The Earth was the center of the Terran civilization up until 2335.
The year 2335 brought a major change in Terran history: The Sol system was sealed off by the destruction of the SD Lucifer in subspace. After the second Great War, the Galactic Terran-Vasudan Alliance might use the data they gathered from the Knossos device to rebuild a Terran portal and restore contact with Earth and the Sol system.
Little is known about Earth or its history during the events of FreeSpace. Earth seems to have been an important part of the Terran industrial base, and the Galactic Terran Alliance's economy apparently suffered greatly after it became separated from Earth. For example, after Earth was cut off, the GTVA lacked the gases to build the Prometheus plasma gun until the discovery of the nebula beyond Gamma Draconis.
Earth's fate after 2335 is unknown. The FS2 techroom description states that, without support from its colonies, Earth would have had to drastically change its political system. Two main theories emerged after the loss of Earth: the system had collapsed into anarchy, or had survived and remained a stable society.
All efforts to communicate with Earth have failed. This is odd, as Earth should have been able to pick up any radio transmissions from its colonies even without subspace communication; the nearest friendly system (Alpha Centauri, controlled by Vasudans) was only five light years away.
Though there is no canon evidence of a return to Earth and the Sol system, it is stated at the end of the FreeSpace 2 cutscene that in decades, the GTVA would be able to build a subspace portal similar to the Knossos and return to Earth. Many campaigns feature returns to the solar system.