Your news story was very cool: thanks for posting it! We've thought something is a bit 'off' about the dynamics of asteroids in the outer solar system for a while, and it's good to have a working model to test.
Cal wrote:... I think they call it a 'scientific consensus', or something ...
I just wanted to address this. Space is very big and very dark. Planets are small and don't make any light themselves. The hypothesised planet, while large, is guessed to be very far away: around 20 times further out than Neptune is. This means it's not the kind of thing we could just 'happen across.'
Why didn't we search before? Well, scientists try to follow the evidence. Up until recently there was no reason to suspect a distant super-Earth would exist in our solar system; then we discovered that they are common in other solar systems, and that outer solar system dynamics are not quite right. Now we have the new data, and a new model, so we can start running experiments. It would have been silly to go looking for a planet before; we would have had no idea where to look, and no reason to suspect the search would be fruitful. Regardless, no scientist (at least when remembering to speak carefully!) would have said
for sure a trans-Neptunian super-Earth does not exist; they would have said that we haven't observed one, and that (at that time) there was little evidence to suggest one existed.
I hope this helps you contextualise how the scientific understanding of this subject has been updated.