One of the things teams adopting agile struggle with is deciding how much to define a plan before you start executing. Have a plan that's too well developed and you end up risking that your team may not be responsive enough to change. Too little of a plan and you may end up changing course excessively, and have no way to measure your progress towards any sort of deliverable.
At the core of this confusion over how much to plan is the reality that plans change, and spending too much time and energy creating a plan that ends up being wrong seems wasteful. But the lack of a plan means that you have nothing to measure progress against. One way to reconcile this is to keep in mind the following quote, attributed to Dwight Eisenhower (and a variant attributed to Churchill):
Plans are nothing; Planning is everything.
If we assume that as a project progresses, that things will change, we can still benefit from talking through the options as a team. Capturing the things that we don't know, but would like too, is useful information, and gives the team a good measure of risk.
The time you spend planning is an important consideration. Constrain the amount of planning time based on the duration of your sprint. If you can't come to an understanding of what the problem is or how to approach it, you have a clue that you're trying to do too much. But rather than throw up your hands, you can aim to have some sort of plan. It might be wrong, but even building the wrong thing can increase your understanding.
For the planning activity to be useful it is important that it not be top-down but that it involve the implementation team, as they are the ones who can speak to the the implementation risks, and can propose creative solutions given the ability to probe about real goals.
One thing that may concern people with the approach of involving the team and stakeholders at the same time is that a planning which raises more questions that providing answers can make some people uncomfortable. Senior managers may be uncomfortable with acknowledging ignorance. Team members may be put off by seeing that there are legitimate disagreements among the product ownership team about some issues. And some people are just uncomfortable when you can't just tell them what to do.
This is a cultural issue that may not be easy to overcome, but agile projects work well because the team can pull together to solve problems when given all of the information, and structure their code and their work to mitigate risk. And if the uncertainty exists it's better to identify it up front.
Regardless of the level of uncertainty about goals and dependencies it is important to exit a planning session with a common vision for the goals and target for when you will re-evaluate the goals. A well run planning activity can helps to focus the team towards a common goal.