Earth

Earth (sometimes called Sol 3 by extraterrestrial civilisations) was a planet in which the Doctor had a "particular interest" (DW: The War Games, DWN: Doctor Who and the Terror of the Autons).

Most of the Doctor's companions were natives of this planet, as well. (DW: An Unearthly Child, The Rescue, The Chase, The Myth Makers, The Daleks' Master Plan, The Massacre of St. Bartholomew's Eve, The War Machines, et al.)

It was the twin planet of Mondas. (DW: The Tenth Planet). It was located in the Sol system in Mutter's Spiral. It was the native planet of humans, Silurians and Sea Devils — as well as other Earth sentients. (DW: An Unearthly Child, Fury from the Deep, Doctor Who and the Silurians, The Sea Devils, The End of the World, EDA: Mad Dogs and Englishmen, TW: Small Worlds) However, it attracted many alien invasion attempts throughout its multi-billion year history. Indeed its birth had little to do with its native species; its core was formed around a Racnoss webstar. (DW: The Runaway Bride)

Location
Earth was located in Sector 8023 of the Third Quadrant. (DW: Logopolis) .It was the third planet in the Sol system, approximately twenty-seven thousand seven hundred light-years from the centre of Mutter's Spiral. At one point the planet was transported to the Medusa Cascade by the Daleks, and was subsequently towed back to its normal orbit in the Solar system by the TARDIS (DW: The Stolen Earth/Journey's End). It was later shifted two light years from this position by the Time Lords and renamed Ravolox circa 2,000,000,000 AD. (DW: The Mysterious Planet)

Moon(s)
For most of its history, Earth had one moon, usually referred to simply as the Moon, a rogue planet which entered the Solar system and was captured in Earth's gravitational field millions of years before the 20th century. (DW: Doctor Who and the Silurians) By the time of the Fourth Great and Bountiful Human Empire, four other moons had been added to the Terran system. (DW: The Long Game)


 * Whether these additional moons were artificially constructed or natural ones moved into Earth orbit is not known.

Geology
Earth was rich in many valuable metals and elements. (DWN: Doctor Who and the Day of the Daleks) It was one of the few sources of quartz in the galaxy (DW: The Pirate Planet) and was also rich in silicon and carbon. (DW: Four to Doomsday)

Geography
Earth's surface changed considerably over the course of its long history, owing to its high seismic activity. When native life first appeared on the planet, for example, what later became the middle of the Atlantic Ocean was a vast, rocky wasteland. (DW: City of Death) What follows is a general description of what came to be known as "Classic Earth". (DW: The End of the World)

Through most of its history, two-thirds of the Earth's surface was covered by water. Several large land-masses existed. Changes to the topography were relatively frequent, owing to erosion and seismic and volcanic activity. Two particularly cataclysmic events occurred when Atlantis, already ravaged first by the Dæmons and then Kronos, sank beneath the waves circa 2000 BC (DW: The Underwater Menace, The Dæmons, The Time Monster) and when Krakatoa erupted in 1883. The latter event was witnessed by both the Third and Ninth Doctors. (DW: Inferno, Rose)

The actions of humans, specifically their pollution, also had a major effect on the geography of the world. By the 26th century, the seas of Earth had been polluted, irradiated, set on fire and boiled off into a thick sludge. (BNA: Ship of Fools) By the 52nd century, winters never occurred due to global warming. (WC: The Frozen) However, in the days of the Second Great and Bountiful Human Empire, the Earth was far less polluted than in the twentieth century. (DW: The Daleks' Master Plan)

Over millions of years, continental drift caused the surface of Earth to change radically, creating such features as the Arctic Desert and the Los Angeles Crevasse. After the 57th Segment of Time and the events of the Ark, the National Trust installed gravity satellites to hold back the expanding Sun, from a now uninhabited Earth. They also shifted the continents back to their earlier positions, a planetary style known as "Classic Earth", which took billions of years. It stayed like this until the planet's final destruction in the year 5.5/Apple/26. (DW: The End of the World)

Sapient native species

 * See Earth sentients.

