Observability of forming planets and their circumplanetary discs II. - SEDs and near-infrared fluxes

Detection of forming planets means detection of the circumplanetary disc (CPD) in reality, since the planet is still surrounded by a disc at this evolutionary stage. Yet, no comprehensive CPD modelling was done in near-infrared (near-IR) wavelengths, where high contrast imaging is a powerful tool to detect these objects. We combined 3D radiative hydrodynamic simulations of various embedded planets with RADMC-3D radiative transfer post-processing that includes scattering of photons on dust particles. We made synthetic images for Very Large Telescope NaCo/ERIS in the Ks, L', and M' bands as well as examined the spectral energy distributions (SEDs) of discs between 1 μm and 10 cm. We found that the observed magnitudes from the planet's vicinity will mostly depend on the CPD parameters, not on the planet's. The CPD is 20-100x brighter than the embedded planet in near-IR. We also show how the CPD parameters, e.g. the dust-to-gas ratio will affect the resulting CPD magnitudes. According to the SEDs, the best contrast ratio between the CPD and circumstellar discs is in sub-mm/radio wavelengths and between 8-33 μm in case if the planet opened a resolvable, deep gap (≥5 {M_{Jup}}), while the contrast is particularly poor in the near-IR. Hence, to detect the forming planet and its CPD, the best chance today is targeting the sub-mm/radio wavelengths and the 10-μm silicate feature vicinity. In order to estimate the forming planet's mass from the observed brightness, it is necessary to run system specific disc modelling.