If an asteroid had impacted Pluto and lobbed off part of it to the point that Pluto was no longer round—something that actually happened to proto-planets Vesta and Pallas in the belt between Mars and Jupiter—then it would make sense to say Pluto stopped being a planet on a particular day. If it suddenly lost a significant part of itself, Pluto would no longer be spherical, and one could legitimately question whether it would still count as being rounded by its own gravity.
Here is the problem. Who said there is a third requirement? Too many people act as though such a requirement was handed down by God on stone tablets. The truth is, it was not—it is simply the opinion of one group of scientists who prefer a dynamical definition, one that focuses on the influence objects have on other objects, over a geophysical definition, which instead focuses on an object’s intrinsic properties.
There is evidence that this “third requirement” was imposed by those who favor a dynamical definition for the specific purpose of excluding Pluto. This means that those who enacted it first decided the conclusion they wanted, then crafted a definition to fit their desired conclusion.
Commonly used by oil companies who commission studies about the connection between fossil fuel emissions and global climate change, this process looks like science but it is not science. Science does not choose a conclusion first, then fit the data to match that conclusion. Researchers paid by oil companies know where their money is coming from, which is why they arrange the data to fit their facts. This is exactly what took place in Prague in 2006 though the motivation was not money but imposing one particular view on the world.
And yes, this also applies to other dwarf planets.
Furthermore, to correct another erroneous assumption still circulating online, there are no known dwarf planets or Kuiper Belt Objects larger than Pluto. While Eris was initially thought to be larger, in 2010, a team of astronomers found it to be smaller than expected after measuring it occult a star. This method is regularly used by scientists to accurately find the size of a celestial object.
Even if there were dwarf planets larger than Pluto—and there may be some yet to be discovered—that would not have any impact on Pluto’s status. If such objects exist, they are all dwarf planets, a subcategory under the umbrella of planets—according to the geophysical definition.
Some people have a notion that our solar system cannot have “too many planets.” There is no scientific basis to this claim. The universe has billions of stars and billions of galaxies. The Milky Way alone likely has billions of planets—more planets than stars. Finding many of one thing does not mean that thing needs to be downgraded because there are “too many.”
Let’s get the message out there!