Some Tests to Establish Confidence in Planets Discovered by Transit Photometry

Increased attention is being paid to transit photometry as a viable method for discovering or confirming detections of extrasolar planets. Several ground-based efforts are underway that target short-period, giant planets such as 51 Peg b, and several missions have been proposed to NASA and ESA to detect planets as small as Earth from spaceborne photometers. The success of these efforts depends in part on the ability to establish appropriate detection thresholds to control false alarm rates and the ability to assess the statistical confidence in planetary candidates drawn from any such search. This latter function attains higher importance for the space-based efforts, where direct ground-based confirmation of terrestrial-size planets is not possible. These tasks are complicated by the need to survey tens of thousands of stars to overcome the limited geometric probability of transit alignment and by the nature of the transit signals themselves. In this paper, we present empirical methods for setting appropriate detection thresholds and for establishing the confidence level in planetary candidates obtained from transit photometry of even a large number of stars. The methods are simple and allow the observer to quickly assess the statistical significance of any particular set of transits.