What distinguishes a planet's steady light from a star's twinkling when observed from Earth?

Answer

Planets appear as tiny disks, making their light steadier.

Stars twinkle because their light is refracted by atmospheric turbulence acting on a pinpoint light source; planets appear as tiny disks, resulting in steadier, non-twinkling light.

What distinguishes a planet's steady light from a star's twinkling when observed from Earth?
astronomystarplanetMoontonight