The empty space past Neptune isn't exactly empty. We know that comets come in two varieties: Those with orbit periods of less than 200 years, which tends to orbit the sun in the same plane as the planets, and those with longer periods which have orbit tilted everyway.
This is something of a problem: comets lose material when they get near the Sun. Over the course of millions of years, these comets should evaporate! And yet here we are 4.56 billion years after the solar system's birth and comets still appear in our skies.
Where the comet come from?
4.5 Billion years ago our solar system formed. Coalescing our of a flat disk of material around the sun, the inner planets were warmer, smaller and rocky while the outer planets were in a region that was colder, and grew huge. Out there in the chillier part of the solar system, water comes in the form of ice mixed in with dust and other stuff. These bits would collide and stick together, growing bigger.
Some grew to hundreds of kilometers across. But there was a problem with these outer planets, They had a lot of gravity, and very chunk of ice getting too close would either fall into the planets and get assimilate or get kicked into a different orbit. It could then plunge in toward the Sun, or get flung out into deep space.
4.5 Billion years ago our solar system formed. Coalescing our of a flat disk of material around the sun, the inner planets were warmer, smaller and rocky while the outer planets were in a region that was colder, and grew huge. Out there in the chillier part of the solar system, water comes in the form of ice mixed in with dust and other stuff. These bits would collide and stick together, growing bigger.
Some grew to hundreds of kilometers across. But there was a problem with these outer planets, They had a lot of gravity, and very chunk of ice getting too close would either fall into the planets and get assimilate or get kicked into a different orbit. It could then plunge in toward the Sun, or get flung out into deep space.
Trillions upon trillions of such iceballs got tossed around the planets, even though they were small compared to the planets, they did have a little bit of mass and gravtiy. So every time planet pulled hard on them, they also pulled a little bit on the planets too. It wasn't much per chunk, but after trillions of events adds up!.
A current model, called the Nice Model where it was proposed, says that the overall effect of all these encounters that Saturn, Uranus, and Neptune slowly moved outward form the sun, while jupiter moved inward.
Neptune would have the biggest effect on these iceballs because it bordered the biggest volume of space where they lived. As Neptune migrated outward, close encounters with these chunks of ice flung lots of them into crazy orbits highly elliptical and tilted with respect to the planets repeated more distant encounters tended to more slowly increase the sizes of the orbits of the iceballs, too.
Astronomers think that this shuffling around of the outer planets is what caused the Lale Heavy Bombardment, the intense shower of objects that cuase screaming down from the outer solar system, scarring planets and moons, a few hundreds will on years after the planets themselves formed. It's not known for sure, According to Astronomers.
The Oort cloud is a region of long period comets Since they orbit the sun on a sphere with no preferred orientation, they come in toward the inner solar system from random directions in the sky. Many newly discovered comets fall into this category. Their orbit can be extremely long; they start their fall from so far away they swing around the sun ar newly escape velocity, and their orbit are close to being parabolic.
The scattered disk is the source of short-period comets. They still are affected by Neptune, which alters their orbits to drop them down closer in. they can orbit the sunon paths between Jupiter and Neptune, meaning eventually they'll have a close encounter with Jupiter, This can send them in closer to the sun, they become short period comets and that's how comets are made.
Until the 1930s, it was pretty much just conjecture but then an American Astronomer, Cylode Tombaugh, discovered the first Kuiper Belt Object, Pluto.
Pluto orbits the sun in an elliptical, mildly tilted path. Its orbit actually brings it closer to the sun than Neptune ( so how come it never collide with larger planets ? )
Pluto orbits cross Neptune's more or less because the orbit is tilted, they never actually physically cross. When Pluto is ar Perihelion, closest to the sun it's well above the plane of the solar system, far from Neptune's orbit. Not only that but Pluto orbit the sun twice for every 3 times Neptune does. Because of this whenever Pluto is closest to the sun, Neptune is always 90 degree in its orbit.
That's many billions of kilometer distance, way too far to affect Pluto. This is mostly coincidence, We've seen how orbital resonance can be forced by tides and by gravity. But in this case, it's due to attrition. Once upon a time, billions of years ago, there were probably a lot of objects out by Pluto, with orbits of all different shapes and tilts.
But the ones that got too close to Neptune got gravitationally tweaked into different orbits, turning them into comets or flinging them deeper into space, the only ones that could survive just happend to have orbits with that 3:2 or 2:1 resonance with Neptune, keeping them far from Neptune's influence. Today's those are the only kind of objects we see with orbits near Neptune. Astronomer call this object Plutinos. They're not really a separate class of object- they're still Kuiper Belt objects, but fun and interesting subset of them.
Once Pluto was found, Astronomers wondered of it might herald a new class of icy object past Neptune. However, it took more than six decades to find the next one. In 1992 QBI was discovered and that opened a sort of gold rush of Kuiper Belt discoveries.
