Did you scan them? Did you look at their descriptions?
EDIT: The reason I ask is that I think what there is to find is Earth likes that say they are terraformed (no lights tho) or that they show signs of bombardment.
I think the story behind who terraformed them, when and why has yet to come out. But that is what I think 'it' is.
Or these are The Missing.
Anyway, how should we think of this quote ? :
"You don't know what I've seen. I've sailed beyond the Formidine Rift, seen the fleets of Octagonal ships blocking out the stars. I've stood on lost worlds, sterilised clean by the Thargoids and erased from humanity's maps. I've seen the graves from the Thargoid wars, countless headstones stretching beyond the curvature of the horizon, little spots of reflected light dottering the abandoned continent ..."
Is it valid at all to be considered as clue ?
At firs I thought it describes one particular world, but now I think it can describe completely different places.
1. Stars blocked by octagonal ships - what can it mean? Star systems where the weapons would be helpful?
2. Sterilised worlds - fits to those hundreds of earthlikes and waterworlds,that look completely normal (apart from their number)
3. Graves from thargoid wars - what graves? USSes with wrecks ? Something to be seen from orbit on worlds? Earthlikes or Ammonia? What is the native thargoid planetary enviroment ?
4. Headstones - well maybe we should look closer on the planets, but not only earthlikes
5. Little spots of reflected light - could be another thing, not about those heads, continent require ocean, so EL or WW, ammonia too probably
I'm not telling those are the main mysteries, but if you'll find those, you'll know you're close.