Invading our Solar System was worth the risk for Tri-Solaris. Because of the Dark Forest nature of the universe they could not just pick a star where they thought there was no life, as life there could just be especially good at hiding. When they found earth they found a luxurious planet containing a race which was ignorant of the true state of the universe and technologically incapable of doing anything about it anyway. On top of all of this, our solar system was very close by :). Their own star system was hellish compared to ours, they needed to leave.
The final book in the series does a good job showing why Earth was a good option for them. Tri-Solarians knew far more about Earth than they could reliably determine about most nearby star systems.
The final book in the series does a good job showing why Earth was a good option for them. Tri-Solarians knew far more about Earth than they could reliably determine about most nearby star systems.