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Earth, known also by its Latin name Terra and administered as the Earth Sphere Commonwealth, is the third planet from the Sun, the densest planet in the Sol system, and the only world in Sol that continues to bear complex indigenous life. It is the homeworld of humanity and the capital world of the Central Galactic Union. Historically, it was also the capital world of the Terran Empire and its predecessor states. It is also a member world of the Union, structured as a hereditary parliamentary monarchy under the Archon of Earth. It briefly was administered as a Federal District after the passage of the Administrative Act but before its admission into the Union as a member state. The modern Earth Sphere Commonwealth retains many offices and positions established under the Terran Empire.

Earth also hosts several independent and sovereign city-states. As a result of Earth's importance in human society, history, and culture, it holds a unique political and economic status, and many scientific standards are based upon Earth's environment and astronomical position. Earth possesses one natural satellite, Luna, also referred to simply as the Moon. Their gravitational interaction causes ocean tides, stabilizes the orientation of Earth's rotational axis, and gradually slows Earth's rotational rate.

According to geological data, the Earth is estimated to be 4.54 billion years old. Within the first billion years of Earth's history, life evolved in its oceans and gradually became more complex before spreading onto landmasses. Since then, life has been able to flourish due to Earth's position from its sun and the planet's physical properties, punctuated by occasional mass extinction events. Earth hosts some of the greatest biodiversity of any human-populated world, with approximately 16 million species, of which 3 million have been described.

Earth's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. Around 71% of the planet's surface is covered in water, and the remaining 29% consists of continents and islands with many lakes and rivers that comprise Earth's hydrosphere. The majority of Earth's polar regions are covered in ice. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the Earth's magnetic field, and a convecting mantle that drives plate tectonics. After a climatological crisis in the 20th through 22nd centuries resulted in raised sea levels and reductions in polar ice, concerted human efforts at environmental preservation and restoration reversed some of these trends by the 25th century.

Humans evolved on Earth around 2 million years ago, reaching their anatomically modern form around 200,000 BCE and their behaviourally modern form around 50,000 BCE. They developed agriculture around 10,000 BCE, industry in the 18th century CE, and became spacefaring in the middle 20th century CE. Colonisation of space began in earnest after a century and a half of exploration and abortive colonisation attempts. First contact with sapient alien races occurred in 2397. Humanity has since become the most prolific and dominant spacefaring race in the Milky Way. Around 4 billion humans live on Earth itself, with another 4 billion on Luna and 4 billion in Earth's orbital habitats and orbital defence satellites. Earth itself once had a peak population of 11 billion in 2194, which was reduced by 2 billion during the Interplanetary War, and reduced further by an exodus of colonists in various stages over the several centuries. The Earth planetary system had a peak population of 21 billion, with 7 billion on Earth itself, in 2862 prior to the Great Pandemic of 2862-2865, which killed almost half of the Earth Sphere population. The population has remained relatively stable since recovery from the epidemiological crisis in the early 3000s. Humanity, on Earth and abroad, boasts a great diversity in societies and cultures. For the first time in its history, nearly all of the species is united in a single political entity, the CGU.

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