Re: "Pluto Now Called a Plutoid"

K_h wrote:
Alan Stern's been making some peculiar statements on this subject even though he really should know better. He says in that article, for example, that "Earth would not be considered a planet if it orbited the Sun beyond Neptune, because its gravitational influence would be insufficient to clear out the Kuiper Belt." By _his own method_ of determining orbit clearing capacity, however, I've shown that it'd be about as capable of clearing its orbit as Mars is (see my previous post linked to above). So if Mars is considered a planet based on its orbit-clearing prowess then Earth in the Kuiper belt would be too.

I agree with Stern that it would not clear out the Kuiper Belt. So, under the IAU's definition it would not be a planet. That is very bad.

Alright, one more time, in more detail.

Back in 2000 Alan Stern and Harold Levison presented a paper to the IAU that described a method for calculating how rapidly an object would clear other objects out of its orbit and proposed classifying planets based on this. The paper can be found here: <http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf>

Calculating the full Stern-Levison parameter is complex, but it is proportional to the mass of the object squared divided by its orbital period so _comparing_ two objects is fairly straightforward. You can find a table at <http://en.wikipedia.org/wiki/Cleared_the_neighbourhood> that lists the Stern-Levison parameters for various objects in the solar system normalized against Earth's Stern-Levison parameter. Notably, the planet that's least able to clear objects from its orbit is Mars, with a Stern-Levison parameter 0.0061 times that of Earth. The dwarf planet that's most capable of clearing its orbit is Eris, with a Stern-Levison parameter 3.5*10^−8 times that of Earth.

A hypothetical Earth-mass object whose orbit lies in the middle of the Kuiper belt (the belt ranges from 30 to 55 AU, so the middle is approximately 42.5 AU) has an orbital period of 277 years. So to compare its Stern-Levison parameter to that of Earth's, you'd do the following calculation:

(((1 earth mass)^2) / (277 years)) / (((1 earth mass)^2) / (1 year))

If you really don't trust me or can't otherwise work it out yourself, Google Calculator can handle it via this link here:
<http://www.google.com/search?q=(((1+earth+mass)^2)+%2F+(277+years))+%2F+(((1+earth+mass)^2)+%2F+(1+year))>

The result is 0.0036. So the hypothetical planet is 0.0036 times as capable of clearing its orbit of debris as Earth is capable of clearing Earth's orbit. This is slightly more than half the orbit-clearing capability of Mars, but more than 100000 times the orbit-clearing capability of Eris. The object should be able to keep its orbit about as clear as Mars keeps its orbit.

This is based on Stern's own work. I don't know why he seems to think such an object wouldn't clear its orbit now, but until he (and you) can present some actual support for the statement that contradicts the above I can only conclude that it _just ain't so._ The numbers don't lie; that hypothetical object is as good an orbit-clearer as an object that is quite solidly classified as a planet, and so would itself be grouped in with the planets on that basis.

So, either point out where I made a math error above, point out a reference where someone's shown the Stern-Levison method to be in error, or give it up. This statement you keep repeating is demonstrably incorrect.

As astronomers learn more and more about other solar systems, more and more unexpected discoveries arise. It is premature to cleaim that it is "unlikely to produce many ambiguous cases".

It doesn't make much sense to claim that something is "unlikely" _after_ it's been thoroughly tested, does it? We'd already know then.

In this case, the theoretical basis for the prediction is quite well understood. An object's gravity grows when it gains mass, which in turn helps it to gain further mass. Do you have some reason to expect otherwise?

My earlier calculations showing that a 0.7-Earth mass _would_ clear an orbit in the Kuiper belt aside, have you heard of the Kuiper cliff? It was also mentioned earlier in this thread, you can read about it here: <http://en.wikipedia.org/wiki/Kuiper_belt#.22Kuiper_cliff.22>. It may be a case of orbit-clearing by a yet-undiscovered planet.

"May be the case" is the operative term. It may be that a 0.7 Earth mass world has not cleared out its orbit, far out in the Kuiper Belt, as Stern has pointed out. If that turns out to be the case then it will be yet another example of why the IAU's definition is so flawed.

Howso? An object that's orbiting _that_ far out seems pretty distinct from the other objects closer in that are called planets. And we're not going to be detecting things like that any time soon, either. By my calculations in order to reduce a 0.7-Earth mass's orbit-clearing capabilities down to the level of Eris it'll have to have an orbital period of about 10,000,000 years. That's a mean orbital radius of 46,400 AU, or 0.73 light-years. Well out into the outer Oort cloud.
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