In our Solar System, the orbits of all known planets fall more or less along the same plane, called the “ecliptic.” http://en.wikipedia.org/wiki/Ecliptic
Why is that?
Is it typical, as far as astronomers know? Could there be star systems were the planets’ orbits describe several intersecting planes?
Correct if I’m wrong but Pluto doesn’t orbit on the same plane as the rest of the planets either. It’s off by about…17 degrees?
AFAIK, astronomers don’t know for sure, but it’s theorized that this is because the planets coalesced out of a spinning disk-shaped cloud of metal, gas, dust, and what-not. (Big spinning things would tend to assume disk-like shapes, because gravity pulls everything in towards the center, but centrifugal force pulls out in the direction of spin. Even smaller spinning objects like planets are deformed slightly out of a true spherical shape by their spin, creative a very slight ‘bulge’ effect that, if its effects were much much much much more pronounced, would turn the planet visibly into a disk.)
Since this is all speculation, and we haven’t really spotted any planetary systems that are are at all like ours yet, it’s hard to know if that’s typical. From an orbital dynamics point of view, I don’t think there would be anything to stop a chaotic planetary system, with planets in all kinds of different ecliptic planes, from existing.
It probably is typical. The cloud of gas and dust from which solar systems are thought to condense generally have a rotational component, and this rotation flattens the cloud into a disk shape–simplistically speaking, centrifugal “force” pushes it outward at the equator, while gravitation from the central mass flattens it at the poles until it makes a big gas-and-dust pancake. Since planets form within the disk, they will naturally all fall within roughly the same orbital plane. I’m sure it’s possible for a solar system to have planets in wildly different orbital planes if it was at some point in its history perturbed by the passing of a large gravitational mass, like a star.
A captured planet is more likely to be off plane, but planets that form along with the sun are likely to be all on plane, since the original “stuff” of the solar system is swriling around a common axis.
And, as noted, Pluto is pretty far off plane.
IANAA, but wouldn’t the gravitational pull from the other planets tend to draw the odd planet into the plane?
Since planets form from the dust and debris orbiting the sun in a disc, it seems to me that they’d form pretty much in the ecliptic.
I doubt it. The planets are pretty far apart. You think the gravity of Mars has any measurable effect on the motion of Earth?
I am not an astrophysicist, but here’s what I remember from various reading (sci-fi and non-fiction) any my semester of astronomy as a college science elective.
The accepted theory on the formation of the solar system describes a cloud of intersteller material spinning in space, and gradually collapsing into a disk. Gravity pulls everything to the middle, but the spinning motion kept matter on more spread out along the plane of rotation. The center of the disk formed the sun, and the planets coalesced out of the rest. Since everything started with fairly similar orbital motion, all the planets probably started out on roughly the same plane.
Of course, once the planets were there, gravity could jostle things around. Planets could capture smaller bodies to become moons. A gravity “slingshot” from Neptune probably explains Pluto getting kicked out of the ecliptic into its angled orbit.
I doubt a star system could have more than one well-defined ecliptic, although planets with angled orbits like Pluto’s (possibly even multiple ones per system) aren’t unreasonable.
And on preview, it looks like a lot of good posts came in while I was composing this, so I’ll leave it at that.
Look up “orbital resonance” sometime.
I can see how planets would tend to form, in the plane defined by the angulatr momentum of the original dust cloud that condenses to form the sun and planets. Most things created are going to be in that plane.
It might be possible for an object from elsewhere to be captured and end up in a completely different plane, but consider the mechanism of capture – an object entering a solar system is, in most cases, just going to swing around in a hyperbolic orbit and shoot right back out again. The only way for something that was originally outside the solar system (having a total energy greater than the potential for the sun) to get captured is for it to interact with something else massive in the system, transferring some of its energy to that. This is more likely the closer its path is to the object’s, and so it’s more likely for objects having trajectories that lie near the plane of the solar system to get captured – they’re getting acted upon at a closer distance and for a longer time and in a more consistent direction that something coming in at a large angle to the solar system. IIRC, they still think that Pluto is a “captured” object, and its angle to the rest of the solar system – not quite the same plane, but not extremely far from it – is consistent with this scenario.
