Question

stars twinkle due to refraction but then why don't planets twinkle

Answer 1

Stars twinkle because they appear as tiny pinpoints as seen from Earth, even through telescopes.

Planets don't twinkle because they are closer, and thus appear larger in our sky, as tiny disks instead of pinpoints.

Answer 2

Unlike stars, planets don't twinkle. Stars are so distant that they appear as pinpoints of light in the night sky, even when viewed through a telescope. Because all the light is coming from a single point, its path is highly susceptible to atmospheric interference (i.e. their light is easily diffracted).