I just finished my mini Astronomy Unit in my Conceptual Physics classes so my students are very space-centric right now. Many of them approached me very excited about the possibility of a(nother) ninth planet that has been in the news. I did tear up a bit with pride when they were able to discuss if it would be a real planet as defined by the International Astronomical Union. We discuss the changing critera for planethood when we read The Pluto Files by Neil deGrasse Tyson and how that mirrors how "real science" is done. If you're interested, my Pluto Files curriculum that follows the book is here.
The IAU's current definition of a planet (as discussed here with a history of Pluto) is:
A celestial body that (a) is in orbit around the Sun, (b) has
sufficient mass for its self-gravity to overcome rigid body forces so
that it assumes a hydrostatic equilibrium (nearly round) shape, and (c)
has cleared the neighbourhood around its orbit.
Although this potential ninth planet, called Planet Nine, has not been directly observed or photographed, evidence of its existence was strong enough to prompt Cal Tech researchers to make the announcement last week. Mike Brown and
Konstantin Batygin found perturbations in the orbits of other Kuiper Belt Objects that they feel must have been caused by a planet ten times or more larger than Earth. This is how Uranus, Neptune and Pluto were found although the movement caused by Planet Nine's orbit is much more complex. This method of discovery is also something we discuss in class; when scientists noticed something off about the planets' orbits they looked for a reason. Investigation driven by curiosity! Below is an infographic from Space.com that summarizes the findings.