The innermost stable circular orbit and shadow in the novel $4D$ Einstein-Gauss-Bonnet gravity

Recently, a novel spherically symmetric black hole solution has been found in $4D$ Einstein-Gauss-Bonnet (EGB) gravity by D. Glavan and C. Lin \cite{Glavan:2019inb}. In this paper, we study the geodesic motions in this spacetime, by focusing on the innermost stable circular orbits (ISCO) for massive particle and photon sphere and shadow. Also, we find that a negative GB coupling constant is allowable in this theory, as in which case the singular behavior of the black hole can be hidden inside the event horizon. Moreover, we find that due to this extension a recently proposed universal bounds on black hole size in \cite{Lu:2019zxb} can be broken.