Sensitivity of Transit Searches to Habitable‐Zone Planets

Photon-limited transit surveys in V band are in principle about 20 times moresensitive to planets of fixed size in the habitable zone around M stars than Gstars. In I band the ratio is about 400. The advantages that the habitable zonelies closer and that the stars are smaller (together with the numericalsuperiority of M stars) more than compensate for the reduced signal due to thelower luminosity of the later-type stars. That is, M stars can yield reliabletransit detections at much fainter apparent magnitudes than G stars. However,to achieve this greater sensitivity, the later-type stars must be monitored tothese correspondingly fainter magnitudes, which can engender several practicalproblems. We show that with modest modifications, the Kepler mission couldextend its effective sensitivity from its current M_V=6 to M_V=9. This wouldnot capture the whole M dwarf peak, but would roughly triple its sensitivity toEarth-like planets in the habitable zone. However, to take advantage of thehuge bump in the sensitivity function at M_V=12 would require major changes.Comment: Submitted to ApJ Letters, 10 pages including 2 figure