Black solid line: probability for the transit of a second putative planet occurring at least once during the Spitzer observations as a function of the orbital period, for periods up to 20.9 d. Black dots: transit probability for a second planet, taking into account the transiting nature of GJ 1214 b (see Sect. 5.2 for details). The red line shows the product of the two probabilities. The green vertical line shows the inner limit of the HZ as computed from Eq. (12) of Zsom et al. (2013).