospheric circulation on the chemistry of the hot Jupiter HD 209458b

“…Transiting exoplanets offer indeed the opportunity to get transmission and emission spectra of the atmosphere by observing at different wavelengths during the primary transit and secondary eclipse, respectively. The interpretation of such spectra, although difficult and sometimes contradictory, allows to identify and get the abundances of the main atmospheric constituents (Charbonneau et al 2002;Tinetti et al 2007;Swain et al 2008Swain et al , 2009Grillmair et al 2008;Sing et al 2009;Madhusudhan et al 2011;Beaulieu et al 2011). Incoming missions such as FINESSE, James Webb Space Telescope, and EChO will in the near future allow to observe the atmospheres of transiting exoplanets down to the size of rocky planets, although to date hot Jupiters offer the best and almost unique chance to get access to the atmospheric composition of extrasolar planets.…”

Spectroscopy of planetary atmospheres in our Galaxy

“…On a large scale, the transmission spectra of hot-Jupiters seem to be dominated by the signature of water vapour (Barman 2007(Barman , 2008Beaulieu et al 2010;Burrows et al 2007Burrows et al , 2008Burrows et al , 2010Charbonneau et al 2008;Grillmair et al 2008;Knutson et al 2008;Madhusudhan and Seager (2009);Tinetti et al 2007aTinetti et al , 2007b, whereas warm Neptunes, such as GJ 436b and GJ 3470b, are expected to be methane-rich (Beaulieu et al 2011;Fukui et al 2013). The analysis of GJ 436b cannot be considered conclusive, though, given the activity of the star (Knutson et al 2011) and the lack of spectroscopic data: only photometric data, often recorded at different times, are available for this target.…”

“…Agol et al 2010;Beaulieu et al 2008Beaulieu et al , 2010Beaulieu et al , 2011Brown 2001;Burke et al 2010;Charbonneau et al 2005Charbonneau et al , 2008Deming et al 2013;Désert et al 2011;Grillmair et al 2008;Knutson et al 2007;Machalek et al 2009;Pont et al 2008;Sing et al 2011a;Stevenson et al 2010;Swain et al 2008Swain et al , 2009b. Parametric models approximate systematic noise via the use of auxiliary information of the instrument, the so-called optical state vectors, which often include the positional drifts of the star on the detector, the focus and the detector temperature changes, the positional angles of the telescope on the sky etc.…”