Why do Stars twinkle and planets don’t?

The nursery rhyme ‘Twinkle Twinkle Little Star’ has always fascinated me more than any other, because unlike most other rhymes, I could relate this rhyme by just looking up to the sky and wonder why do stars really twinkle?

I did not get a good enough answer as a child and so here is my bid to help out.

Now as we know we see a star the moment its light reaches us. But before the light reached our eyes, it has to travel through the Earth’s atmosphere, which is very dense when we compare it to the vacuum of space.  Now as we would have learnt in school, light has the property to bend when it passes through different mediums or materials with different densities.

This bending of light is called refraction, the above test which most of us must have performed in school involves looking at a pencil or straw when dipped in a glass of water. The pencil will appear to bend, this is nothing but light bending when it passes through mediums with different densities, in this case from air through water.

Now the atmosphere consists of a number of different layers. Each layer here has a different density, when light passes through the atmosphere we face refraction again. One should imagine the atmosphere as a maze of mediums of different densities which make the light from a star bend a number of times before it reaches our eyes.

This constant bending of light appears to our eyes as a twinkle, whereas in reality it’s the light bending in unpredictable angles.      This is the reason we will notice stars near the horizon will twinkle more than stars above us, the reason being the stars at the horizon have to pass through more air than the stars above us.

Another fine example of this is the pictures that the Hubble telescope captures are far more detailed than the ones that are taken from the earth. The reason being Hubble is a space telescope and captures the light form the stars before the light passes through the atmosphere.

Now the question regarding planets and why don’t they appear to twinkle. The truth is actually planets do twinkle but this movement is averaged out due to the tiny size of a planet when compared to the enormous stars. When the light of a star passes through the atmosphere it starts to scatter further than the actual size of the pinpoint star that we see, which gives us the image of a twinkle. Whereas when the light of a planet passes through the atmosphere the light gets scattered over an area similar to the size of the planet we see.

We may sometimes see that planets also twinkle, this will occur when we can see planets near the horizon, as there the light has to travel through more air.

There is another theory as well. A hypostasised cloud called the Oort cloud which lies nearly a light year away from our Sun and is mainly composed of ices, such as ammonia and methane. The refraction caused by this cloud could also cause stars to twinkle. Since Planets lie within the boundaries of the Oort cloud, their light is not refracted by the Oort cloud

So the next time you look up to the sky and see a star, imagine the journey the light has taken and the beautiful illusion of the twinkle it creates. There is a lot going on in front of our eyes, we only need to question them.