Humanity's original "sun, this star is what humanity compares all other stars to in their scientific measurements and descriptions of other stellar bodies.
The origin of humanity.  Once a thriving planet of life with magnificent landscapes and dramatic oceans, now it lays shrouded in mystery.  After the environment was devastated by polution that built up throughought the 20th and 21st centuries, gasceous clouds covered the planet, causing the great evacuation of the early 2100s.  Those who were able to make it to other planets on ships prepared by various corporations lived on, and those who did not were left behind, never to be heard from again.  Now, no one knows what lies beneath the toxic fog that was once Earth's atmosphere.
Once the jumping off point for humanity evacuating Earth on their way to their new homes in the cosmos, now Mars remains primarily an outpost for scientists researching the remains of Earth.  There are also rumors that traces of alien ruins are somewhere on the planet, but in humanities race to abandon their home world, these ruins were built over, and may never be proven to have existed.