...Pluto was only verifiably observed (discovered) in 1930. It has not even been a single century.
Stop Mars isn't a planet? As far as we know, basically all tectonic activity there has stopped.
Yeah, I realized that later.
I was thinking of the gas giants. One facepalm for me!
Anyway, Mars had
past tectonic activity, despite its core basically being currently dead, and it has everything else including a very thin atmosphere, so I guess that if a planet had
past or current tectonic activity and everything else, it's still a planet.
However, it's all a bit murky/confusing.
Ceres, as an example, has nothing like that; the only activity it has is chemical reactions that plum out into space on occasion. It's a dead, airless rock like Luna, so it wouldn't be a planet. However, Mercury is basically the same: It's also an airless, dead rock that's blasted constantly with radiation, yet it's still called a planet because it's by its lonesome and orbiting the Sun.
If we found Titan wandering around a stable orbit around the Sun, it'd meet the criteria of being a planet since it has (IIRC) tectonic activity, an atmosphere, and everything else needed to be classed as a planet...
just like Pluto, which isn't classed as a planet. They also say Titan is what Earth looked like very early on its history too, incidentally.
Basically, the whole planet/dwarf planet thing needs to be redone because it's a fucked up mess that was basically hurriedly implemented because "having dozens of planets in our Solar System would be weird", to steal a quote from above.