Out-of-Transit Refracted Light in the Atmospheres of Transiting and Non-Transiting Exoplanets

TL;DR

This paper models out-of-transit refracted light in exoplanet atmospheres, exploring its detectability, especially around late M stars, and discusses potential observations with current and future telescopes.

Contribution

It provides a comprehensive model of atmospheric refraction effects and formalism for detecting refracted light signals in exoplanet systems, including non-transiting planets.

Findings

Refraction signals are detectable mainly around late M stars at >0.5 AU.

Rayleigh scattering limits visible wavelength detection in most systems.

Longer wavelengths increase the likelihood of observing refracted light.

Abstract

Before an exoplanet transit, atmospheric refraction bends light into the line of sight of an observer. The refracted light forms a stellar mirage, a distorted secondary image of the host star. I model this phenomenon and the resultant out-of-transit flux increase across a comprehensive exoplanetary parameter space. At visible wavelengths, Rayleigh scattering limits the detectability of stellar mirages in most exoplanetary systems with semi-major axes $\lesssim$6 AU. A notable exception is almost any planet orbiting a late M or ultra-cool dwarf star at $\gtrsim$0.5 AU, where the maximum relative flux increase is greater than 50 parts-per-million. Based partly on previous work, I propose that the importance of refraction in an exoplanet system is governed by two angles: the orbital distance divided by the stellar radius and the total deflection achieved by a ray in the optically thin portion of the atmosphere. Atmospheric lensing events caused by non-transiting exoplanets, which allow for exoplanet detection and atmospheric characterization, are also investigated. I derive the basic formalism to determine the total signal-to-noise ratio of an atmospheric lensing event, with application to Kepler data. It is unlikely that out-of-transit refracted light signals are clearly present in Kepler data due to Rayleigh scattering and the bias toward short-period exoplanets. However, observations at long wavelengths (e.g., the near-infrared) are significantly more likely to detect stellar mirages. Lastly, I discuss the potential for the Transiting Exoplanet Survey Satellite to detect refracted light and consider novel science cases enabled by refracted light spectra from the James Webb Space Telescope.