Probably a dumb question, but at least in our solar system, are all the planets in the same plane?
My understanding is that yes, most of them are very nearly in the same plane, known as the “ecliptic.” Pluto was an exception, and that is one of the reasons it was booted out of the club.
Of course, this is just from memory of childhood study, not any sort of scholarly testimony, so take it with a grain of salt.
Yes, most planetary orbits are pretty much right on the sun’s equatorial plane (aka the ecliptic). Aside from Pluto, the inclination of planetary orbits is around 5 degrees. Pluto is at 17. There are a couple largish asteroids that are way off - Pallas at 34 degrees inclination, for example - but they are highly exceptional.
Ignorance fought in record time. Thanks for the answers.
I beg to differ. MY ignorance is still running rampant.
This can’t all be coincidence… why do planets tend to be on the ecliptic plane?
Spun out from the same source.
Because they all formed from a spinning disk of stuff around the early Sun. The clouds that form stars tend to collapse into such disks when they start spinning.
Just to make certain the question has been answered, here at thislink, they state the planetary orbital inclinations as:
**Mercury 7.0 Venus 3.4 Earth 0.0
Mars 1.9 Jupiter 1.3 Saturn 2.5
Uranus 0.8 Neptune 1.8 Pluto 17.2**
Yes, I know Pluto has lost its “planethood”, but I included it to verify Gorsnak’s posting.
And of course, Earth is listed as 0.0 exactly because it’s the plane of the Earth’s orbit that all the rest are measured relative to.
It works like this:
If you have a mass of dust, it eventually begins to collapse as the gravity of the various particles pull them together. Now, if that mass has a rotational component (it almost always will, just because it’s so unlikely that the random motions of the particles would exactly cancel out their vectors), then as it collapses, it begins to rotate faster due to the conservation of angular momentum.
Eventually, the centripetal acceleration of the rotating particles balances out the gravitational force, and you get a stable rotating plane of material. But what about the stuff above and below the plane? It doesn’t have the same rotational velocity, and so it begins to be attracted to the disc of material and to the center of the mass, and starts to collapse in towards the plane. Eventually, the mass of dust flattens into a disc.
You could look at it this way - let’s say the Earth was a hollow shell, spinning at a speed such that the surface of it was going orbital velocity at the equator. Now stick a whole bunch of satellites all over it like sprinkles. Now, make the shell collapse instantly into a black hole of the same gravitational pull. What happens to the satellites? The ones at the equator just keep merrily spinning around. But what about the ones at the poles? They have no velocity relative to the center of the earth - they were essentially spinning like a top at the poles. So they fall into the center. As you move away from the poles, the particles still head towards the center, but they do so in an elliptical path Some of them will intersect the equatorial disc and eventually stay there. Some will make their way all the way in to the center.
So, you wind up with a lot of material going to the center of the rotation, where it eventually forms a star. The rest flattens into a disc that spins around the star. And inside the disc, interactions between the particles cause them to start to clump, and eventually large clumps form and start to collect the rest. This forms the planets. One thing interesting about that is because the planets all form from material rotating in the same direction, once the planets form they all spin in the same direction as well. The only exceptions are those planets which had their spin altered through massive collisions with other bodies when the solar system was forming. Venus has retrogade spin, and so does Uranus. But all the other planets rotate in the same direction, although their axis of rotation is not exactly on the plane of the ecliptic because of past collisions, gravitational forces, and precession.
Which direction is that, though? What’s the normal reference frame here, looking down from Polaris? In that event, do the planets revolve clockwise our counter? I suppose they *rotate *counterclockwise. Which way do the planets’ natural satellites revolve, also clockwise?
Were it not based on that, such a measurement would be a stunning coincidence, no? 
And by the way, the moon’s orbit is not precisely on the same plane as the earth’s. That’s why we don’t have solar and lunar eclipses every month.
If it were, we’d never get a good eclipse up north! We’d all have to travel to the equator every month.
Yes.
Viewed from Polaris, all of the planets revolve around the Sun counterclockwise. Most satellites revolve around their planet counterclockwise; the exceptions are mostly small captured asteroids in the outer solar system. Six of the eight planets rotate around their axes counterclockwise; the exceptions are Venus and Uranus and Pluto if you wish to count it.
Wouldn’t the plane of Jupiter’s orbit be more astrophysically significant, given it’s gravitational influence on the rest of the system? Or maybe the plane of the Sun’s rotation?
While this sentence is correct, I thought I’d just note that Polaris is not particularly close to the north axis of the solar system’s orbital plane(s). Polaris is what the Earth’s rotational axis points to, but that’s about 23 degrees off from where the ecliptic axis points.
Possibly, for some purposes — but our coordinate systems were developed for Earth first, because of course that’s where all our observations were made, historically. And still are, much of the time.
Heliographic or Jovian coordinate systems can be more suitable when you’re modelling events related to those bodies. There are a few heliographic coordinate systems in use out there — see here for some examples.
Those are used sometimes, too (though the plane of the Sun’s rotation is harder to define precisely). But it’s clear that wolf_meister’s numbers are relative to Earth’s orbit.
Excellent explanation from Sam Stone. I would only add what I read somewhere: 99% of the solar system’s mass is in the sun and 99% of the solar system’s angular momentum is in the planets (it must most be in the four outer planets–since they are outer and also much more massive than the inner four).
The orbits of comets can be all over the place, too.