When a planet is conjunct the Sun within 8½ degrees, it is said to be Combust. Its energy is considered to be weakened. It is combust or burned up by the Sun.
The Sun's bright rays make it impossible to see that planet, and its physical invisibility reflects its reduced strength as an independent entity beyond the Sun.
Modern Western astrology tends to regard a conjunction with the Sun to be beneficial, with the planet gaining energy from the Sun.
It has also been suggested that a Combust planet is weaker because it is so integrated with the Sun's energy that it is unable to act independently or express its own pure energy. In other words, its energy becomes absorbed into the Sun.