2020MNRAS.491.1795P

Direct imaging is a tried and tested method of detecting exoplanets in the near-infrared (IR), but has so far not been extended to longer wavelengths. New data at mid-IR wavelengths (8-20 µm) can provide additional constraints on planetary atmospheric models. We use the VLT Imager and Spectrometer for the mid-IR (VISIR) instrument on the VLT to detect or set stringent limits on the 8.7 µm flux of the four planets surrounding HR 8799, and to search for additional companions. We use a novel circularized point spread function subtraction technique to reduce the stellar signal and obtain instrument limited background levels and obtain optimal flux limits. The BT SETTL isochrones are then used to determine the resulting mass limits. We find flux limits between 0.7 and 3.3 mJy for the J8.9 flux of the different planets at better than 5σ level and derive a new mass limit of 30 MJup for any objects beyond 40 au. While this work has not detected planets in the HR 8799 system at 8.7 µm, it has found that an instrument with the sensitivity of VISIR is sufficient to detect at least four known hot planets around close stars, including β Pictoris b (1700 K, 19 pc), with more than 5σ certainty in 10 h of observing time in the mid-IR.