"My very excellent mother just served us nothing. That is not nearly as yummy as nice pie. Darn astronomers stealing Pluto from our solar system."
That was what my sister sent me when she heard Pluto had been demoted from planethood. And, of course, she wasn't alone; many people were upset to "lose" Pluto.
Of course, we didn't "lose" Pluto. It's still out there, orbiting the Sun. You can even call it a planet if you want. All that happened is that the International Astronomical Union, for the first time, defined what a planet actually is, and that definition doesn't apply to Pluto.
Why did they do it? They tried not to. In October, 2005, they were set to define a planet as "Any object in orbit around the Sun with a diameter greater than 2000 km." That was a fancy way of saying, "A planet is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, or Pluto," since at the time they thought Pluto was bigger than all of the "potential planets" that have been discovered in recent years. Of course, that definition wasn't very precise, because the Moon (the one that orbits Earth) also orbits the Sun (if we lived on the Moon, we'd probably think of Earth as another planet sharing our orbit), and, with a diameter of 3500 km, it has no trouble squeaking in.
But then we got a good measurement of Xena. Xena, also known as 2003 UB313, is a ball of ice and rock orbiting the Sun in an orbit similar to Pluto's. Earlier this year, NASA pointed the Hubble space telescope at Xena, and found something interesting. Xena has a diameter somewhere between 2300 km and 2500 km. Pluto has a diameter between 2280 km and 2330 km. Xena is right around the same size as Pluto, and quite possibly bigger.
So why did the IAU eliminate Pluto from their definition of planet? Because they're scientists, and scientists have to deal with the real world, not with how they wish things worked. For the word planet to have any scientific meaning, we have to be able to rationally decide if some object is or isn't a planet. In order to do that, there have to be some sort of measurable criteria. Try as they might, the IAU couldn't come up with a definition that seemed important enough for what we've always meant by "planet," but still included Pluto.
But that's the great thing about science. As we find new information, we continuously tweak our explanations to match the data. As time goes by, and we get more information, we refine our predictions and explanations to make sure they still hold up. We constantly update what we think we know to match what we observe.
This is what allows us to make progress. This is why I'm able to write a blog on a computer that communicates through the rest of the world through the air, rather than writing letters that take months to cross the ocean. This is why we now have bathrooms in our houses, rather than in the yard. This is why babies dying at birth is a rare event now, rather than the norm.
And that's why now we know that we have eight planets in our solar system, plus at least three dwarf planets. That's the thing people seem to be missing. We didn't lose Pluto. We gained 3 dwarf planets (besides Pluto, one was formerly an asteroid, and the other was formerly one of those things that we didn't have a name for before). We gained slightly better understanding of the solar system in which we live.