Constraining the radius and atmospheric properties of directly imaged exoplanets through multi-phase observations

Abstract

The theory of remote sensing shows that observing a planet at multiple phase angles ($\alpha$) is a powerful strategy to characterize its atmosphere. Here, we analyse how the information contained in reflected-starlight spectra of exoplanets depends on the phase angle, and the potential of multi-phase measurements to better constrain the atmospheric properties and the planet radius ($R_p$). We simulate spectra (500-900 nm) at $\alpha$=37${}^{\circ }$, 85${}^{\circ }$ and 123${}^{\circ }$ with spectral resolution $R$~125-225 and signal-to-noise ratio $S/N$=10. Assuming a H${}_2$-He atmosphere, we use a seven-parameter model that includes the atmospheric methane abundance ($f_{C{H}_4}$), the optical properties of a cloud layer and $R_p$. All these parameters are assumed unknown a priori and explored with an MCMC retrieval method. We find that no single-phase observation can robustly identify whether the atmosphere has clouds or not. A single-phase observation at $\alpha$=123${}^{\circ }$ and $S/N$=10 can constrain $R_p$ with a maximum error of 35%, regardless of the cloud coverage. Combining small (37${}^{\circ }$) and large (123${}^{\circ }$) phase angles is a generally effective strategy to break multiple parameter degeneracies. This enables to determine the presence or absence of a cloud and its main properties, $f_{C{H}_4}$ and $R_p$ in all the explored scenarios. Other strategies, such as doubling $S/N$ to 20 for a single-phase observation or combining small (37${}^{\circ }$) and moderate (85${}^{\circ }$) phase angles, fail to achieve this. We show that the improvements in multi-phase retrievals are associated with the shape of the scattering phase function of the cloud aerosols and that the improvement is more modest for isotropically-scattering aerosols. We finally discuss that misidentifying the background gas in the retrievals of super-Earth observations leads to a systematic underestimate of the absorbing gas abundance.

arXiv commentAccepted for publication in A&A. 14 pages + 7 of Appendix, 3 Tables, 9 Figures + 6 in Appendix

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