Are all moons that have a retrograde orbit when their planet has a prograde orbit likely to have been captured?
Pluto to regain status as ninth planet...?
- Thread starter Candlelight
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I would guess they would have to be. As far as we know, the only way to get moons is to either create them from the debris left over from planet formation, or capture them. Or do what Earth did, collide with another planet make a moon from the ejecta.
Pluto has a direct orbit (which means he orbits the sun in the same direction of the other planets, i.e. counter-clockwise as seen from the northern pole), but a retrograde rotation (i.e., it spins on its axis in a clockwise motion) as observed from "above" the planet's north pole.
Charon has a retrograde orbit around Pluto, and they are both tidally locked to each other. They always show the same face to each other. Pluto's other satellites also have a retrograde orbit. I don't think this has any effect on the definition of planet: rotational direction of the planets and orbital direction of satellites are not considered in the definition.
No planet (or dwarf planet) in the Solar System has a retrograde orbit (they all orbit in same direction), but a few have a retrograde rotation (Venus, Uranus, Pluto). It is thought to be a consequence of impacts. Virtually all objects in the solar system have a direct orbit, including asteroids. Comets are the only category with a significant retrograde population.
Most moons orbit around their planet in the same direction of its rotation: which means Uranus's and Pluto's satellites have retrograde orbits, while the others have direct orbits). The ones that have a retrograde orbit around a direct-rotation planet are usually small and irregular, and they are thought to be captured asteroids. The exception is Triton, with a retrograde orbit around Neptune.
Very fast-moving satellites like Phobos seems to be moving in a retrograde orbit when observed from the planet, but that's just because they orbit the planet faster than the planet's rotation. Seen from above Mars' north pole, the orbit is the same direction of the planet's rotation (counterclock-wise).
EtA: ninja'd beause I'm a slow typer, and I wanted to check my books before making a fool of myself.
Charon has a retrograde orbit around Pluto, and they are both tidally locked to each other. They always show the same face to each other. Pluto's other satellites also have a retrograde orbit. I don't think this has any effect on the definition of planet: rotational direction of the planets and orbital direction of satellites are not considered in the definition.
No planet (or dwarf planet) in the Solar System has a retrograde orbit (they all orbit in same direction), but a few have a retrograde rotation (Venus, Uranus, Pluto). It is thought to be a consequence of impacts. Virtually all objects in the solar system have a direct orbit, including asteroids. Comets are the only category with a significant retrograde population.
Most moons orbit around their planet in the same direction of its rotation: which means Uranus's and Pluto's satellites have retrograde orbits, while the others have direct orbits). The ones that have a retrograde orbit around a direct-rotation planet are usually small and irregular, and they are thought to be captured asteroids. The exception is Triton, with a retrograde orbit around Neptune.
Very fast-moving satellites like Phobos seems to be moving in a retrograde orbit when observed from the planet, but that's just because they orbit the planet faster than the planet's rotation. Seen from above Mars' north pole, the orbit is the same direction of the planet's rotation (counterclock-wise).
EtA: ninja'd beause I'm a slow typer, and I wanted to check my books before making a fool of myself.
I see that Pluto is believed to have a prograde orbit but a retrograde rotation but it seems that all its moons have a retrograde orbit. I would like to know how this is thought to have come about and whether the orbits/rotations are taken into account when deciding if Pluto is a planet or not. Or is size the only criteria?
Edited to add - looks like these questions have been answered by the Iguana even before I asked them
Edited to add - looks like these questions have been answered by the Iguana even before I asked them
The Mi-Go should be happy about this.
http://lovecraft.wikia.com/wiki/Mi-go
http://lovecraft.wikia.com/wiki/Mi-go
If the definition of planet were corrected to include Pluto, then Ceres would be a planet as well, as would a number of large objects in the outer solar system-- plus many yet to be discovered. And that's exactly why the current definition was created. They didn't like the idea of three hundred planets in the solar system. Not very scientific.
I think it's a remnant of the rotational direction of the accretion disk back when the solar system was first formed.
