Scientists didn’t become pickier - they just later found that Pluto was in a belt of thousands of massive object (called the Kuiper belt), like the asteroid belt but much bigger.
When Ceres was discovered in 1801, it was thought to be a comet, later a planet, but after discovering it was one of many asteroids in the asteroid belt (which it wasn’t big enough to clear), they realized it wasn’t a planet.
When Pluto was first discovered in 1930, it was in a similar situation as Ceres and thought of as a planet, but when other Kuiper belt objects started to be discovered by 1992, they realized Pluto also wasn’t a planet.
They made the definition of a planet more precise after Ceres and then again when they found even more comparable objects. The definition update that changed Pluto’s category was necessary because it would have added a half dozen new planets if they kept Pluto as a planet.
Scientists didn’t become pickier - they just later found that Pluto was in a belt of thousands of massive object (called the Kuiper belt), like the asteroid belt but much bigger.
When Ceres was discovered in 1801, it was thought to be a comet, later a planet, but after discovering it was one of many asteroids in the asteroid belt (which it wasn’t big enough to clear), they realized it wasn’t a planet.
When Pluto was first discovered in 1930, it was in a similar situation as Ceres and thought of as a planet, but when other Kuiper belt objects started to be discovered by 1992, they realized Pluto also wasn’t a planet.
They made the definition of a planet more precise after Ceres and then again when they found even more comparable objects. The definition update that changed Pluto’s category was necessary because it would have added a half dozen new planets if they kept Pluto as a planet.