Several sapient and semi-sapient species evolved on Earth, though the most important and influential ones were the humans and Earth Reptiles. (NA: Love and War, DW: Cold Blood)

Flora and fauna

 * See plants and animals.

In its long history, many species had evolved on Earth. By the 21st century, over three hundred billion species of animals had already evolved and gone extinct. (NSA: The Last Dodo)

Historical

 * Dozens of alien species and influences have affected life on Earth. Therefore, we have noted only the most significant and influential, or, in many cases, the earliest.

Before life
Earth formed circa 4.6 billion years BC around a Racnoss spacecraft which carried the last of their ancient race. (DW: The Runaway Bride) At much the same time, the Sentience was also trapped in the forming Earth. (NA: Nightshade)

Around 3 billion BC, the consciousness of the Sou(ou)shi settled on Earth and went into dormancy. (MA: Venusian Lullaby)

Creation of life
Native life eventually developed due to the explosion of a Jagaroth spaceship circa 400 million BC. (DW: City of Death)

Dinosaurs and Homo Reptilia
Various species of saurian creatures dominated the planet until circa 65 million BC, when a freighter from 2526 time-warped to this period and exploded in the upper atmosphere. The resulting clouds of dirt and smoke obscured the Sun, lowering the temperature and causing mass extinctions. (DW: Earthshock)

An intelligent species of reptiles (whose name is unknown) flourished and then went extinct. (NA: All-Consuming Fire)

100,000 years later, Silurians and Sea Devils, (later known collectively as Earth Reptiles) intelligent reptiles with biotechnology, were also active. (DW: Doctor Who and the Silurians, The Sea Devils) They domesticated dinosaurs and regarded the small, furry, humanoid, semi-intelligent primates around them as pests. (DWM: The Fall of the Silurians)

The reign of the reptiles came to a final end when the rogue planet (later known as Earth's primary satellite, the Moon) entered Earth's orbit. The Earth Reptiles feared it and entered into suspended animation. (DW: Doctor Who and the Silurians)

Apes, regarded by the reptiles as pests, eventually evolved into the species known as human, given a substantial jolt forward when the Fendahl Core arrived in Africa circa 12 million BC. (DW: Image of the Fendahl)

The Doctor believed that the inaccurately named Silurians had lived not in the Silurian Age, but during the Eocene, a far more recent period, despite their use of dinosaurs.

Rise of human civilisation
Human civilisation evolved quickly, assisted (either accidentally or directly) by the Dæmons, the various temporal fragments of Scaroth, the predatory nature imparted to humans by the Fendahl, the occupation by the Silence, and some visits by the Doctor. Scaroth claimed responsibility for the inventions of fire and the wheel; the Silents claimed they were on Earth since that point. (DW: The Dæmons, Image of the Fendahl, City of Death, The Impossible Astronaut)

This predatory and aggressive nature was evident in the many conflicts which humans fought amongst themselves throughout much of their early history, such as during the siege of Troy, the Dark Ages and the American War of Independence. (DW: The Mark of the Rani) The conflicts led to mighty empires and nations such as the Roman Empire, the British Empire,and the United States of America, which had great influence on humanity. It was also exploited by the War Lords, who kidnapped human soldiers from various conflicts in Earth history for use in their war games. (DW: The War Games)

20th and 21st centuries
The conflicts known as the First & Second World Wars and the Cold War would be particularly significant in the 20th century.

