Program: GN-2012B-Q-9

THIS PROGRAM IS NOT FOR TAC REVIEW, IT HAS ALREADY BEEN ACCEPTED IN 2012A. WE RESUBMIT IT IN 2012B PER REQUEST FROM NOAO: TO OBTAIN THE TIMING WINDOWS AVAILABLE FOR THE SECOND PART OF THIS PROJECT IN AUGUST 2012. THIS SHOULD INCLUDE THE PROPOSAL IN THE 2012B TAC PROCESS SO THAT THE 8 HOURS ALREADY ALLOCATED GET ASSOCIATED WITH IT. The Kepler mission has detected an amazingly rich diversity of extra-solar planet candidates. A large amount of them are grouped in multiple system, orbiting the same host star. We have now the opportunity to characterize and compare the diversity of exoplanetary worlds within one multiple system, and with planets from other systems, thus opening a new window to the field of comparative exoplanetology. We propose to observe the atmospheres of the two confirmed Neptune-like planets which orbit the same host star, Kepler-18c and Kepler-18d. Their planetary atmospheres will be probed using transmission spectroscopy observations from 0.4 to 1.0 microns with Gemini-N/GMOS. We will observe the two planet during transit using a recently proved technique for obtaining space-telescope quality exoplanet transit spectroscopy from the ground. These elements are expected to be present in gas phase in the warm atmospheres of these objects. Comparing these observations to giant planet atmospheric models, the overall envelope composition and eventually abundances can be retrieved since models are locked down to the Kepler measurements. Interestingly, the relative composition between these two planets will be derived which will help to constrain planetary formation and evolution theories.