The concept of a chakra as an spinning thing generating energy in the human body is very legitimate and old – and can be found in a few Upanishads. But there is no consensus on exactly where they are or how many of them there are. With modern influence trying to standardize things, we tend to forget that although everyone now assumes there are seven chakras, the actual mystic scriptures present several different opinions.
The concept of planets relating to chakras is still another step removed from what was defined by previous mystics of very ancient times.
Anyway, I do personally think it is a useful tool for understanding the planets in this manner:
Saturn is the mula-chakra – which carries out lower functions and provides all the enduring strength for the rest of the system.
Jupiter is Svadhisthana chakra (naval / reproductive) because Jupiter is concerned with growth, expansion, and children.
Mars is the Manipur chakra (stomach / solar plexus) because Mars gives all the acids and fires to the body – digestion.
Venus is Anahata (heart) because Venus is about sensivitiy, generousity, love, etc.
Mercury is Vishuddha Chakra (throat) because Mercury controlls communication.
The Moon is the Ajna (brow) because this is the mental center, and the Moon is the mind.
Most feel that the Sun joins the moon in the Ajna Chakra. I think it’s more useful to think of the Sun as the Sahasra Chakra (crown of the head) – because the Sun is the soul itself – the source of the highest mental functions: identity, awareness.
Be fully aware that this is a tool and model only. I don’t really mean it literally, although perhaps it happens to be literally accurate. All I really know is that it is accurate and useful as a learning aid for understanding the planets – for those who already understand the chakras (or visa versa, to understand the chakras for those who already understand the planets).
There is also an opinion, which I find agreeable, that Rahu and Ketu are the pathways through which prana travels up and down through the system of chakras.