We now know of more than a thousand Kuiper Belt objects, one called Eris and is very close to Pluto's size and is more massive its probably rockier than icy Pluto. A moon was discovered in 1978 named Charon, it's actually about 1/8th the mass of Pluto itself! while it can be said that Pluto noticeably orbits Charon too. In Reality both circle around their mutual center of mass located between the two.
Four more moons were discovered in Hubble images of Pluto in 2005 and 2012 and there may be more. Pluto is so small and distant that we don't know much about it.
Pluto was found in 1930, long before ant other Kuiper Belt object, because it's much brighter than any other. When it was discovered, it was thought to be about the size of the Earth but better observation showed it to be far smaller than first thought; in fact, it's smaller than the Earth's moon!.
Its surface is unusually reflective, shiny, making it look much bigger than it seems. Most other Kuiper Belt denizens far less reflective and so are far fainter.
Astronomers think that Kuiper belt has 100,000 objects in it larger than 100km wide. The Oort Cloud, surrounding the solar system, may have trillions of icy bodies in it.
The very interesting bodies have been found: Senda and VP113. Sedna orbits take it an incredible 140 billion kilometer from the sun, while VP113 get about half that far out. Both are very elliptical orbits. Neither, however, get close to Neptune, so it's not clear how they get where they are. They may be Oort cloud that was distributed by passing stars long ago, dropping them closer into the sun but no one knows yet.
We can calculate how many Oort cloud objects there should be left over from the formation of the solar system, and it's about 6 billion.
However, calculating how many there are long period comet observations, you mind up getting about 400 billion.
One idea to solve this discrepancy is that the sun has stolen comets from other stars; Comets should from whenever should do, and sometimes the sun passes near other stars, could we be seeing an object from an alien solar system? May be.
There's another explanation, but it's highly speculative Perhaps there's another planet in the solar system well beyond Neptune.
Some very preliminary studies have shown that some long period comets aren't coming in randomly but instead have their orbits aligned in a way you might expect if any distant planet perturbed them.
There is the hand of Kuiper belt object aligned in a similar way. Nasa's wise observatory scanned the skies in infrared and would've seen anything as big as Jupiter or Saturn out to tremendous distances so any hypothetical planet would have to be smaller. And very distant, probably tens of billions of kilometer out. we've seen other stars with planets this far out, so it's physically possible.
This region of the solar system is seriously unexplained it's distant, difficult to reacts, and above all else extremely huge and mumbling empty.
There's still lots of solar systems left to explore.
Once Pluto was found, Astronomers wondered of it might herald a new class of icy object past Neptune. However, it took more than six decades to find the next one. In 1992 QBI was discovered and that opened a sort of gold rush of Kuiper Belt discoveries.
We now know of more than a thousand Kuiper Belt objects, one called Eris and is very close to Pluto's size and is more massive its probably rockier than icy Pluto. A moon was discovered in 1978 named Charon, it's actually about 1/8th the mass of Pluto itself! while it can be said that Pluto noticeably orbits Charon too. In Reality both circle around their mutual center of mass located between the two.
Four more moons were discovered in Hubble images of Pluto in 2005 and 2012 and there may be more. Pluto is so small and distant that we don't know much about it.
Pluto was found in 1930, long before ant other Kuiper Belt object, because it's much brighter than any other. When it was discovered, it was thought to be about the size of the Earth but better observation showed it to be far smaller than first thought; in fact, it's smaller than the Earth's moon!.
Its surface is unusually reflective, shiny, making it look much bigger than it seems. Most other Kuiper Belt denizens far less reflective and so are far fainter.
Astronomers think that Kuiper belt has 100,000 objects in it larger than 100km wide. The Oort Cloud, surrounding the solar system, may have trillions of icy bodies in it.
The very interesting bodies have been found: Senda and VP113. Sedna orbits take it an incredible 140 billion kilometer from the sun, while VP113 get about half that far out. Both are very elliptical orbits. Neither, however, get close to Neptune, so it's not clear how they get where they are. They may be Oort cloud that was distributed by passing stars long ago, dropping them closer into the sun but no one knows yet.
We can calculate how many Oort cloud objects there should be left over from the formation of the solar system, and it's about 6 billion.
However, calculating how many there are long period comet observations, you mind up getting about 400 billion.
One idea to solve this discrepancy is that the sun has stolen comets from other stars; Comets should from whenever should do, and sometimes the sun passes near other stars, could we be seeing an object from an alien solar system? May be.
There's another explanation, but it's highly speculative Perhaps there's another planet in the solar system well beyond Neptune.
Some very preliminary studies have shown that some long period comets aren't coming in randomly but instead have their orbits aligned in a way you might expect if any distant planet perturbed them.
There is the hand of Kuiper belt object aligned in a similar way. Nasa's wise observatory scanned the skies in infrared and would've seen anything as big as Jupiter or Saturn out to tremendous distances so any hypothetical planet would have to be smaller. And very distant, probably tens of billions of kilometer out. we've seen other stars with planets this far out, so it's physically possible.
This region of the solar system is seriously unexplained it's distant, difficult to reacts, and above all else extremely huge and mumbling empty.
There's still lots of solar systems left to explore.
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