Alien invasion An alien invasion was the process of any sapient species invading another planet and attempting to conquer or eradicate the natives . Invasions of Earth . Many extraterrestrial species tried to invade Earth from the 20th to the 22nd century . Human s were a developing race ; by sending space probe s into space , they attracted the attention of more developed races . Organisations such as UNIT , the Torchwood Institute and the Department ( which was created by aliens plotting to invade Earth ) developed defences against these attempts . ( TV : " Spearhead from Space " , " The Christmas Invasion " , TV : " Regeneration " ) Some of the most persistent species who attempted to invade the Earth include : Other species that attempted to invade Earth include : Several ( if not most ) of these were prevented ( for the most part ) by the Doctor in various time periods , ( TV : " The Dalek Invasion of Earth " , " The Invasion " , " The Unquiet Dead " , " The Christmas Invasion " ) but in the 21st century a group of neighbours and friends led by the Doctor ' s former companion Sarah Jane Smith did their part in preventing alien invasions . ( TV : " Invasion of the Bane " , " Eye of the Gorgon " , " Secrets of the Stars " , " The Temptation of Sarah Jane Smith " et al . ) While this group dealt with smaller threats , the larger invasion attempts were usually dealt with by the Doctor . ( TV : " Journey ' s End " ) Later in the 2050s , K9 Mark 2 became part of a group which safeguarded the Earth in the same fashion as Sarah Jane . ( TV : " K9 " ) Invasions of other planets . During the human colonisation period of the mid - 3rd millennium to the 51st century and beyond , humanity invaded various planets for their empires , including at least half of Mutter ' s Spiral , including Delta Magna ( TV : " The Power of Kroll " ) , the Althosian system ( PROSE : " The Pit " ) and many of the worlds in neighbouring galaxies such as the Andromeda Galaxy . ( TV : " The Ark in Space " ) The Vardan s and Sontaran s attempted to invade Gallifrey , the Doctor ' s home planet . ( TV : " The Invasion of Time " ) Motives . There were many motives for alien invasion . Some species invaded due to the destruction of their homeworld , ( TV : " Terror of the Zygons " , " Rose " , " The Parting of the Ways " , " Invasion of the Bane " , " The Day of the Doctor " , et al ) or the rendering of their homeworld uninhabitable , ( TV : " The Tenth Planet " , " The Seeds of Death " , et al ) whilst others invaded to expand their empires . ( TV : " The Christmas Invasion " , et al ) One particular case had the Earth invaded as part of a plan to power a weapon . ( TV : " The Stolen Earth " / " Journey ' s End " ) The Sontaran Commander Kaagh believed the common reason was that human s had ' no guts ' , remarking that it was ' no wonder ' that half the species from Earth to the Verlixix Spiral Cluster had plans to invade the planet . ( TV : " Enemy of the Bane " ) The Twelfth Doctor , after hearing from a human that there was a horror movie called " Alien " , commented that with that " really offensive " attitude it was " no wonder everyone keeps invading you . " ( TV : " Last Christmas " ) Some unknown power , organisation , or person had classed invasions by type . A type F invasion was said to be aesthetically motivated . ( PROSE : " The Well - Mannered War ) "