Abstract

Characterization of the Atmosphere of Exoplanets with MIRI

Pierre-Olivier Lagage (CEA Saclay)

Absil O., Barrado D., Bailey J.I. III, Boccaletti A., Bouwman J., Coulais A., Decin L., Dicken D., Gastaud R., Glauser A., Guedel M., Henning T., Glasse A., Kamp I., Krause O., Lahuis F., Mueller M., Nehme C., Olofsson G., Pye J., Rouan D., Royer P., Scheithauer S., Topinka M., Vandenbussche B., Walker H., Waters R., Wright G.

The characterization of the atmosphere of exoplanets is one of the three large programs (100 hours each) to be conducted in the framework of the Guaranteed Time Observations (GTO) of the European MIRI consortium. The aim of the program is twofold : 1) to constrain the atmospheric models of exoplanets with no equivalent in the Solar system, 2) to constrain giant planet formation models from the determination of the molecular composition of the atmosphere (C/O, metallicity) or of the bolometric luminosity in the case of isolated young giant exoplanets.

The wavelength coverage of MIRI (5 – 28 microns) is particularly well adapted to get precise measurements of the bolometric luminosity of “warm/cold” exoplanets, to determine the molecular composition of the exoplanet atmosphere, to characterize the nature of clouds or hazes which may be present in the atmosphere, …. Indeed several dust features due to silicates and other dust species are present in the 10 microns region; most of the key molecules possibly present in the atmosphere of an exoplanet have features in the mid-Infrared; some, such as NH3, O3, C2H2, have their strongest bands in the mid-IR. So far spectroscopic studies of exoplanet atmospheres in the mid-IR wavelength range have been sparse. Only a couple of transiting exoplanets has been observed spectroscopically with Spitzer. No observations beyond 5 microns of an exoplanet detected by direct imaging have been yet reported. Making possible observations over a large spectral range is definitely an advantage of the JWST.

The capabilities of MIRI to characterize exoplanet atmospheres will be illustrated by going through the various observing MIRI modes to be used for GTO exoplanet observations. We plan to perform spectroscopic observations of transiting exoplanets of various masses, gravity, temperature, using the slitless low resolution spectroscopic mode of MIRI (wavelength range: 5 -12 microns; spectral resolution: 100 at 7.5 microns); the mode has been specifically developed for exoplanets observations. Most of the observations will be eclipse observations, complemented, when possible in terms of high enough signal to noise ratio in one transit, by transmission observations; a phase curve of one exoplanet (WASP 43-b) is under consideration. We plan to perform coronagraphic observations with the three phase masks specifically developed to detect the NH3 line at 10.65 microns expected in the spectra of young giant exoplanets at temperature lower than about 1000K. We plan to use the Integral Field Unit medium resolution spectroscopic (MRS) mode (5-28 microns, spectral resolution between 1000 – 3500) to observe wide separation planetary mass companions, as well as one or two free floating planets. Note that we also plan to use the MRS to observe a sample of brown dwarfs.

Talk