In my setting I want to destroy the world in a way where people have:
- Plenty of notice of the impending demise
- No chance of stopping it
- They can't engineer a way to survive on earth
- The rest of the inner solar system is also rendered uninhabitable
I plan to do this by having a large extra solar object, that was previously undetected, on an intercept course with our solar system and will likely be captured into an eccentric orbit of the sun in the inner solar system. This would hopefully cause all the inner planets from mercury to mars to either be thrown out of their orbits or eaten by the new arrival. I was planning on having this object be a brown dwarf or a super Jupiter.
The problem I'm not sure about is: if an object that big was under a century away I think we could have seen and discovered it already.
So my question is: What is the largest astronomical object that could be overlooked, assuming modern or near future technology, until it was only a few decades away from intercepting our solar system?