Implicit Biases in Transit Models Using Stellar Pseudo Density

The transit technique is responsible for the majority of exoplanet discoveries to date. Characterizing these planets involves careful modeling of their transit profiles. A common technique involves expressing the transit duration using a density-like parameter, $\tilde{\rho }$ , often called the "circular density." Most notably, the Kepler project-the largest analysis of transit light curves to date-adopted a linear prior on $\tilde{\rho }$ . Here, we show that such a prior biases measurements of impact parameter, b, due to the nonlinear relationship between $\tilde{\rho }$ and transit duration. This bias slightly favors low values (b ≲ 0.3) and strongly disfavors high values (b ≳ 0.7) unless the transit signal-to-noise ratio is sufficient to provide an independent constraint on b, a criterion that is not satisfied for the majority of Kepler planets. Planet-to-star radius ratio, r, is also biased due to r - b covariance. Consequently, the median Kepler DR25 target suffers a 1.6% systematic underestimate of r. We present a techniques for correcting these biases and for avoiding them in the first place.