SCRUM – How to do sprint planning when very little is known about the tasks?

My team is trying to adopt SCRUM but we are unsure about how the whole process works when too little is known about the tasks to plan effectively.

Imagine you have a the following user story:

I as a user want to predict X using machine learning given a prepared data set

Now the SCRUM guidelines dictate that in the 1h sprint planning meeting this user story should be broken down into tasks, estimated how long each task will take and then each task assigned to a team member. It's obvious that this user story is way too large to be done by a single team member. But at the same time it is not clear how it could be broken down into subtasks that can be divided upon the team members. The sensible thing to do would be to first research what machine learning methods may be viable in principle and then task each team member to investigate and build a model with one of those methods. But this can not be done during the sprint planning meeting, so it is unclear how to divide the task. Additionally, none of the team members are familiar with machine learning, so the time estimate depends on how many people work on it since everyone needs to study it.

Since SCRUM dictates that the sprint backlog can not be changed during the sprint, we can't break the big task it down later.