Biased Extrasolar Planet Radii from Field Transit Surveys

The precisions of extrasolar planet radius measurements are reaching the point where meaningful and discriminatory comparisons with theoretical predictions are possible. However, care must be taken to account for selection effects in the transit surveys that detect the transiting planets for which radius measurements are possible. Here I identify one such selection effect, such that the number of planets with radius Rp detected in a signal-to-noise limited transit survey is ∝ R α p , with α ∼ 4 − 6. In the presence of a dispersion σ in the intrinsic distribution of planet radii, this selection effect translates to bias b in the radii of observed planets. Detected planets are, on average, larger by a fractional amount b ∼ α(σ/〈Rp〉) 2 relative to the mean radius 〈Rp〉 of the underlying distribution. I argue that the intrinsic dispersion in planetary radii is likely to be in the range σ = (0.05 − 0.18)RJ , where the lower bound is that expected theoretically solely from the variance in stellar insolation, and the upper bound is the 95% c.l. upper limit from the scatter in observed radii. Assuming an arbitrary but plausible value of σ/〈Rp〉 ∼ 10%, and thus b ∼ 6%, I infer a mean intrinsic radius of close-in massive extrasolar planets of 〈Rp〉 = (1.03± 0.03)RJ . This value reinforces the case for HD209458b having an anomalously large radius, and may be inconsistent with coreless models of irradiated giant planets. Subject headings: planetary systems – planets and satellites: general – stars:individual (HD 209458)