Using Stellar Spectral Energy Distributions to Measure Exoplanet Parameters

The ability to make accurate determinations of planetary parameters is inextricably linked to measuring physical parameters of the host star, in particular the stellar radius. In this paper we fit the stellar spectral energy distributions of exoplanet hosts to measure their radii, making use of only archival photometry, the $Gaia$ parallaxes and $Gaia$ extinction maps. Using the extinction maps frees us of the degeneracy between temperature and extinction which has plagued this method in the past. The resulting radii have typical random uncertainties of about 2 per cent. We perform a quantitative study of systematic uncertainties affecting the methodology and find they are similar to, or smaller than, the random ones. We discuss how the stellar parameters can be used to derive the properties of both transiting exoplanets, and those where only a radial-velocity curve is available. We then explore in detail the improvements the method makes possible for the parameters of the PanCET sample of transiting planets. For this sample we find the best literature measurements of the planetary radii have mean uncertainties about 40 per cent larger than those presented here, with the new measurements achieving precisions of 2 per cent in radius and 10 per cent in mass. In contrast to much recent work, these transiting exoplanets parameters are derived without using theoretical models of stellar interiors, freeing them of the assumptions those models contain, and any priors for stellar age. As the data used are available for the whole sky, the method can be used for self-consistent measurements of the planetary parameters of a very large fraction of known exoplanets.