The discovery of these Pluto-sized "planets" makes it clear that we need to decommission Pluto to a planitoid. That will leave 8 planets in the Solar System. Eight is a nice number. Two to the third power.
There was another one published today in an effort to avoid getting scooped (2003 UB313) that is unambiguously larger than Pluto and very possibly larger than Mars. That makes the "what is a planet" game far more interesting. Preliminary size range estimates are 4000-10,000 km diameter, but there is an expectation it will probably be at the lower end of that range.