tokens: light images with HST /STIS at 260 au, and estimated the mass of a potential planet to be 0.4 M jup (based on some simple analytical derivation). Isella et al. ( 2016 ) also detected a small dip in the integrated CO brightness profile at ≈22 (see their Figure 1, or Figure 5 in Liu et al. 2018 ). In our model, the gap appears in scattered light for a planet mass larger than 2 M jup , but remains undetected in the synthetic CO maps. The final profile of a planetary gap establishes itself on a viscous timescale (thousands of orbits with a viscosity of 10 −3 ), however, and the gap width and depth in our models are only lower limits. The effect of the planet appears fainter in the 13 CO channels maps than in the 12 CO maps, even if the planet is located in the midplane. This is due to optical depth and vertical temperature gradient effects: the 12 CO is coming from a vertically narrow and warm layer above the midplane, while the 13 CO is originating from a deeper, thicker layer, where the disk is almost vertically isothermal, resulting in a uniform emissivity that washes out some of the kinematics signal. Are we seeing the signature of an embedded planet? Can we exclude wishful thinking? The strongest evidence is that the perturbation to the disk kinematics is highly localized in both space and velocity. This excludes any mechanism that merely produces axisymmetric rings in disks. This excludes, for example, ice lines (Zhang et al. 2015 ), self-induced dust traps (Gonzalez et al. 2015 ), instabilities (Takahashi Inutsuka 2014 ; Lorén-Aguilar Bate 2015 ), and zonal flows (Flock et al. 2015 ). A spiral wave could in principle result from the disk self-gravity, but multiple, large-scale spirals would be expected in that case (e.g., Dipierro et al. 2015 ), which the localized deviation seen in HD 163296 would seem to exclude. The localized nature of the kinematic perturbation, that it occurs close to the gap found by Grady et al. ( 2000 ), and the similarity to the signatures predicted by our hydrodynamic models is strong evidence for a young protoplanet in a gas-rich disk. However, confirmation with direct imaging is the only way to be sure. The relatively large planet mass and its known location in the disk means direct imaging follow-up might be able to detect it, depending on how embedded it is in the disk. So far, no point source has been detected at the location of the potential planet with near-IR adaptive optics systems. A 2 M Jup planet is consistent with the upper limits (for an unobscured planet) obtained by adaptive optics systems, such as SPHERE (Muro-Arena et al. 2018 ) and Keck L’ (Guidi et al. submitted). Using the formalism developed in Pinte et al. ( 2018 ), we find that the velocity kink is located at a distance of ≈260 au, and an elevation above the midplane of ≈70 au. Assuming that the potential planet is located in the midplane, it would be at a projected distance of 2.3 ± 02 and PA = −3° ± 5° from the star, where we
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