Was this planet ever intended to be played in the multiplayer? When we land on the planet it immediately desyncs our multiplayer game.

It was intended to be multiplayer compatible. I'm sorry to hear this. It was tested (and we did reach the planet without desyncing), but I can only do so many tests because it requires multiple people. The event system has been reworked since the last round of multiplayer testing, so it is possible that this is the cause.
1) What version of Rubia are you on?
2) Exactly when does the desync occur? Many functions are called when trying to land on the planet. Knowing exactly when during descent you have a desync would help tremendously.
3) Do you have a desync report?

I got on a multiplayer run again, this time with Samario to stress test the landing on Rubia from a few save files. We had no desyncs. Rubia appears to be multiplayer-compatible as far as we can tell.

I would need your save file to be able to diagnose your issue, and probably also mod list.

My friend started to attack flying meterors on the Rubia with his gun and laser defenses, it immediatelly causes a desync

That helps, but I would really need a save file AND desync report. Multiplayer testing requires me to get other people together, and their patience with me is dwindling after the last time we spent an hour of failing to reproduce your bug. Without a save file and desync report, I cannot justify getting a group together to test the fixes.

The most I can say is that I have heard about desync issues around personal laser defense, but no one provided anything I needed to track it down and fix it.

I fixed desync issues with another user, and changes should be available in the newest version. Thank you for your patience.