Astronomy Cafe (#1), Pluto and Charon barycenter

[GrapesOfWrath]

Another recycled thread.

Sten Odenwald's Astronomy Cafe (aka The Space Scientist, aka Ask the Astronomer) is the number 8 link on the BA's Top Ten Picks, and I enjoy reading the Space Scientist pages, but I've found some errors.

The Space Scientist has a list of errors webpage, but it is incomplete. In fact, the last paragraph says that he'd corrected Where is the center of mass of the Pluto-Charon system located? to say that the Pluto/Charon barycenter was 700 miles beneath the surface of Pluto. Pluto's radius is less than 700 miles, and he meant 700 kilometers, of course, as it does say in the answer, but even that is wrong.

The answer has the following equation:
19,700 x (0.000123/(0.0024+0.000123) = 19,300
The left hand side is actually equal to 960, more or less, not 19,300, which would put the barycenter 200 km below the surface of Pluto, instead of 700 km.

Still, the mass of Charon appears to be three times as great as that, and the barycenter is actually 1200km above the surface of Pluto.

[GrapesOfWrath]

Dang. I guess these two questions rightly belong in Bad TV! Bad Movies! No doughnut! Sorry about that.

[GrapesOfWrath]

I added #3 to that forum.

[jkmccrann]

Pluto & Charon are thus a truly double-planetary system then are they not? So I have to wonder why they aren't referred to as such in the current literature, and is this because the definition of a `double-planet' has not yet been settled?

[mantiss]

Well since the status of "Planet" for Pluto is debatable, I think it is best that the qualifier "double planet" remains unused for the time being.

[jkmccrann]

I'd definitely have to agree with that mantiss, as I don't consider Pluto a planet either and feel it will be reclassified in due course, but would that make it a double KBO? Sounds rather cumbersome and i guess the Pluto-Charon system is still unique in at least that regard?

[hhEb09'1]

I'd definitely have to agree with that mantiss, as I don't consider Pluto a planet either and feel it will be reclassified in due course, but would that make it a double KBO? Sounds rather cumbersome and i guess the Pluto-Charon system is still unique in at least that regard?

unique enough to make it a planet, maybe. Charon is about an order of magnitude smaller, mass-wise, than Pluto.

The whole barycenter issue is deceptive--I think the barycenter of my baseball and Sirius is outside of the surface of Sirius. If not, I'll just toss it farther away.

[jkmccrann]

Yes, but since when has your baseball been in orbit around Sirius? Anyway, i think we're talking apples & oranges if we start arguing viewpoints like that. I don't think you can draw the analogy you've drawn and use it in relation to Pluto-Charon.

[hhEb09'1]

If the Sun is in orbit with Sirius, then surely so is my baseball.

But you're right, analogies are in the eye of the beholder. I think I can quantify the argument a little bit. Pluto is about 8 times the mass of Charon, which is 9 Pluto diameter away. If it were only 4 1/2 diameters, the barycenter would be inside Pluto. The difference is smaller than the Earth diameter--but under that scheme it would mean the difference between Charon being a moon, and being part of a "double".

[Kaptain K]

And if it was only 2.4 diameters, it would be a ring and the question would be moot!

[Planetwatcher]

Weither or not Pluto and Charon is a double planet system, is as moot of an arguement as weither or not Pluto is considered a planet.
While everyone is entitled to their own personal opinion, in the face of modern astronomy, and those who make the official designations of the science, personal opinions differing from the mainstream are just that.

Officially Pluto is a planet, weither anyone likes it or not. Just as officially, Charon is Pluto's only known moon, again weither anyone likes it or not.

If opponents to the official designations are willing to spend 15 to 20 years in college, and achieve at least half a dozen or more different PhDs, then write and publish a whole bunch of very advanced scientific research papers which could revolutionize the science, then I suppose their opinion could could be seriously regarded as credible.

But short of that, all of us on here are just expressing personal opinions. Unless of coarse we are quoting the respected scientists who have gone to the lengths which we haven't.
This doesn't mean we are not entitled to our opinions, just that they don't matter.

Just so that I'm not misconstrued, I am not speaking here as a moderator, but just another member like everyone else. So if someone does not agree with me, that's just fine.

It may well be that some day Pluto is redesignated as something else. But today is not that day.

[tony873004]

Pluto / Charon animated GIF to scale from Gravity Simulator: