In order to make everything work proper here is how to proceed!

1. Once you get coordinates for Sol, send Explorer with Ion Shield.
2. First go to Terra and investigate - Events unfold and little surprise
3. After Terra go to Mars and Rebuild Mars Guardian Station (bring Constructor with Ion Shield) and research what you find there!
Do not colonize Mars to acquire the station that way - repair it straight away!
4. Events now take their course - it will be couple of years before the grand finale but its awesome when it comes !
5. Investigate and clear the rest of the Sol System. Few goodies and some background story waiting!

DO NOT DEVIATE FROM THE ABOVE!
Do NOT open and manipulate Terra, Mars Guardian Station, Terra Space-Port, Mars or the Abandoned ships in the Game Editor! Do NOT!
Their attached Events will NOT fire anymore!!!

IMPORTANT:
When you generated the galaxy before starting the run check around your HW for a storm bubble with a star in the center in 350M to 450M (in 10x10 galaxy) distance around your HW.
If you find it inside a bigger game Nebula abort and generate a new game!

You could also open the editor to check for event name <Ancient Human Capital> to find the one.
Of course this gives you a direction but unfortunately Sol spawning inside a big Nebula will defeat the Return to Terra arc.
Hopefully the developer will add some error handling to avoid story planets with their own storms to spawn in the big game Nebula´s.