[lsadam0]

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.

[ajuc]

If you're afraid that a planet will have life, then why choose a planet where you are sure there is life?

They were betting their existence on a species they knew already uses MAD as a basis for its' survival not figuring out that it could use MAD to defend themselves.

Their system was interesting, but ultimately not much worse than ours, once you leave the planet. And they have to leave the planet anyway.

Once you have nanomaterials allowing cheap access to space - putting your population in space habitats is cheap and easy. We could do it now, if we had space elevator. The cost of materials and energy is negligible compared to invasion of another star system. Especially with such conveniently storeable population :)

> Tri-Solarians knew far more about Earth than they could reliably determine about most nearby star systems.

There's nothing stopping them from figuring it out. You don't have to invade a system, you can send a stealthy probe. I would assume in their circumstances it would be a wise thing to do if they really don't want to live in space further away from the center of gravity of their star system.

Besides, if the dark forest is so important - why invade Earth at all and play with fire? You can kill everybody on Earth without bothering to show up or sending any warnings. Send a big meteor their way or make a deadly disease that looks like flu, infect everybody, and on set date kill all hosts. Much cheaper, and no risk of ceasing to exist.

What they did in the books was very risky and wasteful for no good reason.

[lsadam0]

Spoilers Ahead!

> betting their existence on a species they knew already uses MAD

If ant colonies depend on MAD for continued existence, it means nothing to us. This example was used multiple times throughout the series. They didn't even make a singular bet on Earth. The invasion was just one avenue they chose for survival.

> You don't have to invade a system, you can send a stealthy probe.

In the dark forest of the three body universe you could send a probe, but you learn nothing if it finds no life in a system. The only reliable, actionable information a probe can provide you is if life is found. Finding an absence of life just as easily means that the life in the system is so far advanced beyond you that you cannot detect it. It would be far riskier to bet your future on a system in which you found no life. Not to mention that you are making a huge gamble on the stealthiness of said probe. The probe could be the very thing that makes you the target of a dark forest strike. Earth already appeared safe, so why take further action that carries real risk of notifying some other civilization that you exist? You can't be reliably stealthy because some other race can be technically beyond you.

> Send a big meteor

Earth was an un-imaginable eden to Tri-Solaris, it's made clear that they wanted the planet intact.

> or make a deadly disease that looks like flu

This requires a much longer timeline as multiple trips are required to carry out the plan. The Tri-Solarans were facing a very real threat of extinction that made a shorter plan worth the risk. It's made very clear that they did not wish to exterminate humanity.

Earth was valuable because in all probability the rest of the universe did not know it existed. Given that you are saying they were stupid for not building space habitats, you must not be aware of what actually happened to the Tri-Solarans :). Besides, those space habitats present their own form of danger to their inhabitants. Actually, the danger they pose is worse than existing on a planet.