New constraints on the HR 8799 planetary system from mid-infrared direct imaging

Direct imaging is a tried and tested method of detecting exoplanets in the near infrared, but has so far not been extended to longer wavelengths. New data at mid-IR wavelengths (8-20{\mu}m) canprovide additional constraints on planetary atmospheric models. We use the VISIR instrumenton the VLT to detect or set stringent limits on the 8.7{\mu}m flux of the four planets surrounding HR8799, and to search for additional companions. We use a novel circularised PSF subtractiontechnique to reduce the stellar signal and obtain instrument limited background levels andobtain optimal flux limits. The BT SETTL isochrones are then used to determine the resultingmass limits. We find flux limits between 0.7 and 3.3 mJy for the J8.9 flux of the differentplanets at better than5{\sigma}level and derive a new mass limit of 30 MJupfor any objects beyond40 AU. While this work has not detected planets in the HR 8799 system at 8.7{\mu}m, it has foundthat an instrument with the sensitivity of VISIR is sufficient to detect at least 4 known hotplanets around close stars, including\b{eta}Pictoris b (1700 K, 19 pc), with more than5{\sigma}certaintyin 10 hours of observing time in the mid-IR.