I suspect that's down to how the solar system was formed billions of years ago with all the rotational direction of debris (accretion disk) around Sol, would explain why the planets orbit in the same direction,
As for why planets have the same spin perhaps it is down to the same reason, if the planets orbited in the opposite direction, perhaps they would have the opposite rotation to what we have. But I'm not an astrophyscisit
As for why planets have the same spin perhaps it is down to the same reason, if the planets orbited in the opposite direction, perhaps they would have the opposite rotation to what we have. But I'm not an astrophyscisit
That's basically correct. In fact, even the sun rotates around its axis in the same direction as the planets orbit around it.
Again, that's essentially correct. If you consider a section of the accretion disk in the primeval Solar system that will be coalesce to form a planet, the external part of the section would have a slightly higher velocity than the internal part. Long story short, that would create a differential in velocity which in turn would make the newly-formed planet spin in a specific direction (the "outer side" of the planet leading the "inner side"). I am not sure I am explaining it well without pictures.
A star can spin in any random direction. (Not just that, but their axis can point in any random direction. In fact, the rotational axis of the sun forms an angle of 60 degrees with the galactic rotational axis: viewed from the galaxy plane, the sun rolls more than it spins).
But the planets around a star, barred some kind of mindbogglingly catastrophic impact event, orbit in the same direction of the rotation of the star, and they rotate on their axis in the same direction as well (again, barred a catastrophic impact: but since just in our solar system we have 2 planets spinning in the opposite direction on a total of 8, we can argue it's not a completely unlikely event).
EtA: Found this. It's actually pretty cool if you think about it.
But the planets around a star, barred some kind of mindbogglingly catastrophic impact event, orbit in the same direction of the rotation of the star, and they rotate on their axis in the same direction as well (again, barred a catastrophic impact: but since just in our solar system we have 2 planets spinning in the opposite direction on a total of 8, we can argue it's not a completely unlikely event).
EtA: Found this. It's actually pretty cool if you think about it.
Huh? That seems wrong. Mercury zips around the sun at high speed, while distant planets take a much more leisurely pace.
I think he meant higher velocity, not faster orbital period.
I would run 10m still faster than an olympia runner would do 100m.
Same would Mercury complete it's short orbit in shorter time than Neptune it's much longer arboit despite being technically faster.
An internal part is surrounded in the plane by the disk, so the gravitational attraction on an internal part due to all the outer parts of the disk largely cancels. On the other hand, for an external part there is no cancellation of the attraction from the rest of the disk, which is totally interior to it.
In contrast, in the solar system, almost all of the matter is concentrated at the sun. The sun is interior both to Mercury and to distant planets, so the cancellation effect doesn't happen.
I was talking about tangential velocity.
To go a bit further into it:
The primordial accretion disk around the sun spun, but not as a rigid body. Due to Kepler's Law, the tangential velocities of a section in the disk was proportional to the reciprocal of the square root of the distance from the sun.
When a centre of agglomeration is formed, it draws in material from sections of the disk that are both closer and farther from the sun. When the "outer" material moves closer to the centre of agglomeration (which is, in this case, closer to the sun), its tangential velocity increases to preserve angular momentum. But, when the "inner" material moves closer to the centre of agglomeration, it also moves farther from the sun, so its tangential velocity decreases to preserve angular momentum.
The net result is that the final planet, composed of material coming from both closer and farther from the sun, spins in the same direction of the primeval accretion disk, and ultimately, the same direction the orbital motion of the planet itself (in our solar system, counter-clockwise).
I hope it's clear enough even without diagrams. God, how I miss a blackboard when this happens.
I love the retro feeling of blackboards. And using big, unwieldly wooden rulers and compasses to draw lines in pastel-colored chalk. We still have them in some of our oldest classrooms. I so feel like a proper professor when I do that.
Whiteboards make me feel like a marketing consultant. Shudder.
Whiteboards make me feel like a marketing consultant. Shudder.
SmartBoards are the best.
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