As you can see from my comments above, I am not at all wedded to the notion of having nine planets in our solar system. In fact, I like the way Alan Boyle, in his book The Case for Pluto, describes our solar system’s planet count: Four (terrestrials) plus four (jovians) plus more (dwarf planets). The number of dwarf planets is in flux as more are being discovered. So far, there are five recognized by the IAU that are known to be large enough to be spherical, and three more were just discovered. There are others, such as Quaoar and Ixion, for which we do not yet have sufficient data to determine what I see as the crucial factor in determining whether an object is a type of planet–that is, whether it is large enough and massive enough to be rounded by its own gravity. As for Charon, I can see it being counted jointly as a dwarf planet (part of a binary dwarf planet system) and as a moon/satellite planet since the barycenter between Pluto and Charon is not within Pluto but between the two bodies.

Dynamicists can make a case for eight planets; however, the IAU definition does a poor job at accomplishing this, with its vague terms requiring objects to “clear their orbits” and its complete exclusion of exoplanets.

It’s easy to count eight (per Pluto’s demotion) and that is a non-arbitrary criteria.

It’s easy to have 11 (including Ceres, Pluto, and Chiron) – although you’ve got to set some limits (distance or eccentricity) to leave out the other Trans-Neptune objects.

I don’t disagree that we do have a wide range of planets – gas giants, rocky, etc. and that the dwarf planets (or whatever label you wish to use) do count as planets, just as asteroids count as “microplanets”.

When there was less knowledge about the solar system it was easy to label objects as planets, moons, asteroids, and comets. But as we gain more knowledge we’ve got to have more sub-categories and objects which can fit into multiple categories, and those labels will also change as further knowledge is gained.

This is just plain not true. First, Pluto is not a “minor planet.” The term “minor planet” is a synonym for asteroids and comets, objects too small to be rounded by their own gravity. Dr. David Weintraub discusses this is his book “Is Pluto A Planet.” These objects are planetoids. There IS a strong scientific, sensible definition of planet that includes not just Pluto but all dwarf planets. It’s called the geophysical planet definition. According to the geophysical planet definition, a planet is any non-self-luminous spheroidal body in orbit around a star. In other words, if the object is large enough to be rounded by its own gravity, it is not a “minor planet” but a dwarf planet, which is a subclass of planets, at least according to this definition. The problem is only if one assumes there are only two classes of planets, terrestrials and jovians. If one accepts that there are three rather than two classes of planets, with dwarf planets being the third category, then it is not at all difficult to have a sensible planet definition that includes Pluto. Yes, such a definition also includes other small spherical bodies orbiting the Sun such as Ceres, Haumea, Makemake, and Eris. Just like we have a wide spectrum of stars, as demonstrated in the Herzsprung-Russell Diagram, we also have a wide spectrum of planets.

It should be clear to anybody looking with an unbiased view that the continents are North America, South America, Antarctica, Australia, Africa, and Asia – with Europe just a subset of Asia just as Patagonia is a subset of South America. (Greenland is no where large enough to be considered a continent – it’s just a flaw in Mercator projection maps which makes it look so large.)

Likewise looking at the rainbow it’s clear that the visible colors are Red, Orange, Yellow, Green, Blue, and Violet. Indigo is just a transition between Blue and Violet, no more of a color than Yellow-Green or Orange-Yellow (unless you’ve got a 64 box of Crayola crayons of course). From what I’ve read Newton thought that 7 was a mystical number (seven seas, seven days of the week, seven planets in his time (Mercury, Venus, Mars, Jupiter, Saturn, Sun, and Moon – the Earth didn’t count), etc.) so when he discovered that white light was actually the combination of all colors he “found” seven colors in the rainbow. It’s interesting to note that most modern pop-art rainbows (the Apple logo, the gay flag, etc.) use six colors – I suspect that’s because most folk can’t figure out which crayon (or Panatone) color to use for Indigo.

The point I’m making is it’s all arbitrary – it depends on who makes the definitions. Pluto is a planet and Europe is a continent if you want them to be planets and continents.

Culturally Pluto is a planet, the only one discovered by brute force. But from an astronomical point of view it’s incredibly difficult (if not impossible) to make a sensible definition of what is a planet which includes Pluto but not the other minor planets.

Personally I like Issac Asimov’s suggestion in 1980 (shortly after Charon was discovered). Since Pluto was so much larger than any (known) minor planet at the time, and so much smaller than the major planets it should be put into its own category – a mesoplanet (from “meso” the Greek word for middle). Any future objects could join Pluto in that class. This suggestion was made about two decades before such objects were discovered in the outer asteroid belt, Kuiper belt, and even further objects which orbit the sun.

The reason Pluto was assumed to be the size of the Earth when it was discovered in 1930 was a combination of factors – its brightness, its distance, and assumptions about how reflective it’s surface is (is it rocky or icy). There was also another assumption – that a planet beyond Neptune’s orbit was needed to account for anomalies in Uranus’s orbit. (It turns out there were no anomalies, just the limitations of the accuracy of observations of Uranus and Neptune’s orbits – including observations made in the 1800s manually, before the use of film for astronomical purposes).

With each serious observation of Pluto from 1930 to 1980 the maximum possible size shrank. My college astronomy professor suggested that if the trend continued Pluto would effectively vanish by the end of the 20th century. It wasn’t until the discovery of the moon Charon that hard estimates could be made for Pluto’s size and mass.

Rick

As for Plutos status as a planet — it was somewhat controversial. Who’d have thought that astronomers were all political, but they are. It seems at the IAU meeting where they voted to demote Pluto, there was still a lot of disagreement.. and some have thought the vote may have been “rigged”. There’s 10,000 IAU members, 2700 showed up for the 10 day IAU conference. But the vote was late the last day, after many had already left, and only 424 members actually participated in the vote.

Mickey, Minnie, Donald, et all just live across town from me (Disneyland is about 8 miles from my house.) Knott’s Berry Farm is also quite close.

Rick

2) KD5ZS…
Yes. Goofy is an antropomorphic dog, As Mickey is a 3 foot tall antropomorphic mouse.

Pluto may take 248 years to orbit the sun, but its surface is changing at a much faster rate, new images reveal.

http://www.newscientist.com/bl.....pluto.html

I always think of this exchange from “SOAP” when Billy Crystal’s character Jodie
came out of the closet to his mom:

Jodie Dallas:(listing famous people who were gay) …Plato was gay.
Jessica Tate:(pause) Wait… Mickey Mouse’s dog was gay?
Jodie Dallas: Yeah, that’s right mom. And Goofy was his lover.