“Planets are combust when they move so close to the Sun, from our Earthly vantage point, that their rays become consumed or burnt. Each planet has a specific orb of sensitivity to the Sun’s light, anytime it falls within that orb, its rays will be combust…
Combustion is from a group of “avasthas”, which measure a planet’s temperament. The Sanskrit word for combustion is “kopa” which literally means “angry”, though I generally consider “frustrated” to be more appropriate.
The effects of combustion can be illustrated by this example.
Imagine a 2-liter pot of water on the stove. If the pot of water is filled close to the top, it will take a long time for the water to come to a steady boil. The less water in the pot the faster the water will boil. A combust planet is like a planet with not a lot of water in the pot. The more severe the combustion (the closer to the Sun it is) the closer it is to boiling, and the more sensitivity will be surrounding the use of that planet and the easier it is for us to become angry and frustrated with its use.
The water element is related to stability, peace and contentment. This translates to the capacity to receive something unpleasant with patience and love. The combust planet has difficulties in this area, as it is prone to lash out in frustration in an attempt to protect the sensitivity.
Mercury and Venus especially are prone to combustion, as they never move more than approximately 45 degrees from the Sun. The increased frequency of these “personal planets” combustion also reveals the nature of combustion to primarily affect our personal relationships.
*NOTE: Mercury and Venus are not considered malefic when combust and retrograde as they are closest to the Earth at that time and become very powerful.”