Jupiter

Jupiter, officially the Kingdom of Jupiter, is a planet nation that is an out-of-Earth country, the fifth planet from the Sun, and the largest in the Solar System. It is a gas giant with a mass one-thousandth that of the Sun, but two-and-a-half times that of all other planets in the Solar System combined. Although mostly uninhabitable, human colonies have existed on some of Jupiter's rocky plateaus since the early 1980s, with the capital and most populous city being Gallileia. Since its colonisation, Jupiter has played an important role in the functioning of Earth. Its gaseous abysses have been used since 1988 as dumping grounds for human waste, helping to keep the Earth clean. The planet is one of the brightest objects visible to the naked eye in the night sky, and has been known to ancient civilisations since before recorded history. It is named after the Roman god Jupiter.

The first human expedition to Jupiter took place between 1979 and 1981, with multiple scientific fleets testing its ability to harbour life and become a second human colonisation. In 1982, the United States declared Jupiter as an American territory, sending 200,000 willing colonists over. Republican Senator Howard Baker became the first governor of the planet, headquartered in the new capital city of Gallileia. In 1986, the planets growing and differentiating society claimed independence from the United States and registered as an independent out-of-Earth nation under correspondence with United Nations. That year, Jupiter was established as a state ruled by a Duchy, with former Governor Baker as the first Duke of Jupiter. Infamous for its many leaders, the current Duke of Jupiter is Kerhs, a member of the Sandler Dynasty.

Wikipedia also has an article on Jupiter

Human discovery
Humans have known of Jupiter's existence for millennia, having been recorded in history for centuries and known of by ancient civilisations for many years prior. It was first viewed and described in detail by Italian astronomer Galileo Galilei, whom the nation's capital is named after, in 1610 through a small telescope.

Pioneer 10 was the first spacecraft to visit Jupiter, making its closest approach to the planet on December 4, 1973; Pioneer 10 identified plasma in Jupiter's magnetic field and also found that Jupiter's magnetic tail was nearly 800 million kilometres long, covering the entire distance to Saturn.