This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

The Formation of our Solar System and Long Period Comets

by Dr Jamie Love Creative Commons Licence 1997 - 2011

The things you learned in June about orbits, will help you understand the reason for the orbital properties discussed in this month's lessons. If you find the following information is too complicated you can either review the lesson on orbits or simply accept these properties as true. I hope you will make an effort to use what you learned from Kepler's Laws of Planetary Motion in order to help you understand this month's lessons.

Our Solar System is the product of the planetary ring nebulas produced by dying giants and the remnants of novas and supernovas. All of our carbon, oxygen, magnesium, etc. were created by a star. Over time the materials ejected from some of these explosions were drawn together by their mutual gravitational attraction and formed nebulas. About five billion years ago one of those particularly thick nebulas started to contract into a central body that was to become our Sun.

As materials fell towards the "ProtoSun", a small proportion fell slightly off center to the main mass and ended up swirling around in an orbit. More mass followed and a disk of material started to form around our ProtoSun. Astronomers call this disk an accretion disk because materials started to accrete (accumulate) within this swirling disk. Most materials continued to fall into the ProtoSun but some entered orbits and joined the accretion disk.

Eventually some large masses started to form in the disk as materials condensed from the nebula. Gravitational attraction caused complex tug-of-wars among the large masses in the accretion disk and they started to make large bodies in a process that astronomers call the accretion process. At first only clumps of a few centimeters in size accreted, but eventually these clumps attracted other clumps and formed masses measured in kilometers and then hundreds of kilometers in size.

It's through this process of accretion that "protoplanets" formed. After many millions of years several masses had "won" the gravitational tug-of-war and they became planets.

Each planet's gravity attracted more materials from its neighborhood and the accretion process continued. Each planet acted as a vacuum cleaner collecting all the local debris and turning the accretion disk into planets.

Hey, is that why most of the planets are near the ecliptic?

Yes! Exactly.
The accretion disk "defined" the plane upon which the planets formed. Of course, it wasn't perfect, but the orientation of the accretion disk provided the orientation for the orbits of the planets which formed from it. That's why the inclination of the orbits of planets are closer to each other than you would expect had they been randomly created.

When the ProtoSun went through its T-Tauri stage it blew away a lot of the small debris remaining in the inner Solar System however there still remained hundreds, if not thousands, of "miniplanets" orbiting the Sun. As time went by most of these small worlds collided and condensed into larger worlds. This was the time in the Solar System's history that astronomers call the "Age of Great Bombardment". It must have been very exciting! This age began when a planet's surface first became solid because that is when it is fair to say that a planet has been bombarded. (Before that time, there was just "accumulation by accretion".) It's hard to put a date on the end of the Age of Great Bombardment because the bombardment still continues (!) but not as frequently or as massively as it once did. It's fair to say that the major bombardment occurred in the first half billion years of the Solar System's history.

Bombardment still continues but at a lower level of activity. Collisions are now much less frequent and involve a lesser body hitting a full-fledged planet. Here on Earth, the scars of bombardment are quickly erased by our weather and active geology. Wind and rain erode impact features and the volcanoes and subduction of the Earth's crust obliterate impact basins. There are still a few signs of the most recent impacts on our planet because the Earth's weather and geology haven't quite wiped out those signs - yet.

Our lovely neighbor, the Moon, is another story. It does not have weather and it has been "dead", geologicaly speaking, for billions of years. The Moon retains and displays its bombardment history very well. Astronomers study the Moon's surface features in order to better understand the Age of Great Bombardment. Images from our interplanetary satellites also provide us with useful information about the bombardment in other parts of the Solar System. It appears that bombardment was thorough. There is evidence of some massive collisions.

Indeed, our Moon is probably the result of a massive collision!
Several billion years ago, when the Earth was still pretty molten (from accretion and previous collisions), something BIG collided with it. The force of this impact was so great that it threw into space a huge amount of the Earth's mantle (the outer layer of materials above the Earth's core). This Earth-mantle debris formed a ring around the planet, similar to the one around Saturn, and over the subsequent millions of years the ring condensed to form our Moon.

How do you know that happened?

Well, nobody was around to see it happen but astronomers and geologists have pieced that story together as the most like explanation for the origin of the Moon.
1. Samples returned from the Moon are similar in isotopic composition to materials found in the Earth's mantle.
2. Moon rocks are also similar in age to that of the Earth so they probably solidified at about the same time. [This information comes from an understanding of radioactivity and half-lives.]
3. Also, the Earth's core is much denser than the Earth's mantle. That's because the Earth's core is made of very dense materials like iron and nickel. The Moon, on the other hand, has a density (and composition) more like that of the Earth's mantle and the Moon has no core at all (to speak of). [The evidence for core and mantle compositions comes from careful analysis of the way seismic waves (shock waves) travel through the Earth or Moon.]
All this evidence is taken together as supporting the hypothesis that the Moon was made from the Earth's mantle and that material was released from the ProtoEarth by a massive collision. Computer simulations show that this hypothesis is feasible and consistent with the time scales involved.

