Slashdot Mirror
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.
Maybe you should RTFA. The SPACE.com story is talking about in a few billion years, when the barycentre of the Earth-Moon system has moved above the surface of the earth. That would make the Earth and the Moon double planets. In a few billion years. The IAU FAQ you quoted was more concerned about right now.
Why can't something be both a planet and a moon? As far as I understand it, the new IAU definition of a planet is something that's in orbit around a star, is not a star, and is large enough for gravity to make it roughly spherical. A moon is something that's in orbit around a planet. So you could argue that our Moon is already a planet (it's in orbit around the Sun as well as the Earth). The same would apply to many other large moons in the solar system.
I think Asimov said something like this in one of his thousand books. His argument was that, unique among (known) moons, Luna's orbit is always curved towards the sun, making it more of a coplanet than a proper moon. But I can't remember which book it was, unfortunately.
--
I don't want to rule the world... I just want to be in charge of mayonnaise.
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?
I think you're confusing the term "humans" with "human civilization as we are familiar with it."
The odds of current civilization lasting another thousand years may be low, for the reasons you cite. The odds, however, of us successfully wiping out so much of the population that humans are no longer a viable species within the next thousand years are, in my opinion, fantastically low. We breed too fast, we're spread over 30% of the planet's total area, and we're highly adaptable to changing conditions.
Frankly, I fully expect some descendant species of humans to be living here pretty much right up until the planet is inside the sun.
Reality has a conservative bias: it conserves mass, energy, momentum...
Moving away at its current speed, it would have taken 10bn years to move from the Roche limit to its current position. (In rough figures: 4*(10^8)m / (0.04m/year) = 10^10years)
But the moon is drifting away due to tidal effects. So it would have been drifting faster in the past. Taking that into account, the MAXIMUM possible time the Moon could have been orbiting earth is less than 1.5bn years.
So how come many scientists think the Earth-Moon system is 4.5bn years old? Maybe they just haven't done the math.