Contamination from a nearby star cannot explain the anomalous transmission spectrum of the ultra-short period giant planet WASP-103b

TL;DR

This study reanalyzed data of WASP-103b to determine if a nearby faint star caused its unusual transmission spectrum, finding that contamination from the star cannot explain the spectral anomalies observed.

Contribution

The paper provides a detailed characterization of the faint star near WASP-103b and demonstrates it does not account for the planet's anomalous transmission spectrum.

Findings

Faint star has a mass of 0.72 +/- 0.08 Msun and is likely bound to the system.

Contamination from the faint star has minimal effect on planetary parameters.

The anomalous spectral features of WASP-103b remain unexplained by nearby star contamination.

Abstract

The planet in the WASP-103 system is an excellent candidate for transmission spectroscopy because of its large radius and high temperature. Application of this technique found a variation of radius with wavelength which was far too strong to be explained by scattering processes in the planetary atmosphere. A faint nearby star was subsequently detected, whose contamination of the transit light curves might explain this anomaly. We present a reanalysis of published data in order to characterise the faint star and assess its effect on the measured transmission spectrum. The faint star has a mass of 0.72 +/- 0.08 Msun and is almost certainly gravitationally bound to the planetary system. We find that its effect on the measured physical properties of the planet and host star is small, amounting to a planetary radius larger by 0.6 sigma and planetary density smaller by 0.8 sigma. Its influence on the measured transmission spectrum is much greater: the spectrum now has a minimum around 760 nm and opacity rises to both bluer and redder wavelengths. It is a poor match to theoretical spectra and the spectral slope remains too strong for Rayleigh scattering. The existence of the faint nearby star cannot therefore explain the measured spectral properties of this hot and inflated planet. We advocate further observations of the system, both with high spatial resolution in order to improve the measured properties of the faint star, and with higher spectral resolution to confirm the anomalous transmission spectrum of the planet.