Pick a Peck of Planets, Why Not Pluto?

Why Isn't Pluto Still a Planet?

Pluto hasn't shrunk really, though perhaps it is smaller than we thought it was. The main thing is that the Solar System became more populated -- in particular beyond the orbit of Neptune. There have been a number of large spherical bodies similar to Pluto which have now been discovered beyond the orbit of Neptune and Pluto. (They now want to refer to Pluto as 134 340)

The mass of Pluto was determined and it was found to be a body of mostly ice as well as being small. The mass could be determined because it was discovered that Pluto had a satellite -- Charon. By determining the orbital period of Charon and knowing the distance between the Pluto and Charon they calculated the mass of the two bodies. Pluto's mass and structure is much different from Mercury which is the smallest planet excluding Pluto.

It wasn't the size or mass that meant the change in status, but the fact that there are a number of other bodies in the area of the orbit of Pluto. The fact that there are other objects -- smaller ones even -- in the orbit of Pluto, in its "orbital neighbourhood" (Approximately 1000 objects in the Kuiper belt to date.) that it has been demoted from planetary status.

It was with the discovery of the large bodies up to the size of Pluto and even larger in the extra-Neptunian space of the Kuiper Belt that Astronomers felt a need to come up with an astronomical definition for "Planet".* The International Astronomical Union (IAU) came up with this definitions that in the solar system a planet is a celestial body that:

A Planet**

• is in orbit around the Sun
  
• has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape
  
• has "cleared the neighbourhood" around its orbit

There was originally consideration of creating "dwarf planets" as a subset of "planets" however this was dropped because it would lead to the addition of several dozen planets in the Solar System.* That would have started off with the Asteroid Ceres, Pluto, and the Trans-Neptunian bodies Makemake and Eris**.

Non-satellite celestial bodies in the Solar System have been classified as "Planets" "Dwarf Planets" and "Small Solar Bodies".

Dwarf Planet**

• is in orbit around the Sun
• has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape
• has not cleared the neighbourhood around its orbit
  
• is not a satellite

I assume that the "is not a satellite" is assumed with Planets.

Small Solar System Bodies**

• all other objects except satellites, orbiting the sun

Personally I think the definition is a bit short sighted. I think too much is based on location of the celestial body and so does not directly account for the hundreds of extra-solar planets that have apparently been discovered around other stellar bodies. I think also that there is a bit of underestimating of people with some sort of fear of their being more than 9 planets... more than 12... (Pluto, image to left -- image from Wikipedia)

Consider this while there are 10 provinces and 3 territories in Canada and 6 states in Australia, (hoping I have that right there have been some changes since I graduated from high school) there are 50 states in the United States. I don't think anyone considers redefining what a "state" is in the US in order to reduce the number of states.

Perhaps there does need be classifications of types of planets? We already do that with "gas giants" "terrestrial planets" "ice planet" and perhaps "super earth" and "hot jupiter". We might add "dwarf planet" or others?

Celestial bodies might be defined in a few ways -- by tradition, religion, and politics; by location and orbital dynamics; by mass, radius, and composition; or perhaps others... While there is perhaps some politics in the IAU decision it seems based on orbital dynamics and mass. That is fairly reasonable.

I do think perhaps a bit short sighted. I think that a person has to look at what is important. Why are we defining what a planet is? If the point is to limit what is called a planet to 8 or 9 bodies, then the definition that was decided on works. If the definition is to classify celestial bodies, I think they could do better. Definitions based on structure and composition would be far better to my mind. Orbital dyanmics might be important for locating and organizing planets when we have a large number of them.

Image above shows a size comparison between Earth's Moon, Neptune's Moon Triton, and several large Trans-Neptunian Objects (TNO). Note that 2005 Fy9 is now known as Makemake with Eris, Pluto, and Makemake being the largest three TNO known thus far and the ones they were speaking of as being in that subset of planets. -- Image from Wikipedia - Trans-Neptunian objects

My own personal bias says, if I were to stand on it or were in orbit around it, would I think it was a planet? Would I think I were orbiting it or orbiting with it? I was going to say "If I were standing on it, would I feel if I were on a planet?" but of course one can not do that on the gas giants -- some of which it is debatable if they even have a surface to stand on -- and Venus which is so very inhospitable that we can't even drop a probe on to last for very long. Still, referring to the terrestrial and "ice" planets we could imagine standing on Venus' surface or even Titan's. We would stand on a surface and look to some sort of horizon -- though I guess again that Venus' would be weird due to high atmospheric pressure.

