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We discovered evidence for a possible additional 0.75 R_Earth transitingplanet in the NASA EPOXI observations of the known M dwarf exoplanetary systemGJ 436. Based on an ephemeris determined from the EPOXI data, we predicted atransit event in an extant Spitzer Space Telescope 8 micron data set of thisstar. Our subsequent analysis of those Spitzer data confirmed the signal of thepredicted depth and at the predicted time, but we found that the transit depthwas dependent on the aperture used to perform the photometry. Based on thesesuggestive findings, we gathered new Warm Spitzer Observations of GJ 436 at 4.5microns spanning a time of transit predicted from the EPOXI and Spitzer 8micron candidate events. The 4.5 micron data permit us to rule out a transit athigh confidence, and we conclude that the earlier candidate transit signalsresulted from correlated noise in the EPOXI and Spitzer 8 micron observations.In the course of this investigation, we developed a novel method for correctingthe intrapixel sensitivity variations of the 3.6 and 4.5 micron channels of theInfrared Array Camera (IRAC) instrument. We demonstrate the sensitivity of WarmSpitzer observations of M dwarfs to confirm sub-Earth sized planets. Ouranalysis will inform similar work that will be undertaken to use Warm Spitzerobservations to confirm rocky planets discovered by the Kepler mission.

A Search for a Sub-Earth Sized Companion to GJ 436 and a Novel Method to Calibrate Warm Spitzer IRAC Observations