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Our Moon Could Become a Planet
anthemaniac writes "Earth's moon is drifting away from us more than an inch every year. In a few billion years, if the system survives, the moon would be reclassified as a planet under the new IAU definition. You gotta wonder if the astronomers who dreamed this definition up had thought of that."
The energy to lift the Moon's orbit comes from the rotational energy of the Earth, which is limited. As the Moon gets higher the Earth rotates slower. There may not be enough energy to lift the Moon high enough to qualifty.
If the moon going further away from the Earth causes the barycenter of Earth to drift outside its surface, then the Earth will be orbiting a point outside itself, with its orbit becoming greater the farther the barycenter drifts, until it peaks at one point. This is similar to Pluto constantly orbiting a point outside itself, as illustrated in this NASA chart hosted by Wikipedia. I think that when a moon begins to have that effect, it should be classified as a planet.
Currently, the Earth's barycenter is three-fourths of the way to its surface, causing it to sort of wobble, rather than fully orbit an invisible point. This is like an analogy: This is like a Chippendale stripper doing a pelvic thrust, rather than running around in a circle.
Earth's orbit around the sun currently makes the sun wobble in a barely perceptible fashion. Jupiter's orbit around the sun, however, causes the sun to orbit a point about 7% above its surface. I think that there should be a new class of planets for the purposes of describing a planet that makes a star orbit itself in this manner.
Clearly, all brown dwarfs orbiting a star would also have a similar or greater effect. The best way to describe it, in my opinion, would be by merely affixing "co-orbital" to describe a planet altering the sun's orbit in this fashion, or a brown dwarf orbiting a star doing this.
If this causes a planet to be "co-orbital" for only part of its orbit, or a natural satellite to be a planet for part of its orbit, in some eccentric situations, that's fine with me. There's one other issue with the new definition that makes me uncertain, though. EL61 is a "minor planet" that has a very oblong shape caused by its own orbit around the sun. If it were in a slower, closer orbit, its own gravity would almost certainly be enough to warp it into a nearly spherical shape. Should EL61 be considered a planet, despite its problem?