In 1969, the Eleventh Doctor made the humans subconsciously turn against the Silence and drove them off the planet, leaving behind their network of underground tunnels and several of their crafts. (DW: Day of the Moon) Scaroth would be killed in the 1970s. (DW: City of Death)

The space race, partially inspired by the Silence (DW: Day of the Moon), was of great significance: while Earth had been visited by aliens many times throughout its history, the launching of space probes made such visits more frequent, as Homo sapiens began drawing attention to themselves. (DW: Spearhead from Space) These visits also tended to be more openly hostile. Earth was saved from conquest or outright annihilation often only through the intercession of the Doctor or the secret (prior to the 21st century) organisation Torchwood. An attack by the Great Intelligence would finally force the Earth's governments to agree on a unified response to aliens: the United Nations body known as UNIT. (DW: The Web of Fear, The Invasion) Despite this increase in invasions, a number of aliens arrived that were benevolent.

The governments of Earth, as well as the Doctor, UNIT, Torchwood, and (by the 21st century) Sarah Jane Smith and her Ealing group, tried to keep these invasion a secret. UNIT was officially a counter-terrorist group and several incidents explained as terrorism and accidents, though this caused political scandals throughout the 1970s. (MA: Who Killed Kennedy) Torchwood used mind-wiping drugs to cover up some incidents (DW: Everything Changes), while Sarah Jane used her computer Mr Smith to deliberately falsify evidence and news reports. (SJA: Revenge of the Slitheen, DW: The End of Time) The Seventh Doctor would remark that these coverups were helped by the fact humans didn't want to notice, and were performing self-deception on themselves. (DW: Remembrance of the Daleks)

In the early 21st century, starting with the Slitheen infiltration of 10 Downing Street (DW: Aliens of London), alien incidents became far more blatant. The governments and UNIT would have a random policy of alternately admitting that some aliens existed (DW: The Christmas Invasion, The Sound of Drums, WEB: UNIT website, BFU: The Coup) and trying to desperately cover things up, such as claiming the Battle of Canary Wharf (DW: Doomsday) was because of terrorists planting drugs in water. (TW: Everything Changes) Many humans acknowledged the aliens, while others tried to claim they were fake (DW: Smith and Jones). However, several further events, most notably the forced relocation of Earth and a global Dalek assault (DW: The Stolen Earth/Journey's End) and the 456 crisis (TW: Children of Earth) served to finally convince the populace at large that humanity was not alone in the universe. Subsequent events, however, in particular the manifestation of the Time Field, removed some of these events from the memories of many, if not all humans (DW: Flesh and Stone, The Big Bang); the Doctor ultimately rebooted the universe and Earth's history, but it remains unclear as to whether humankind once again remembers these events (DW: The Big Bang) even though at least one fixed point in history - the exploration of Mars by Adelaide Brooke in the mid-21st century - is tied to memories of the Dalek Invasion of 2009.

In 2011, Earth's political and social climate changed heavily following Miracle Day, where the human race stopped dying as a result of a morphic field and yet continued to age. (TW: The New World, Rendition) This ended when Jack Harkness' blood was fed into the Blessing's antipodes. (TW: The Blood Line)

From 2019 to at least 2059, Earth went through a period of chaos: climate change, ozone degradation, the "oil apocalypse", the Great Cataclysm and a Korven invasion (DW: The Waters of Mars, K9TV: The Eclipse of the Korven). Humanity faced extinction, but not only survived, they achieved major advances in space travel (DW: The Waters of Mars) and developed weather control technology (DW: The Moonbase). In 2058, Earth's first colony on Mars was established (DW: The Waters of Mars); humanity would go on to create an early form of transmat (DW: The Seeds of Death) and lightspeed travel (DW: The Waters of Mars). From 2086 to 2088, Earth would win the genocidal Thousand Day War against Mars. (NA: Transit)

In 2089, Earth made its first interstellar flight. In 2095 a full-scale human colonisation of Mars began to reduce the overpopulation of earth. (BNA: Beige Planet Mars) Four years later, the governments of Earth, in answer to global warming, had taken vast sections of the Arctic and Antarctic and placed them inside huge domes called Snowglobes. (NSA: Snowglobe 7) By the year 2100, Earth had colonised a few nearby star systems. This had truly began humanity's great diaspora across the Universe. (DW: The Waters of Mars)

