Astrologers chuckle when we hear people say that Pluto is no longer a planet.  Try telling that to someone going through a gnawingly agonizing Pluto transit!   But ever since Pluto was reclassified (some say “demoted”) a few years ago, a small but very vocal contingent of astronomers is fighting to classify Pluto as a planet once again.

One of these astronomers is Alan Stern, who was interviewed bySpace.com:

Just because Pluto orbits with many other dwarf planets doesn’t change what it is, just as whether an object is a mountain or not doesn’t depend on whether it’s in a group or in isolation.

What we see in the Kuiper Belt is a third class of planets, the dwarf planets, or DPs. Most, like Pluto and Eris, have primarily rocky compositions (like Earth), moons, and polar caps, atmospheres, seasons and other attributes like the larger planets. They’re just somewhat smaller.

Back before the Kuiper Belt was discovered, Pluto did look like a misfit that didn’t belong with either the terrestrials or the giant planets. Turns out that was exactly right, but now we know why: Pluto looked like a misfit because our technology back then couldn’t see that it was just the brightest and easiest to detect of a large new class of planets.

In fact, that’s why it’s clear Ceres [the largest object in the asteroid belt] was a planet all along, but was misclassified for a time because we didn’t have enough similar examples to recognize dwarf planets as their own category. Today, however, it’s clear the DPs outnumber both of the other two planet classes we know of in our solar system — the giants and the terrestrials. Which types look to be the misfit now, versus the norm?

Some people seem to be uncomfortable with that fact. I see it as just another step in the Copernican revolution that began by displacing the Earth from the center of the universe.

What we see in the Kuiper Belt is a third class of planets, the dwarf planets, or DPs. Most, like Pluto and Eris, have primarily rocky compositions (like Earth), moons, and polar caps, atmospheres, seasons and other attributes like the larger planets. They’re just somewhat smaller. Back before the Kuiper Belt was discovered, Pluto did look like a misfit that didn’t belong with either the terrestrials or the giant planets. Turns out that was exactly right, but now we know why: Pluto looked like a misfit because our technology back then couldn’t see that it was just the brightest and easiest to detect of a large new class of planets. In fact, that’s why it’s clear Ceres [the largest object in the asteroid belt] was a planet all along, but was misclassified for a time because we didn’t have enough similar examples to recognize dwarf planets as their own category. Today, however, it’s clear the DPs outnumber both of the other two planet classes we know of in our solar system — the giants and the terrestrials.

Which types look to be the misfit now, versus the norm? Some people seem to be uncomfortable with that fact. I see it as just another step in the Copernican revolution that began by displacing the Earth from the center of the universe.

For another view, see “How I Killed Pluto and Why it had it Coming” about Mike Brown, the discoverer or Eris who tried to kick Pluto out of the solar system.