From The Stargate Omnipedia
Small, unassuming balls of light, not much smaller than marbles, inhabiting the moon of M4C-862
. The creatures are capable of traveling through objects at will, and appear to possess a rudimentary, instinctual intelligence.
The entities of light are perfectly harmless and do not pass through mortal flesh unless one of two factors occurs:
1. Cadet Hailey's theory: The planet's energy fields are in a state of magnetic flux when a rogue object, either a small asteroid or meteor, is pulled over the poles, causing the light aliens to go into an angry frenzy.
2. Major Carter's theory: The beings are capable of communicating and relaying threats to one another. Not only does this mean they're intelligent, but they are also terratorial and possibly sentient.
Either theory is likely, but the S.G. teams being attacked by the creatures were not going to wait around and find out. When penetrating the body, the light beings essentially "cook" the surface of the flesh, and possibly the inside of the body as well. Repeated attacks can be fatal. Their first victim was a scientist named Thompson.
The S.G. teams determined an erratic disturbance in an electromagnetic field, such as a human's, repels the creatures. Jack O'Neill volunteered to be zapped by a zat'ni'katel in order to reach the Stargate and dial Earth.
For a matter of minutes O'Neill's bio-electric alteration repelled the creatures, but not long enough. It took a rescue by Teal'c with the zat gun to continue to fight off the creatures until the Stargate was active. Once done, the enormous super-conductor created it's own, greater field of electromagnetism, allowing the teams to escape.
See also: Pegasus light aliens
HOMEWORLD - M4C-862
FIRST APPEARED - Prodigy
Prodigy - Scientists discover light aliens within the woods of M4C-862 and decide to capture one of the creatures in a magnetic isolation container.