Transits of Transparent Planets - Atmospheric Lensing Effects

TL;DR

This paper reveals that atmospheric refraction in transparent planets can significantly alter transit light curves, creating measurable shoulders that allow direct atmospheric scale height measurements, especially for planets with orbital periods over 70 days.

Contribution

It provides analytical expressions for atmospheric lensing effects in transits and demonstrates their potential to measure planetary atmospheric scale heights.

Findings

Refraction can add measurable shoulders to transit light curves.

Atmospheric lensing effects dominate for planets with periods over 70 days.

Refraction effects can be as large as 10^{-4} of stellar light.

Abstract

Light refracted by the planet's atmosphere is usually ignored in analysis of planetary transits. Here we show that refraction can add shoulders to the transit light curve, i.e., an increase in the observed flux, mostly just before and after transit. During transit, light may be refracted away from the observer. Therefore, even completely transparent planets will display a very similar signal to that of a standard transit, i.e., that of an opaque planet. We provide analytical expression for the amount of additional light deflected towards the observer before the transit, and show that the effect may be as large as $10^{-4}$ of the stellar light and therefore measurable by current instruments. By observing this effect we can directly measure the scale height of the planet's atmosphere. We also consider the attenuation of starlight in the planetary atmosphere due to Rayleigh scattering and discuss the conditions under which the atmospheric lensing effect is most prominent. We show that, for planets on orbital periods larger than about 70 days, the size of the transit is determined by refraction effects, and not by absorption within the planet.