,We all used to think there were nine planets. But in 2006, the solar system was left with only eight planets, when Pluto was no longer classified as such. But is it still possible there is a planet out there beyond Neptune—possibly way beyond it?
maybe - but the IAU's definition for a planet indicates that body must clear its orbital path of debris (excepting any natural satellites or or those otherwise under its gravitational influence) - and that immediately rules out any trans-Neptunian objects. possibly there is something between the Kuiper belt and the Oort cloud but our tech is not good enough to detect such a contender for planet as yet.
The hypothetical "planet 9" that astronomers have been actively searching for during the past several years is expected to orbit somewhere between the Kuiper belt and the Oort cloud — and we absolutely do have the technology to detect it, it's just painstaking work that takes a lot of telescope time (which is in short supply since lots of astronomers are all competing for it) and is difficult to automate.