According to www.iflscience.com, stars twinkle because of the turbulence in our atmosphere, which shifts their light slightly. This effect is more noticeable with stars since they appear as point sources of light. On the other hand, planets don’t twinkle as much because they are closer to Earth and appear as tiny disks rather than points. This makes their light less affected by atmospheric turbulence.
“There are five planets visible to the naked eye in the sky: Mercury, Venus, Mars, Jupiter, and Saturn. Venus and especially Mercury do not stray too far from the apparent position of the Sun. As their orbit is inside Earth’s own, there is no geometrical arrangement for them to get very high from the horizon, so you’ll find them in the east or west depending on where they and the Earth are in their respective orbits.” And what about the other three planets? “The other three planets are all further from the Sun than Earth, so they can get to the zenith, the highest point in the sky – but you can always find them around the ecliptic. This is the imaginary plane in which the Earth orbits the Sun, and no planet is too many degrees off that. Finding that line is easy, but even easier is to find the star-like objects that don’t twinkle, as you do not need any knowledge of cardinal directions to do so.”
So why don’t planets twinkle? “Planets might appear to our eyes as equally tiny points, but they are close enough that they are actually little disks. Given the extended size of these disks, the turbulence of the atmosphere doesn’t affect them as much, and so their light appears not to twinkle – making them very distinct in the sky compared to stars.”
Read more here: www.iflscience.com/why-do-stars-twinkle-but-planets-dont