Floating discs often depicted behind or above the face of a divine being in a bursting golden color are commonly believed to represent light, glory, and dawning in religious context. Yet a halo at first glance with no prior knowledge, seems to simply resemble the shining sun, similar to the solar disc Egyptian god Ra is usually shown with. Ra combines all aspects as that of the sun to science, as a necessity for life, bringer of warmth and growth.
Upon closer inspection, the solar disc is not just imagery of the sun, as it is also wrapped with a serpent coil called a Kundalini; This is representative of potential energy rising to open the Chakras, on the path to rebirth; As the sun does each day. The halo in Egyptian reliefs depicts a life-giving sun, and appears to signify this more than designations to Ra as glorious or divine as those aspects are implied as part of the nature of Ra, the god of the sun. Religions other than Egyptian tend to utilize this iconography to identify individual glory, the light, and divinity.
In Buddhism, a colorful circular disc emanates from the body of Buddha after achieving enlightenment. A halo's color of light symbolizes the level of spiritual elevation, similar to the rising and setting sun, as the sun appears to change the sky's color throughout the day. Sunlight in its purest form is white, containing a full spectrum of colors, of which only certain wavelengths are visible depending on the current atmospheric conditions. Enlightenment is a gentle awakening of conscious energy, a way of connecting to the universal mind, becoming whole with nature and the divine. This idea of self-realization frees an individual from bounds of consciousness when awakened, opening the mind to a new world.
To many people, prominence of the halo is most recognized from Christian artworks dating back as far as the 4th century to designate the divine. Essentially this element commonly found in religious artwork was an interpretation of light and glory emanating outward. The halo often appears like the sun is located directly behind the head, mimicking its power and life-giving nature. Even-though light travels on a dimensional plane, artworks tend to depict the halo as a flat disc. Like Buddhism, these icons represent a level of divinity, imbued with the energy of God. The bible refers to the halo growing in size during Christ's transfigurations in prophet revelations, another key aspect to reaching enlightenment. 
Modern ancient alien theories about the halo point in a different direction, contending the circular disc of light is actually representative of an ancient astronaut helmet in a simplistic sense, like looking at a bubble over someone's head face-on. Perhaps this means our ancestors were ancient astronauts themselves yet...
Somehow alien astronaut theory neglects the related stories, legends, and modern enlightenment practices still thriving today in this sense; The foundation of why our ancestors encircled heads of the divine with an all powerful light similar to the sun. This points to a greater likeliness, as in the case of ancient aliens, Earth was once visited by beings from another world who were capable of emanating light outward from their heads, and mankind interpreted this in representation as those beings as divine from the heavens, since they indeed descended from up above.
