After the earth was used up. Mars' barely terraformed pale green planet proved to be still too hostile and small to accommodate the earth's remaining population. The Mars and Earth governments decided to collaborate on an exodus to a new star system. Dissolving all countries into one world effort to leave the solar system, the Human Nation was formed. Cannibalizing the Earth's last resources, the HN built the ships that would one day bring them to new worlds.
We being a sentimental species, all monuments, buildings and cities were scanned in incredible detail and uploaded into a grand simulation. Every book ever written, the oldest recordings still intact, even the whole of the internet were all copied and pasted into the supercomputers of a very important highly protected and shielded ark ship. Everything. It was the most involved project of the most involved effort in human history. The ark ship was duplicated into 5 ships, all identical in content to make sure no information was lost in the mass exodus.
30 years after Exodus (A.E.), the ark ships are headed in the direction of a star system that seems to be more ideal than the original. The system has 3 earth like planets(sufficient gravity, atmosphere, and liquid water) that are all well within the habitable zone. (One of which they will name Newton because its gravity is nearly that of Earth's). The astronomers, cataloguing every turn and looking at every star system from an ever changing perspective, find a new star system 5 light years away from their intended target. The system had a much wider habitable zone with at least 3 earth like planets but the astronomers were still counting.
As the years went by and the news got out, there were some people who started to see the newly discovered system as a more viable option. The majority of the science community, however, saw that the system had too many unknowns and was not properly vetted for safe living conditions and advised the population to stay the course. The popularity grew, still for the new system and so the HN council had to come to a decision. It was decided that the purpose of the exodus was to keep the population going and it was in its best interest to spread to new worlds. The HNC gave everyone a choice: stay together or split up.
The overwhelming majority chose to split the population and go off to live in both systems. The ark ships each contained enough mining equipment and settlement startups to start on a new planet and live long enough to be self-sufficient thanks to Mars' experience and the portability of the life support systems on all ships. The HNC allowed people a year to say goodbye and shuttled them between ships to allow for a clean split. The process and negotiations set the exodus back 4 years (since you can't shuttle at lightspeed) but it was a clean split for the most part.
There were some casualties of spirit when kids and young adults still under their parents care had differences in opinion. There were also riots when, in the interest of preserving genetic diversity, some were forced to go or stay.