Radial velocity
Dynamical simulations covering a period of 107 years show that a second, low-mass, planet could only orbit stably if it were no more than 0.2 AU away from the star; in the simulations, these planets show oscillations in eccentricity up to an eccentricity of 0.25. Radial velocity observations rule out any such planet whose mass is greater than 4 Neptune masses.