Earth

Earth, sometimes called Terra, is the homeworld of Humanity and the capital of the Terran Federal Republic. It is the second most populous planet in the Galaxy. Earth has immense cultural, societal and religious significance to humans. Additionally, it is the political, economic and diplomatic heart of the Terran Republic. The planetary administration is the Commonwealth of All Peoples of Earth. 
Earth has hosted sapient life for the longest time of any homeworld: whilst other spacefaring species appeared around 50,000 years ago, humans appeared over 250,000 years ago. Humanity was however one of the last species to achieve meaningful space travel. Alien scholars consider humans to have entered their space age in the early 2200s, over a hundred years after Earth was unified by a single government. Earth has also unlike most other species' homeworlds hosted nation-states rather than a permanent single government. Humanity is notorious among species for being incredibly divided. However, with centuries spent under Terran federal rule, these divisions faded away, and Humans from Earth are now considered similar societally to many aliens when it comes to internal divisions. 
Earth is different from most other planets in and out of the Federation, as most of its population is concentrated in arcologies. These encompass over half the population, and can house over half a billion people. The largest one of these by volume, the Tower of Babel is considered one of the Six Galactic Wonders, and is home to over half a billion people. The remaining population is spread out in the so-called Old Cities, which were built before the Unification. These house most of the government institutions and most prestigious institutes and universities. Additionally, Earth's districts retain major elements of pre-Terran cultures to a much greater extent than any other population centre in the Federation. 
Earth today is home to over 9 billion people. This however is not the peak population Earth reached, as in the late 2030s earth's population was nearly 10 billion. The population was drastically reduced during the mid-21st century crisis, notably during the Great Famine, where the population of Africa and a large chunk of Asia was reduced nearly tenfold. The Unification Wars further reduced Earth's population, and by the time of the Unification the population was only around two billion. Earth's population has steadily grown ever since, with no large bumps in population since the first years of the Federation. Earth has the highest national emigration rate of any planet in the Federation, but it also has the highest international immigration rates. Earth has every sapient species represented on it. Since the formation of the UCO and the end of the Great Galactic War, millions of aliens have migrated to Earth. They now make up around 10% of the total population, or around 910 million people. The most numerous are the Dermkha, Surben and Tesrat, which are biologically and anatomically close to humans.
Where Earth is the homeworld of humanity and a planet of idyllic beauty and wondrous biodiversity, Terra is the ultimate symbol of the Terran Federal Republic. It stands as the single most powerful planet in the known galaxy. In this role, it is known as Caput Imperii. Its symbol is a crowned lion rampant d'or on sable, in one hand holding a sword and in the other the Milky Way.