u guys say the original planet defi. is not good because our Moon could become a planet this way...?

Question

i encourage u to read the words of the proposal it say the IF THE BARYCENTRE OF THE MOON AND PLANET IS INSIDE THE MOON, IT IS A MOON, WHICH MEANS ALL THE MOONS IN THE SOLAR SYTEM WOULD STILL BE A MOON, except CHARON!

so dont tell me the original one is not good because the moons would all become planets then...they wont, read a little!

Answer 1

Under the proposed definition, the Earth's Moon actually will become a planet in several billion years.

The Earth's tidal bulge leads the moon a tad. The resulting gravitational potential is slowly transferring angular momentum from the Earth to the Earth's Moon. (See wiki for a detailed discussion at http://en.wikipedia.org/wiki/Tidal_acceleration ). The net result is the moon moves further out and orbits more slowly and the rotation of the Earth slows a bit. Eventually the Earth will be tidally locked to the Moon, just as the moon is already locked to the Earth. This locking will occur when the lunar orbit and one terrestrial rotation takes 47 days.

Once the Moon's orbit is about 43 days (before the locking is complete) the barycenter of the system will be at the surface of the Earth's crust.

You can see this 43 day thing for yourself keeping in mind that the moon is 1/80th the mass of the Earth, so once it is over 80 earth radii distant (which corresponds to an orbit of about 43 days) the barycenter will no longer be inside of the Earth. The timescale for this to occur is a billion years or three so for most practical reasons, who cares!!! For aesthetic reasons, this part of the proposal bothers me. I would still vote for it, but would like to see this double planet thing modified.

Also, how do you define "inside" when one of the objects is gaseous? The atmosphere just keeps getting thinner and thinner? Do you select one optical depth at visible wavelengths as the "surface"? This does not matter in our solar system, but the definition is worded such that it is supposed to apply to extra-solar planets too.)

Edit: I just ran across an article in Space.com. I knew I could not have been the only person to have thought of this. http://www.space.com/scienceastronomy/060817_moon_planet.html

Answer 2

Act 1, Scene 1, - SPLATT is on Y!A, in his study. He is reading a question on the Astronomy site. He types.....

I hadn't heard of the Barycenter part of the definition. Interesting. I'll have to think about it.

How about providing the reference for barycenters for Earth and Moon.

But I think you meant it's inside the 'planet (larger body)', the other is a moon.

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Act 1, Scene 2. A few minutes later...... Thought about that Barycenter.

The Earth's average density is 5,515.3 kg/m³ (Wiki).

Gold is 19,300 kg/m³ (Wiki).

The current Barycenter is about 2900 miles from the earth's center. The surface is about 3960 miles from the center.

If the earth were made of pure gold of the same total mass as currently, the orbit of the moon would be little affected. But the volume of the earth would be less than 1/3 the current volume. The new radius would be about 2675 miles. That would make the Barycenter outside the new surface of the Earth.

Pluto has a density of about 2030 kg/ and a radius of about 715 miles. The Barycenter is about 1200 miles from Pluto's center. If Pluto's density were to drop to a density of Saturn (about 700 kg/ m³) it's radius increase would put it well beyond the 1200 mile range. That's enough to put the Barycenter inside the planet.

I don't think that the planet/moon decision for Charon or Luna should be affected by the composition of the planet it accompanies. I would not include the Barycenter as a criterion.

Answer 3

finally someone smart.

thank you JackZack