I was walking to work today and noticed the moon and started thinking of planetary motion. Which got me thinking about lunar eclipses-- and it occured to me suddenly that everytime I see a picture of the Universe it’s a 2d picture. Even 3d models that I see are basically a representation of the 2d pictures.
So it occured to me that every picture I’ve seen of our universe shows all the planets rotating around the sun on the same plane. But the sun being spherical has 360 degrees that the planets could actually rotate around. Being as that the planets are pretty dang far apart it would seem more likely that this was the case, and not that the planets were working in the same horizontal plane.
But then, why would EVERY picture I’ve ever seen show it this way and not at least mention that it is a representation of the planetary order.
So what’s the deal… are the planets lined up in a neat little row, or do they rotate around the sun at several different angles? If they did line up in a row how in the Universe did something so random end up so orderly? The gravitational pull to each other? Anybody out there know of at least a good website?
Podkayne, Chronos, Bad Astronomer and a couple others should be along to make me look stupid shortly.
The current theory, IIRC, is that planets and other bodies orbiting stars accreted from a disk of gaseous material rotating around the star. They were all in the same plane to begin with, since the rotation of the disk resulted in it being flattened out. Any bodies that might have formed in other planes were in unstable orbits and were probably flung away or fell into the sun.
I believe that the variation of planetary orbits from the ecliptic is, for most of them, only a few degrees. Pluto has an extremely eccentric orbit, though.
Very astute question. The none planets do not all revolve in the same plane but eight of them are pretty close to the same. Pluto’s orbit is quite eccentric compared to the others which sometimes makes it closer than neptune and the plane of its orbit is tilted quite a bit from the rest.
Most of the planets orbit in roughly the same plane (the ecliptic, which uses the Earth as a reference IIRC). This is a result of the accretion disc from which the planetary solids formed.
By the way, you mean “solar system” and not “universe”. That’s a common error. People confuse them all the time on TV etc.
The basic idea is that the solar system started as a cloud, which flattened out as it collapsed. It formed a disk, from which the planets and Sun formed.
Hmph. 200 million years compressed into two sentences.
oop-- forgot to quote this stuff… so I’ll do it generically
Very astute question.
Thanks. Although it kind of makes me realize what a geek I am. Here I am, on my free time… not thinking about football or baseball or women or anything… but the movement of the planets and their relation to the sun.
Thank you Bad Astronomer-- I was about to inquire… “accretion disc-- WTF?” But think I got it now.
Thanks to everybody else. I think it makes more sense now… :-S
You aren’t any more a geek than anyone else. My daughter is 4 and can explain about fresh water on Europa and Sulphur volcanoes on Io. This isn’t a comment on her but a sad one about me!
Accretion disks are a really simple concept if you think about them in the right way. If you start off by throwing a bunch of gaseous junk at a star or other gravitational source, each piece of the cloud will rotate in some random direction, because it’s unlikely to actually crash into the relatively small star. The orbits will initially be elliptical and in all different planes. But the bits and pieces in orbits that intersect will eventually crash into each other, and when they do, will tend to all become more circular, and the orbits will go towards the average of the original rotation planes. Over millions and millions of years of this, the material all ends up going in a circle in a single plane.
Then it accumulates into planets, which will revolve around the star in that same plane.
The same process explains the circular, very flat rings of Saturn (and other planets). I read recently that Saturn’s rings are only around 50 meters thick!
True, except that the rings probably did not form from the original proto-planetary disk. They might have formed from a former moon that was busted up from a collision with another object.
FWIW, the Nine Planets website (see above link) says that the rings are less than 1 km thick.
Does the “asteroid belt” between Mars and Jupiter generally lie in the same plane as Mars and Jupiter, or is it more spread out over the surface of an imaginary sphere?
I have a related question. All the planets have elliptical rather than circular orbits right? Now I’m guessing these orbits aren’t concentric. That is, the long axes of each planet’s orbit don’t all line up. If this is true, and all orbits are almost in t he same plane, why isn’t there overlap between orbits? Shouldn’t there be collisions between between planets when an outer planet is is at perhelion (closest point to the Sun) whilst an inner planet is at aphelion (furtherest point from the Sun)? Is this just too unlikely because of the size of the solar system or is it because the system has found a stable state where all the large collision possibilities were accounted for at an early point in the life of the system?
Also, I can understand how all the planets woul d line up in a similar plane due to accretion disc origins but I can’t see why one would orbit in the opposite direction. Venus goes the other way right? Or is it the direction of revolution that I am thinking about?k
All the planets revolve in the same direction, which makes it much more convenient to launch probes at them (thanks God! ;)). Venus’ rotation is retrograde; it spins backwards (and not very quickly). Also, Uranus’ axis of rotation is “sideways”, and technically it has retrograde rotation (its “north pole” is about 3 degrees “below” its orbital plane).
The planets’ orbits, Pluto’s excepted, are not elliptical enough to “cross”. Mars’ is elliptical enough that it gets about twice as close to the Earth on some approaches than on others, but that’s about the extent of it.
