It's a good book, but while some elements are good sf it's not all hard sf. They're looking for a new planet and didn't even send a probe 50 years ago?
Because the trisolarans didn't know who was out there until they received a message from Earth. They were worried that if they sent a probe to another star then a more advanced civilization perhaps hiding around that star would see the probe arrive, trace the source and annihilate them.