How do the planets differ from the stars? The planets revolve round the sun; the stars do not belong to the sun's "mandala" [they are not part of the solar system]. If you hold a diamond in your hand and keep shaking it about, it will glitter. The stars glitter in the same way and twinkle, but the planets do not twinkle.
The sun and the stars are self-luminous. The stars dazzle like polished diamonds. The planets Jupiter and Venus shine like the bigger stars but they do not twinkle. The sun too has the brilliance of the stars[it is in fact a star]. If you gaze intently at the sun for a moment the watery haziness surrounding it will vanish. Then it will look like a luminous disc of glass floating in water and it will not be still. The moon is not like it. I will tell you how to prove the sun twinkles. Observe the sun sun's light pouring down from an opening in the roof. Observe similarly moon's light also coming into your room. You see the sun's rays showing some movement but not the moon's. The planets are also like the moon.
If the star is a big one, we may be able to see its light refracted into the seven colours(vibgyor), like the colours emanating from a brilliant diamond.
The sun is called "Saptasva" (one with seven horses--the sun god's chariot is drawn by seven horses). It is also said that there is only one horse drawing the chariot but it has seven different names. "Asva" also means "kirana" or ray. So "Saptasva" could mean that the sun emits seven types of rays or colours. It is of course the same light that is split into seven colours. In the Taittiriya Aranyaka it is clearly stated that the same "asva" or ray has seven names: "Eko asvo vahati saptanama. "
The stars are self-luminous, while the planets shine by reflected light. The light of the stars is not still. That is how we say, " Twinkle, twinkle little star ". The stars rise in the east and set in the West. The planets too travel westward but they keep moving a bit towards the east every day. It is like a passenger walking westward on a train speeding eastward. The seven planets thus keep moving eastward.
For a general background, please see here