In the 22nd century, Earth suffered the rise of large corporations which would occasionally wage active war against each other, energy crises and the collapse of the tourist trade. In 2156, Earth Central declared bankruptcy and the corporations (under the Earth Alliance of Corporations) took control of the planet. Despite this, Earth's colonisation of the galaxy continued to advance. The Adjudicators were established to enforce Earth law on the colonies. (NA: Lucifer Rising)

Dalek invasions
The 22nd century Dalek invasion proved devastating to Earth. First, the Daleks wiped out fifteen colonies (NA: Lucifer Rising); next, they released a plague across the Earth. Over a six month period, Africa, Asia and South America were almost wiped out; humanity was barely able to resist when the Daleks arrived. (DW: The Dalek Invasion of Earth) New York City and many other cities were destroyed. (DW: The Chase) For ten years, the remaining inhabitants of earth were terrorised by the Dalek forces, who enslaved many and turned others into Robomen, while the Daleks attempted to turn Earth into a mobile dreadnought. Only a handful of resistance groups existed.

Following the defeat of their grand scheme (DW: The Dalek Invasion of Earth) and the shattering of their solar system blockade in the Battle of Cassius, the Daleks left Earth. The planet remained divided into feudal kingdoms for a time, as Earth refused assistance from the colonies (EDA: Legacy of the Daleks); large conglomerates, however, stepped in to rebuild the Terran Security Forces. (PDA: The Final Sanction) Thirty years after the invasion, the Earth Council existed as a fledgling government, but anti-alien organisations like Earth United began to attract many youths to their cause. (BFA: An Earthly Child)

Humanity travels to the stars
By the 23rd century, Earth had recovered and a major "break-out" of colonisation took place. (MA: The Romance of Crime) In time the planet formed a unified government and an Earth Empire emerged. This dominated most of Mutter's Spiral for over 500 years. This empire at times came into military conflict with other space powers, particularly the Draconians (DW: Colony in Space), Cybermen (DW: Earthshock) and the Korven (K9TV: The Korven). It often employed oppressive and disenfranchising policies on the indigenous inhabitants of its colonies. (DW: The Power of Kroll, The Mutants) Alongside Draconia, Earth would fight and win the Second Dalek War. Before the war, Earth and the Empire was overseen by a President (DW: Colony in Space), but from the late 26th century it was ruled by a Divine Empress. (MA: Cold Fusion)

Unrest among the subjugated alien races, instabilities in the Empire's political and economic structure, and ecological damage to Earth eventually led to the dissolution of the Empire (DW: The Mutants, MA: The Sorcerer's Apprentice), which in time gave way to the Galactic Federation, headquartered on Io. (NA: Legacy)

In the 29th century, Earth's environment was largely destroyed by solar flares. Humanity fled the planet on giant spaceships, returning when the solar flares stabilised. Autonomous nations returned during the crisis, as countries like the United Kingdom constructed their own, specific refuge ships. (DW: The Beast Below)

By 4000, Earth and its dominions were ruled by a Guardian of the Solar System, with the Space Security Service handling military intelligence. Following the death of the incumbent Guardian, Mavic Chen, (DW: The Daleks' Master Plan) however, his descendants would bring down the Galactic Federation's democracy (MA: The Crystal Bucephalus) and the Earth-ruled Second Great and Bountiful Human Empire took its place (DW: Planet of the Ood). The Empire would be succeeded by the Earth Alliance. (BFDE: Invasion of the Daleks) However, the brutal Dalek invasion of 4162 (BFDE: Dalek Empire I) would see Earth conquered and occupied by Daleks once again and would result in the Great Catastrophe that devastated Earth and its colonies. (BFDE: Dalek Empire II)

