Pluto

It’s not a planet.

Not this again.

It's smaller than the moon (Earth's moon, that is). I'm honestly amazed that anyone could even discover it in 1930. Respect for what it is, but it's not a planet.

This is a plot to erase Venetia Burney and nothing more.

It's that much older now, and things shrink as they get older.

To answer OP's question more directly - astronomers started to realize that there were many other bodies in the solar system with Pluto's general characteristics, and rather than call all of them planets, they demoted Pluto. Also, Pluto's orbit is just too erratic. We do need minimal standards.

Venus creeps me out

It’s a sphere. It has moons. It’s a planet.

Our planet is going to end up like Venus if we’re not careful

Is it just the largest chunk in the asteroid belt or what?

Venus is beautiful and dangerous. A true femme fatale.

Pluto is still just as potent in astrological charts. The U.S.A. is undergoing a Pluto return as we speak. Pluto will kick your ass wherever it shows up.

Ask Dr. Tyson...

I ran into Pluto on MainStreet at Disneyland a couple of years ago and told him that I was rootin’ for him to get his planet back. He just waved at me—I’m not sure he understood but I felt good about it anyway.

For R10 - not in the asteroid belt, though there are asteroids that qualify as dwarf planets. It's a Trans-Neptunian object, of which there are several and several yet to be discovered, no doubt. Pluto is the gateway to what we have yet to learn about the solar system and the universe, I give it that much.

“Pluto: Why did its status as a planet go down the shitter?”

We threatened copyright infringement. Pluto is ours.

Because Uranus is an insatiable bottom.