The Similar Seven: A set of very-alike exoplanets to test correlations between system parameters and atmospheric properties

TL;DR

This study presents a homogeneous set of seven similar exoplanets to investigate correlations between system parameters and atmospheric aerosols, finding a tentative link between host star metallicity and aerosol levels.

Contribution

The paper introduces a carefully selected sample of very similar exoplanets with homogeneous parameters to test atmospheric correlations, which was lacking in prior research.

Findings

Most system parameters are very similar across the sample.

A tentative correlation between stellar metallicity and planetary aerosol levels was identified.

Predictions suggest planets around metal-rich stars have higher aerosol coverage.

Abstract

Studies of exoplanetary atmospheres have found no definite correlations between observed high altitude aerosols and other system parameters. This could be, in part, because of the lack of homogeneous exoplanet samples for which specific parameters can be isolated and inspected. Here we present a set of seven exoplanets with very similar system parameters. We analyze existing photometric timeseries, Gaia parallax, and high-resolution spectroscopic data to produce a new set of homogeneous stellar, planetary, and orbital parameters for these systems. With this we confirm that most measured parameters for all systems are very similar, except for the host stars' metallicities and possibly high energy irradiation levels, which require UV and X-ray observations to constrain. From the sample, WASP-6b, WASP-96b and WASP-110b, have observed transmission spectra that we use to estimate their aerosol coverage levels using the Na I doublet 5892.9{\AA}. We find a tentative correlation between the metallicity of the host stars and the planetary aerosol levels. The trend we find with stellar metallicity can be tested by observing transmission spectra of the remaining planets in the sample. Based on our prediction, WASP-25b and WASP-55b should have higher levels of aerosols than WASP-124b and HATS-29b. Finally, we highlight how targeted surveys of alike planets like the ones presented here might prove key for identifying driving factors for atmospheric properties of exoplanets in the future and could be used as a sample selection criterium for future observations with e.g. JWST, ARIEL, and the next generation of ground-based telescopes.