Planet of Origin: Unknown
The plan 9 aliens are a humanoid species, looking almost undiscernable from normal humans, who have developed an advanced spacefaring civilisation, generally peaceful but ready to eradicate species which the predict might become a threat before they do. They travel through space using saucer-shaped ships equipped with heavy force field and operate from a network of at least 7 space stations.
They are known to have visited Earth in 1959 to study mankind and make contact with the planet's leaders. Their attempt was met with fear: as soon as their flying saucers were sighted in the sky over the united states of America, mainly Washington and California, they were greeted by heavy fire from the army.
To force mankind to better dispositions, they decided to use Plan 9, which consist of using a technology allowing the resurrection of the recently dead by stimulating their pituitary and pineal gland with remote controlled electrodes and using them to create an army of zombies.
After interviewing some humans, the judged the species dangerous as their development was putting them on a track to develop solarbonite, a substance capable of "exploding sunlight molecules" (photons?) which they would be too immature to have at their disposition: "It is capable of exploding the particles that make up sunlight itself. If contained in a weapon, it can set off a chain reaction that explodes sunlight, along with anything the exploding sunlight touches, that will eventually destroy the universe, possibly at the speed of light itself."
The aliens ruled that mankind should be destroyed to prevent this from happening. Luckily, the humans they got in contact with managed to thwart their effort and the alien ship exploded while leaving the planet.
Source: Plan 9 from Outer Space, by Ed Wood Jr.
Alien space station 7
Alien saucer over California.