The Earth is a planet in the solar system which is featured in Plazma Burst: Forward to the Past, Plazma Burst 2 and Plazma Burst 2.5, and is the home of the Civil Security. Its population during the games' events is roughly 7-8 billion humans. The planet is also the original home of the Human faction on Mars, but how often they communicate back to Earth is unknown.
Earth has seen many attempts to formally unite, but these attempts ultimately failed and led to separation of countries and other territories.  It is not mentioned in the games what countries currently exist on Earth, or if any new attempts to unite the world have succeeded or not.
Factions and relations
The Civil Security are the Earth's main army. They are the major representatives of Earth and its technology, as they use a wide variety of weapons, gear, and vehicles. The Civil Security have unknown goals, but judging by their name and the fact that they function as an army, it could be seen that they are there to protect civilians in a militaristic manner. Whether this protection is wanted or not is also unknown.
The Usurpation Forces are a group of aliens which hail from a completely unknown planet in an undiscovered area of the universe. They launched an assault against Earth with undescribed reasons, and are shown to be much more technologically advanced than humans are. However, despite this, they heavily use stolen human technology for unknown reasons.
The Human Soldiers are a small faction on Mars, who utilize the Grubs found there. It is currently unknown why they are there, or who they communicate back to at Earth, if anyone.
The Falkoks are a generally peaceful faction of aliens from an unknown area. They do not see any benefits from capturing Earth, due to the fact that Earth's gravity is different than their native planet, which affects them in unknown ways, and therefore leave it alone. Unlike the Usurpations, they are just as advanced as humans, which could also be another factor of their neutrality. They are also found on Mars, and have possibly come into conflict with the Human Soldiers also stationed there.
The Earth's inhabitants can be considered very advanced. The previously mentioned Civil Security has developed vehicles such as the Hound Walker-CS and firearms such as the CS-BNG. Another former faction, Correction Nine, is responsible for the creation of the Time Machine, a machine which allows people to travel backwards and forwards in time.
Ultimately, the humans' curiosity and advancement in technology led to their demise and their planet's capture, as long-range portal tests helped the Usurpation Forces pinpoint the location of the Earth.