Left: the mass and semimajor axis of an additional planet in a circular orbit whose angular momentum equals the angular momentum deficit (AMD) for the HD 17156 system. The dot indicated the known mass and semimajor axis of HD 17156b. Right: eccentricity of HD 17156b as a function of the semimajor axis for the additional planet. The horizontal dashed line indicated the current eccentricity of HD 17156b.