The Commonwealth of Australia is a country in the Oceania region of the southern hemisphere, consisting of the world's smallest continent, the island of Tasmania, and over eight thousand smaller islands. It is the world's sixth-largest country by total area. Australia has been inhabited for about 50,000 years by Aboriginal and Torres Strait Islander peoples. After discovery by Dutch explorers in 1606, Eastern Australia was claimed by the British in 1770, and officially settled as a British penal colony in 1788. As the population grew and new areas were explored, six largely self-governing Crown colonies were established within Australia over the course of the 19th century. On 1 January 1901, the six colonies federated and the Commonwealth of Australia was formed. Since federation, Australia has had a stable liberal democratic political system that functions as a federal parliamentary democracy and constitutional monarchy. It is a member of the Commonwealth of Nations and a Commonwealth realm. Australia has a population of about 20 million, concentrated mainly in the coastal cities of Sydney, Melbourne, Brisbane, Adelaide and Perth. A highly developed country and one of the wealthiest, Australia is the world's 12th-largest economy and has the world's fifth-highest per capita income.