I first became interested in astronomy about the time of the Apollo Moon landings. At that time competing theories were that the Moon had come from somewhere else, captured as a preformed world, or it had condensed from the accretion process in its present orbit. Rocks collected by Apollo provided evidence for the collision hypothesis to explain the Moon's origin. This is just another example of how "new" data can reshape our views of the universe.

Nowadays, this collision process is fairly dull and only of historical (geologically historical) importance. A few billion years ago, the Age of Great Bombardment came to an end and so too did the formation of the planets in our Solar System.

So the accretion process turned the accretion disk into the planets and gave us our tidy Solar System.

Well, sort of.
You see, the accretion process is not terrible efficient. It leaves behind a lot of debris. This leftover material makes up an important part of our Solar System - comets, meteoroids and asteroids.

The remainder of this lesson and the next is about comets. (We'll cover meteoroids and asteroids shortly in a lesson specifically about them.)

There are two types of comets - long period comets and short period comets.
Let's look at the long period comets first.

The giant nebula that created our Solar System covered a huge amount of space. (And I do mean "space". ) Only the material very close to the center of the condensing mass was captured by the ProtoSun or the accretion disk. Materials farther away ended up orbiting the Solar System at all kinds of odd, but distant, orientations. These orbits have nothing to do with the primordial accretion disk. In other words, these distant leftovers condensed into clumps of materials that can be orbiting the Sun at angles very far from the ecliptic.

The most distant leftovers may be in an orbit as far as one light-year (over 9 trillion kilometers) from the Sun! When you consider that the Sun's nearest star neighbor is only 4.3 light-years away you can understand that these fragments are about as far away as you can get and still have any connection with our Solar System. The pull of the Sun's gravity at a distance of a light-year is pretty weak but the pull is still there and these chunks are expected to take millions of years just to complete a single orbit around the Sun!

That's pretty far away. How do we know they are there?

Good question.

In 1950 a Dutch astronomer named Jan Oort carefully measured the paths of comets and concluded that many of them started their journey towards the Sun from trillions of kilometers away. [This involved detailed analysis of the motion of the comets and a detailed understanding about the physics of motion and the geometry of ellipses and hyperbolas.] Oort concluded that a cloud of comets lies far off in space, slowly orbiting the Sun. The "Oort Cloud" has never been observed directly but astronomers believe it exists and believe that it is the "home" (place of origin) of most long period comets. Long period comets are simply comets whose highly eccentric orbits carry them close to the Sun (and us) very infrequently. That is, their orbital period (the time to complete one orbit) is very, very long. By definition a long period comet takes at least 200 years to complete one orbit.

Astronomers estimate (guess) that there are about a trillion comets in the Oort Cloud!

Collisions with another comet or the tug of a passing star cause a comet to be ejected from its distant orbit and move inward towards the Sun. Most of them probably swing very wide of the Sun as they settle into eccentric orbits whose perihelion (closest approach to the Sun) is still beyond the orbit of Pluto. However, some of them swing into highly eccentric orbits that bring them into the inner parts of the Solar System before swinging back out again towards their aphelion.

Don't get confused here. Comets in the Oort Cloud are in very distant orbits and have never been seen directly. Those comets take millions of years to complete one orbit and those orbits are not very eccentric. Indeed they are as close to a circular orbit as you are likely to ever find. However, when a comet is ejected from the Oort Cloud it can take on a highly eccentric path requiring only a few thousand years or less to complete its orbit and it may have a perihelion that takes it close enough to us to be seen.

A few thousand years is a pretty long time so these are called "long period comets" (even though the comets hidden far away in the Oort Cloud have even longer periods of orbit). Because no one lives for thousands of years, long period comets are poorly documented and appear unexpectedly. That's because we don't know their orbits until they have been discovered.

The 1990s have been blessed with some spectacular long period comets.

My favorite was Comet Hyakutake. It was discovered by a Japanese amateur astronomer, Yuji Hyakutake, on the evening of 30 January 1996. Yuji was the first to report his sighting to the proper authorities so it was named after him. (Cool, huh? ) Using a powerful pair of binoculars (25 power magnification and with an outer lens of 150 millimeters) he found the comet when it was only of magnitude 11. Over the following months Comet Hyakutake brightened as it came closer to the Sun. Like many long period comets, Hyakutake followed a path far off the plane that had once been the accretion disk. It arched northwards through our night sky and on the 24th of March (1996) it came within 15 million kilometers of the Earth. (That's only 40 times the distance from the Earth to the Moon!) At that time it appeared to be near Polaris and had a magnitude of -1 with a beautiful gossamer-like tail that arched through 100o of our sky.

This photo was taken by Rick Scott and Joe Orman at about that time.

Comet Hyakutake reached perihelion (just 34 million kilometers from the Sun) on the 1st of May 1996 and has been moving away from the Sun and fading ever since. Astronomers have carefully measured Hyakutake's path and estimate that it will not return for another 15,000 years.
Certainly Comet Hyakutake can be classified as a long period comet!

What about "short period comets"? Where do they come from?

Ah, well short period comets are another matter and you will learn all about them in our next lesson. Feel free to take a break or continue on to learn about short period comets and comets in general.




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.