Earth would recover yet again, forming advanced national alliances before a new Ice Age and World War VI struck it in the 51st century. However, it still triggered the Great Breakout of space colonisation and created the Time Agency. (DW: The Talons of Weng-Chiang, The Invisible Enemy)

Later events
Earth, in its later history, was abandoned numerous times by its human inhabitants, first when solar flares bombarded the planet, leaving Earth uninhabited for 10,000 years. (DW: The Ark in Space) The population of Earth was evacuated in vast starships, while a small number of people were left behind aboard Nerva Beacon to begin the repopulation of the planet once conditions on the surface were suitable for life. Humans elsewhere carried on with another period of colonial expansion under GalSec. (DW: The Ark in Space, The Sontaran Experiment) While Earth was uninhabited and damaged, it was occupied by the Charrl, who renamed the world Antýkhon. Humans who had survived the flares, referred to as "Hairies", had mutated to adapt to the depleted atmosphere and soil caused by the Charrls' industrial activities. The Charrl occupation lasted some 3,497 years. (NA: Birthright) Nerva Beacon's evacuees returned to Earth after ten thousand years had passed. They recolonised the planet. (DW: The Ark in Space)

Some time after, Earth was visited by the Usurians, who relocated the dying human population first to Mars and then Pluto. Generations later, humans had thrown off the yoke of Usurian exploitation and returned to Earth. (DW: The Sun Makers)

By the year 200,000, Earth was at its height, covered with megacities and a population of 96 billion. It was the centre of the galactic domain of the Fourth Great and Bountiful Human Empire that stretched across a million planets and species. The Empire had been secretly manipulated and its progress hindered (DW: The Long Game) by Dalek survivors of the Last Great Time War. Earth underwent a century of social collapse and pollution before a full-scale invasion commenced in 200,100, including a bombing campaign which disfigured the continents. The Dalek fleet was destroyed by Bad Wolf. (DW: The Parting of the Ways) About the year 2,000,000,000, Earth was relocated two light-years from its proper position by the Time Lords to prevent the escape of aliens from the constellation of Andromeda who had stolen information from the Matrix on Gallifrey. Most of the humans remaining on Earth were wiped out. Those that survived degenerated to a primitive, tribal culture. The planet came to be known as Ravolox, with Earth being completely forgotten. The inhabitants of Ravolox eventually came to know of the true nature and history of their planet and to rebuild their civilisation. (DW: The Mysterious Planet)

At some unknown point in the future, the planet was known by the name Tellus and owned entirely by the mining company, Tellac Inc. During this period, the president of the company was the political leader of the planet as well. (AK9: K9 and the Missing Planet)

Earth's destruction
In the 57th segment of Time the last humans left Earth because of the increasing danger that it would fall into the Sun, fleeing to planets such as Refusis II and Frontios. (DW: The Ark, Frontios) The now uninhabited Earth was preserved by the National Trust. Over the following billions of years it was restored to a 'classic' Earth until it was finally destroyed by the expanding Sun in the year 5.5/Apple/26. (DW: The End of the World) However, humanity had long since relocated to a planet which they called "New Earth". (IDW: Agent Provocateur) Humanity outlived their original homeworld for many trillions of years, up until the end of the Universe itself. (DW: Utopia)

Political
Until well into the 31st century, Earth was divided into myriad nation-states of varying size, population, tactical strength, and sociopolitical, economic, and religious ideologies, all at any given time allied to or warring with each other. The first moves towards planetary union actually came in the form of military conquest and annexation of nations by leaders such as Cyrus the Great and Alexander the Great, with the Old Roman Empire, the Han Empire of China, and the Great Khanates of the Mongols among the earliest superpowers. From the 17th to the 22nd centuries, much of the military and economic strength of Earth was controlled by the British Empire, and France, and later, the United States of America, a former British colonial possession, as well as the Soviet Union (Later the Russian Federation) and China.

