HD 40307b, the first transiting Super-Earth?

We stand on a great divide in the history of exoplanet discoveries. On the one side are the Spitzer observations of hot Jupiters that have revolutionized the burgeoning field of exoplanet characterization. On the other side are the Earth analogs with surface temperatures suitable for liquid water. Spitzer has a tremendous opportunity to bridge the divide by detecting the first transiting Super-Earth. HD 40307 is a bright (V=7.2, K=4.8) nearby (12.8 pc) K2 dwarf with three newly-discovered very low-mass exoplanets (Mayor et al., 2008). The innermost planet has a minimum mass of 4.21 M_Earth. With a 4.3 day period, the transit probability is 8% if the orbital plane is randomly aligned. We propose 10.5 h of IRAC-ch4 in subarray mode to monitor HD 40307 during the expected transit time. The first detection of a transiting Super-Earth would enable the first robust identification of a terrestrial-like exoplanet. With a measured radius and mass, the interior composition of the planet can be constrained. Spitzer is the only existing instrument able to perform this challenging detection. There is furthermore a compelling reason to conduct this particular search now, while Spitzer still has cryogen. This system is very favorable for observation (being only 12 pc distant), and a temperature measurement for the Super-Earth---by detection of the secondary eclipse---will be possible, once a transit is confirmed. Due to the nearly 40-degree orbital coverage we will obtain, our proposed observations will teach us about the presence and properties of the planet's atmosphere even if HD 40307b does not transit. We point out that in the case of a transit, the transit probability for the two other Super-Earths in the system is high. The possible payoff of our proposed 10.5 h program is thus extremely large.