Question

why do planets not twinkle.?

Answer 1

planets unlike stars are not distant objects 
stars appear to shift  whenever there is a disturbance in the atmosphere
similarly,planes also shift  but  is not noticeable to us
like this, they dont  'twinkle' for us.

Answer 2

Explanation:

Planets are much closer to the earth and are seen as extended sources. So, a planet may be considered as a collection of a large number of point-sized light sources. Although light coming from individual point-sized sources flickers but the total amount of light entering our eye from all the individual point-sized sources average out to be constant. Thereby, planets appear equally bright and there is no twinkling of planets.