Dual Planet systems. Earth and Moon? Pluto and Charon? Xena and Gabrielle?

Question

As we sweep around the sun, are we moving in a straight line as a single object, or, since the Earth and Moon are the closest in size of any 2 planet system, are we like a couple of ballroom dancers moving as one unit around a central locus as we sweep across the dance floor? I hope I used locus correctly.

Answer 1

There will soon be a conference in which the true definition of what constitutes a planet will be determined. After that conference, we could see several changes.

Your analysis of the dual planet systems is fairly accurate (and might I add elegant?). And based on the criteria that (1) a planet must orbit the sun, and (2) a planet must have sufficient mass to form a spherical shape, (these are the traditional criteria) then it is possible that all of these bodes can be labeled as planets including the moon. The real questions that remain are the oblateness (spherical shape) of these bodies.

What few people know (although you seem to be aware of it) is that technically speaking the moon is also a planet. The location of the center of gravity between the moon and the earth is irrelevant. What is releveant is that the gravitational attraction between the moon and the sun is roughly 4.5x10^20 Newtons and the gravitational attraction between the moon and the earth is only 2.34x10^20 Newtons. That is roughly twice the force.

The real tell is simply this: if the sun and earth parted ways, who would the moon follow? If it follows the earth, then it is truely a moon and should not be labeled a planet. If it stays with the sun...well, you know the rest of it.

I guess we'll just have to wait and see what decision is made on the official definition of the moon.

Answer 2

I love the dance floor analogy. Yes we are locked like dancers. The biggest difference between The Earth and the Moon and Pluto and Charon (which are closer in size to each other not the earth/moon) is that with the earth and moon the center of the shared gravity still resides within the earth. In the case of pluto/charon the center is outside of pluto so they are both revolving around a common center of gravity. This is why you may soon see pluto and charon both counted as planets while the moon will remain just a sattelite of earth even though it is much much larger than Charon.

Answer 3

Our solar system resides in a part of the Milky Way galaxy known as the Orion Arm. The "arm" is one of several that are typical of spiral galaxies. In such an arm, our solar system orbits the central mass of the galaxy much like a planet orbits the sun. However, the Earth and Moon do affect one anothers positions along the orbital path of the Earth, but to a negligible extent. The Earth is -for lack of a better term- steady in its path while the Moon orbits around our planet.

Answer 4

The International Astronomical Union (IAU) may adopt Xena as the name for UB 313. Michael Brown of the California Institute of Technology who discovered the planet nicknamed it Xena after the warrior princess of TV fame. He chose Xena because it would be the 10th planet which is "X" in roman numerals. Gabrielle is the name for the moon of Xena

The 12 planets in our solar system listed in order of their proximity to the sun would be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, and Xena (UB313). Pluto's largest moon, Charon; and the asteroid Ceres would be categorized as planets. Pluto would be categorized as both a planet and a pluton. Both Pluto and Charon each are large enough (massive enough) to be spherical. Both bodies independently satisfy the definition of “planet”. The reason they are called a “double planet” is that their common centre of gravity is a point that is located in free space outside the surface of Pluto.

The center of gravity for the Moon's orbit is in the center of the Earth. The Moon truly orbits the Earth.

The new definition of a planet: any round object larger than 800 kilometers (nearly 500 miles) in diameter that orbits the sun and has a mass roughly one-12,000th that of Earth. Moons and asteroids will make the grade if they meet those basic tests.

The growing category of "plutons" - Pluto-like objects that reside in the Kuiper Belt, a mysterious, disc-shaped zone beyond Neptune containing thousands of comets and planetary objects.
Plutons are distinguished from classical planets in that they reside in orbits around the Sun that take longer than 200 years to complete (i.e. they orbit beyond Neptune). Plutons typically have orbits that are highly tilted with respect to the classical planets (technically referred to as a large orbital inclination). Plutons also typically have orbits that are far from being perfectly circular (technically referred to as having a large orbital eccentricity). All of these distinguishing characteristics for plutons are scientifically interesting in that they suggest a different origin from the classical planets.

Answer 5

These are not dual planet systems - they are planet and moon - duh