A smaller asteroid... planetoid? ...would be different where you probably would not feel like you are standing. I can not be sure on that of course, but it is a feeling. I also think that you would feel like you were not orbiting a smaller body but orbiting with it -- orbiting the Sun, or another Planet or another Asteroid/Planetoid.

Personally I do not really agree with the differentiation between planet and satellite.*** (natural satellite rather than artificial satellite) I really do not believe that Mars would be any less a planet if it were to be orbiting Jupiter. True the formation of the Solar System would be different, but the celestial body itself would be the same. If Titan were to be in an orbit by itself, one that had "cleared the neighourhood around its orbit, between Uranus and Neptune, or somewhere else cold in our Solar System, it might be considered a planet by the definition of planet the IAU came up with.

We already have something similar in astronomy. Stars are still considered stars if they come singly such as the Sun or in pairs like in binary stars or with triple systems. If two or three stars orbit another they are still called stars. If a star orbits a star which is in turn orbiting another star, we still call it a star. Why differentiate non-stellar bodies in this way?

We do have a definite division between luminous and non-self luminous bodies. Brown-dwarfs being the smallest of normal stars which are just a bit bigger than the largest of gas giant planets. Perhaps there is overlap but there is a division which makes some sense.

There also is some sense in a division between bodies which have enough "self-gravity to overcome rigid body forces so that it can assume a hydrostatic equilibrium (nearly round) shape"* and those that can't. Now where those "Small Solar System Bodies" end and dust and then gas begin, I am not sure. But there are division points.

Still... it does give an idea of there being a continuous spectrum of masses between gas, dust, small bodies, planets, and stars -- with those being further divided with meteors, asteroids, terrestrial planets, dwarf planets, gas giants, brown dwarfs, red dwarfs... and the whole main sequence of stars. There are also other categories of stars that are different stages in main sequence stars' lives. But white and black dwarves; neutron stars and black holes; red giants and super giants and other parts of the different size stars lives are not really a part of this article.

The main thing to my mind which really says "planet" is that it is large enough to pull itself into a relative sphere. That also means it has enough gravity to allow you to stand on it and reasonably orbit it.

Still, by my personal definition, there would be many planets in our Solar System because not only would the traditional 9 planets be considered planets, but also Ceres (Image to right -- image from Wikipedia) -- which was considered a planet for half a century before reclassification as an asteroid (1800's) -- and a few other asteroids; and many of the larger satellites of Jupiter, Saturn, Uranus, and Neptune including Titan (Saturn) which has an atmosphere. There would also be the Trans-Neptunian celestial bodies which are similar to Pluto and also large enough to pull themselves into relative spheres.

We could always refer to the "Classical Planets" and the "Traditional Planets". The Classical Planets would be the planets visible with the naked eye excluding the Sun and Moon which were considered planets by the ancients and adding the Earth which I do not believe was considered a planet. The classical planets would be Mercury, Venus, Earth, Mars, Jupiter, and Saturn. The "Traditional Planets" would be those of the 20th century, or at least after the discovery of Pluto which would be the Traditional Nine Planets that most of us grew up with including Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

Then of course we have the IAU Planets -- Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

I guess it would be silly to include the 19th & 20th Century list of planets -- Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, and Pluto -- but I had to just because I am me.

Perhaps being Canadian, I can understand not wanting to have to memorize the names of over two dozen planets -- after all I am comfortable with 10 provinces and now 3 territories. I know that my American cousins are a bit more familiar with knowing 50 states and their capitals as well as all the names of the presidents -- in order I have been told. I know that despite being a bit keen in the area of Astronomy as you might have noted if you read my column -- I do not know the names of all the major satellites of all the planets and once planets of the Solar System. I probably would have a good idea of which planet a given satellite orbited if you gave me its name... perhaps fumbling the ball a bit with Triton and Titan and Charon and Chiron -- the later an asteroid. Okay now I would know Titan is around Saturn and Triton... I would fairly certainly say orbited Neptune. Charon I would be good to go with Pluto... though I keep mixing the name up with Chiron... I might also mix up the Saturn and Jupiter families...

It would be worth trying... I have to try harder to remember the names of all my cousins too... of course now a lot of my cousins have children... But we aren't demoting any of them to acquaintances or strangers.

Later!
~ Darrell

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* 2006 definition of a planet - Wikipedia.

** Dwarf planet - Wikipedia.

*** I am going to use the term Satellite rather than "Moon" because I think that the Earth has a natural Satellite named the "Moon" and other Planets have natural Satellites which each have their own names. It is a distinction I am making and I do not know that anyone else does.
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