The earliest peaceful attempt at unifying the planet under one governing body was the formation of the League of Nations at the close of the First World War. The 1930s saw the formation of LONGBOW, an organisation set up under the League to handle matters of international security, occasionally running into threats of an extraterrestrial nature. While never made public, this was the first acknowledgement by the world's leaders that humans were not alone in the universe and their neighbours were not always friendly. LONGBOW ceased operations after it and the League failed to prevent the outbreak of the Second World War, when Adolf Hitler sent German troops to invade Poland.

The United Nations was formed at the end of the Second World War, in hopes of succeeding where its predecessor had failed in preventing global conflict. After numerous alien attacks, especially those by the Yeti (DW: The Web of Fear) and the Cybermen (DW: The Invasion), the UN, at the suggestion of Colonel (later Brigadier) Alistair Gordon Lethbridge-Stewart, drew up the charter for the United Nations Intelligence Taskforce, whose duty was essentially the same as LONGBOW's had been.

The development of nuclear weapons and the Cold War increased international tensions in the late 20th century. At many times, nuclear war seemed imminent. This threat was somewhat alleviated when the great powers entrusted their nuclear launch codes to the United Kingdom for safe-keeping (naturally - everywhere else was just foreign) (DW: Robot), and again when the planet came under the rule of the World Zone Authority in the early 21st century (DW: The Enemy of the World)

As a result of Miracle Day, some countries like India and Pakistan reduced tensions with one another, while others like North Korea started mobilising troops and the legal definition and practical use of attempted murder and suicide werethrown into chaos. (TW: The New World, Rendition, The Categories of Life, The Middle Men) Various governments established the categories of life, throwing category 1s and 2s into overflow camps and incinerating category 1s inside Modules. (TW: The Categories of Life) This quickly became public knowledge. (TW: The Middle Men)

Some time before 2050, a Korven known as Lomax hijacked the British government and ruled over Great Britain. He also used the British government to gain control of many other parts of the world. Those under the British Government's control lived under a corrupt, totalitarian regime. Lomax and his subordinates were killed during their invasion attempt. (K9TV: The Eclipse of the Korven)

The United States of America and Eurozone began showing hostilities to each other (EDA: Trading Futures) and by 2084, there were two vast power blocs in conflict and the threat of nuclear war. (DW: Warriors of the Deep)

These tensions ended with the Martian assault of 2086, when Paris was destroyed. A common, alien enemy united humans and planted the seeds for a united Earth. (NA: Transit) A brief relapse into warring feudal states followed the Dalek occupation before the Earth was again united.(EDA: Legacy of the Daleks). It was also around this time that Earth formed the Alliance with Alpha Centauri and other nearby systems, its first formal alliance with alien species. (NA: Original Sin, MA: Lords of the Storm)

The united Earth government quickly became corrupt and on the payroll of powerful financial concerns like INITEC, the Galatron Mining Corporation, the AMORB Project and especially IMC, looking the other way as these organisations victimised indigenous species and human colonists alike on the outer worlds. (DW: Vengeance on Varos, Colony in Space) In time the business policies of these corporations and the political and diplomatic policies of the Earth government became parallel and laid the foundations for the Earth Empire. Law and order was enforced by the Adjudicators. Although Earth was governed by a democratically elected leader, civil liberties in the mid-third millennium were marginalized and critics of the government were frequently sent to the Moon, which was converted from a civilian colony to a prison. (DW: Frontier in Space) In the last centuries of the third millennium, the Republic became an Imperial monarchy, with the Holy King (or Queen) ruling Earth and its empire. The last Holy Monarch left control of the Earth to a ruling council upon her death, shortly before the Solos Incident. (NA: Original Sin, DW: The Mutants)

