Twinkling, or to use the astronomical term, scintillation, is a product of the atmosphere, not the observed body. With clear, cold air scintillation is minimal, but given an atmosphere that is thermally unstable, all objects twinkle. Planets however, are actually tiny discs rather than a point light source, so in marginally unstable air, their scintillation averaged out over a larger area, so they appear to less twinkly.
That makes sense since they are not the source but a reflection of the source which also twinkles from a very far distance! LOL I see double so I see lots of stars.