I’ll bet Mars does have a measurable effect on the motion of the Earth.
IIRC, when astronomers studied the orbit of Uranus, it didn’t follow exactly the orbit we would expect, so someone (a famous astronomer I don’t recall) hypothesized that there must be another planet of a certain mass at a certain distance out there causing the discrepency, which led to the discovery of Neptune. The numbers still didn’t crunch perfectly, which in a similar manner led to the discovery of Pluto.
IANAA either, but it seems elementary to me that gravitational forces, to a lesser/or greater degree, effect all of the planets, w/ the Sun being the dominate force. Over time this would seem to logically draw all the planets into the same plane.
When you have planets out of plane with each other, any gravitational interaction between them will tend to change the orbital planes. That destabilizes systems without an ecliptic. You can still get some odd behavior within an ecliptic:
Newly Forming Solar System Has Planets Running Backwards
You’re half right. Uranus did slow down briefly after its discovery, and that is how they found Neptune. It was pure luck that Uranus and Neptune were in just the right places at just the right time, otherwise Neptune would have had to wait until a couple of people with powerful telescopes did another inventory of the night sky, and then compared their charts.
The discovery of Neptune spurred a flurry of observation and searching, which is how the asteroids were found.
Pluto has almost no gravitational effect on Neptune, and probably none which could be measured by modern equipment. In 1930, Clyde Tombaugh discovered Pluto while he was performing such an inventory mentioned above in search of Percival Lowell’s mythical Planet X.
Regarding whether a captured planet could come into the plane with the other planets in a solar system: I would suppose it could be possible, given a captured orbit that is near (but not too near) the orbit of one of the larger resident planets and enough time. I’m sure a proper astronomer would be along who could give an actual answer.
Neptune was discovered in 1846; the largest of the asteroids (Ceres) was discovered in 1801. The discovery of Uranus, and the way it fit the “Titius-Bode Law”, did inspire a search for the missing planet between Mars and Jupiter, but the discoverer of Ceres (Giuseppe Piazzi) wasn’t specifically looking for it.
You seem to be underestimating how precisely things can be measured. The precession of Mercury’s perihelion is observed to be about 5600 arcseconds (about 1.5 degrees) per century relative to the equinoxes. A large part of this is due to the precession of the equinoxes themselves; but about 10% of it is due to the gravitational attraction of Venus, Earth, and Jupiter. And these numbers could be calculated and measured precisely enough that until Einstein came along, scientists were deeply concerned by a discrepancy of 43 arcseconds per century.
Hmm…
Hmm…
Maybe that’s why Uranus circles the sun in the opposite direction of the other planets.
I think you’re confusing orbital direction with rotational direction. You might also be conflating this with the fact that all the gas giants have satellites in both prograde and retrograde orbits.
That, or I’m being whooshed.
With Uranus the rotational direction is unclear, due to the fact that the axial tilt is about 90 degrees. But that extreme a tilt might by itself mean that a seriously non-planer solar system is possible.
I recall that comets orbit out of the plane of the solar system, and with the new planets being discovered in the Kuiper belt, maybe one of them may be out of the plane too.
Am I ignorant to think that the natural result of prolonged chaotic orbital collisions is for the orbiting matter to resolve into a disc shape?
Wouldn’t the sun’s planets form into a disk for exactly the same reasons that saturn’s and jupiter’s rings form into disks, albeit on a different time/volume/mass scale?
That is to say, if you hypothesize a gravity well, and random mass orbitting it, and then simulate, after sufficient time, shouldn’t you end up with a disk orbitting roughly in the direction of the combined motion of the original mass?