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Short-period super-Earths and Neptunes are now known to be very frequentaround solar-type stars. Improving our understanding of these mysteriousplanets requires the detection of a significant sample of objects suitable fordetailed characterization. Searching for the transits of the low-mass planetsdetected by Doppler surveys is a straightforward way to achieve this goal.Indeed, Doppler surveys target the most nearby main-sequence stars, theyregularly detect close-in low-mass planets with significant transitprobability, and their radial velocity data constrain strongly the ephemeris ofpossible transits. In this context, we initiated in 2010 an ambitious Spitzermulti-Cycle transit search project that targeted 25 low-mass planets detectedby radial velocity, focusing mainly on the shortest-period planets detected bythe HARPS spectrograph. We report here null results for 19 targets of theproject. For 16 planets out of 19, a transiting configuration is stronglydisfavored or firmly rejected by our data for most planetary compositions. Wederive a posterior probability of 83% that none of the probed 19 planetstransits (for a prior probability of 22%), which still leaves a significantprobability of 17% that at least one of them does transit. Globally, ourSpitzer project revealed or confirmed transits for three of its 25 targetedplanets, and discarded or disfavored the transiting nature of 20 of them. Ourlight curves demonstrate for Warm Spitzer excellent photometric precisions: for14 targets out of 19, we were able to reach standard deviations that werebetter than 50ppm per 30 min intervals. Combined with its Earth-trailing orbit,which makes it capable of pointing any star in the sky and to monitor itcontinuously for days, this work confirms Spitzer as an optimal instrument todetect sub-mmag-deep transits on the bright nearby stars targeted by Dopplersurveys.

The Spitzer search for the transits of HARPS low-mass planets - II. Null results for 19 planets