There are a lot of large starships in sci-fi but their influence on a solar system is rarely deeply explored and kept fairly limited. For example, in Eve-Online fluff, titans are considered massive enough to alter tides on nearby planets but that's about the full extent of their effect. (I'm assuming that bit might be exaggerated since they aren't even remotely close to the size of the moon.) The question I have though is, at what mass would a starship being in close proximity cause more than just a small problem. (i.e. Creation of rogue planets, undesirable tidal/tectonic effects, rendering a planet uninhabitable or collision of a starship with a planet)
The rules for this are fairly simple:
- Bigger = objectively better. More mass = more ship. Whoever has the largest vessel and the most of them wins hand over fist over their opponent. (Obviously not true in reality but we're using sci-fi logic here)
- All engineering/practical issues are considered solved. Funny issues such as multiple day trips to travel from end to end of the vessel as well as tensile strength and thrust issues from such a large ship are not being considered.
- Destruction of the ship or the system is considered as a failure by both sides in the conflict as they seek to take and hold the system instead.
- Mass negation by exotic matter/etc is not possible
As a wild guess, I would think one solar mass would easily destabilize any star system's orbits. An additional wrinkle I foresee would be that you could have a much smaller vessel, but the total fleet mass of both sides combined could also be sufficient to pose a threat to the systems existence. I suppose at that point you would have an effective mass "budget". I'm interested to hear what you guys think.