With the advent of the Second Ice Age and continuing unrest among the colonies, Earth's government collapsed at the turn of the fourth millennium. Feudalism returned on the planet and on many of its colonies. An aristocratic government remained in control of Earth when the Galactic Federation was formed in the later years of the millennium. (DW: The Curse of Peladon) This aristocracy gradually gave way to more democratic forms, but this new era of freedom was short-lived as the Solar System came under the sway of the Chen Dynasty after 4000. (DW: The Daleks' Master Plan, MA: The Crystal Bucephalus)

By 4126, Earth was the centre of the Second Great and Bountiful Human Empire, a vast and powerful tri-galactic power. It is known that Ood slavery was common and accepted during this period. (DW: Planet of the Ood) By 4162, however, it had been replaced by the Earth Alliance, a powerful Earth-centred bloc. This was the main power fighting another Dalek invasion of the galaxy (BFDE: Invasion of the Daleks), though Earth itself was conquered for much of the war (BFDE: Dalek War: Chapter One). When it was liberated, it became a victim of the Great Catastrophe.

After this, the nations of the earth became completely independent once more. A Sixth World War loomed by the year 5000 between the superpowers of the Icelandic Alliance and Supreme Alliance. This ended with the Supreme Alliance victorious and Iceland occupied. Other important nations at the time included the Commonwealth of Australia, which itself was occupied and the Philippines, which liberated Iceland and defeated the Supreme Alliance. (DW: The Talons of Weng-Chiang) At the same time, possibly due to this, Earth was responsible for a new period of space exploration and colonisation. (DW: The Invisible Enemy)

By the 100th century, the Earth was facing civil wars and by the 102nd century it was reformed into the Junta. (PDA: Synthespians™)

Information on what forms of government existed on Earth during its later periods was scant, owing to the frequent evacuations and resettlements by humanity which occurred. During Earth's period as the planet Ravolox, the few surviving humans there appeared to live in a tribal matriarchy somewhat similar to that of the ancient Celts. (DW: The Mysterious Planet)

Economy
Variations and hybrids of capitalism, communism and socialism were practised by Earth humans throughout their history. No information is currently available on what economic forms were utilised by the reptilian species which preceded humans. Towards the end of Earth's history, Trees which traced their ancestry to the tropical rain forests had acquired sizeable fortunes, mostly in real estate. (DW: The End of the World)

From the 20th to at least the 40th century much of Earth's economy was dependent on oil, a non-renewable resource, much to the annoyance of the Fourth Doctor. (DW: Terror of the Zygons, The Infinite Quest)

In 2008, the Earth entered recession. Three years later, the Earth's economy spiralled further following the Miracle, leaving Ireland and Greece bankrupt. (TW: End of the Road, The Gathering)

Place in the Universe
Earth was the target for many alien invasions. The conquest of the planet was often for one of three reasons:
 * The need for resources
 * The expansion of empires
 * The replacement of a homeworld

The Sontaran invasion of Earth, as well as a previous claim to the planet, were motivated by the actions of the Rutan-Sontaran War. (DW: The Sontaran Stratagem/The Poison Sky, The Time Warrior). The Nimon sought control of Earth due to its position in space allowing for multiple expansions in their Great Journey of Life (BFA: Seasons of Fear).

Species that planned on turning Earth into their "New..." planets included the Zygons (DW: Terror of the Zygons, NSA: Sting of the Zygons), the Cybermen (DW: Silver Nemesis), the Nestene Consciousness (DW: Rose), the Daleks (DW: Evolution of the Daleks), the Pyroviles (DW: The Fires of Pompeii), and the Saturnyns (DW: The Vampires of Venice)

Behind the scenes

 * The first shot of the first scene of Rose (the first story following Doctor Who's extended hiatus) featured a dramatic zoom from Earth orbit into Rose Tyler's apartment. A number of Doctor Who stories have since opened with this same dramatic zoom, including The Eleventh Hour.
 * The End of the World depicts Earth orbiting its original Sun, so presumably at some point between the year 2,000,000,000 and 5,000,000,000 the planet was relocated back to its original location and its name reverted from Ravolox back to Earth.