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We have investigated the information content in reflected-starlight spectraof exoplanets. We specify our analysis to Barnard's Star b candidatesuper-Earth, for which we assume a radius 0.6 times that of Neptune, anatmosphere dominated by H$_2$-He, and a CH$_4$ volume mixing ratio of5$\cdot$10$^{-3}$. The main conclusions of our study are howeverplanet-independent. We set up a model of the exoplanet described by sevenparameters including its radius, atmospheric methane abundance and basicproperties of a cloud layer. We generate synthetic spectra at zero phase (fulldisk illumination) from 500 to 900 nm and spectral resolution R$\sim$125-225.We simulate a measured spectrum with a simplified, wavelength-independent noisemodel at Signal-to-Noise ratio S/N=10. With an MCMC-based retrievalmethodology, we analyse which planet/atmosphere parameters can be inferred fromthe measured spectrum and the theoretical correlations amongst them. Weconsider limiting cases in which the planet radius is either known orcompletely unknown, and intermediate cases in which the planet radius is partlyconstrained. If the planet radius is known, we can generally discriminatebetween cloud-free and cloudy atmospheres, and constrain the methane abundanceto within two orders of magnitude. If the planet radius is unknown, newcorrelations between model parameters occur and the accuracy of the retrievalsdecreases. Without a radius determination, it is challenging to discern whetherthe planet has clouds, and the estimates on methane abundance degrade. However,we find the planet radius is constrained to within a factor of two for all thecases explored. Having a priori information on the planet radius, even ifapproximate, helps improve the retrievals. We urge exoplanet detection effortsto extend the population of long-period planets with mass and radiusdeterminations.

Directly imaged exoplanets in reflected starlight. The importance of knowing the planet radius