Although comets and asteroids are all members of the Solar System, their orbits are not the same. Asteroid orbits tend to remain in the plane of the Solar System (ecliptic) or near it, while comet orbits seem to have no preference; they can be oriented in any direction. Also - most comet orbits are much more eccentric than asteroid orbits. Some look much more like cigars than circles. Comet orbits also tend to be long. The famous comet Hale-Bopp of a few years ago has a period of over 2,000 years. Some long-period comets have orbital periods of over 10,000 years.
Remember Kepler's Second Law. The comet moves fastest when it is nearest to the Sun and slowest when it is farthest out. This means that comets spend most of their time far from the Sun; in the case of real long-period comets that is far beyond Pluto. Out there is the Oort Cloud. These comets join many others whose orbits are less eccentric and don't approach the Sun.
A comet is a lot of show from a very small object. The actual body of a comet is a lump the size of a small asteroid, although it contains a large amount of volatile materials not found in asteroids. We are not able to actually see these lumps because they are too small. The only way to get good pictures of them is to send spacecraft to fly close to them and send back pictures. Your book has a picture of Halley's Comet obtained in this way (see Figure 4.11).
Some comet pictures...
As comets make their short pass by the Sun and liberate lots of gas and dust, they leave behind a trail of particles that escaped from the nucleus with the gas. These particles will spread out along the comet's orbit and form a stream. If the Earth's orbit happens to pass through such a stream, we will get a meteor shower.
Also consider what happens to a comet over a long period of time. Each time the comet approaches the Sun, it loses a little bit of its mass to evaporation. Ices evaporate (sublimate) and release dust in the process. After enough passes byt he Sun, the ices are mostly depleted, and the comet stops releasing the clouds of dust and gas.
Meteor showers are actually streams of comet remnants. As the volatile material of comet nucleus evaporates, it releases small particles of solid material which continue in the comet's orbit. Over time these particles spread out along the orbit to make a meteor stream. These streams are the origin of meteor showers. Since these particles tend to be small, the showers tend to not make holes in the ground. The particles are completely consumed during their plunge through the atmosphere. The MUCH larger objects that DO make holes in the ground tend to be asteroid fragments and are not found in streams; they are loners in their orbits.
In addition to all the comets, asteroids, meteoroids and planets, there is a large amount of dust particles orbiting the Sun. Disks of dust like this have been observed around other stars. You can actually see this dust if you know how. You need to look in the west in late spring, just after the end of twilight in the evening; do so from somewhere away from city lights and smog. You will notice a faint glow extending up from the horizon. It look definitely different from twilight, as it forms a thin sort of triangular shape standing on the horizon. This is the Zodiacal Light. This photo was taken at Teide observatory in the Canary Islands.
Here's an easy mnemonic for remembering how the planets revolve and rotate. Hold your RIGHT hand out; point your thumb up and curl your fingers. The thumb is defined to point NORTH and the fingers then point in the direction of rotation and revolution. Look down at the end of your thumb, this is counter-clockwise. Therefore, when you look down at the North pole of anything, you would observe it rotating counter-clockwise.
When someone is trying to come up with a scientific mode to explain how our Solar System was formed, they must be certain that the model accounts for ALL the observed properties of the system. There are 9 interesting properties. See the list starting on page 115. Read them carefully.
Any model for the formation of the system must account for all 9 of these properties. They describe such a neat and orderly system, something NOT the result of random accidents. The nebular contraction model seems to be one that can account for those 9 properties.
The space between stars is populated with clouds of gas and dust particles. Such a cloud may contain several solar masses of material. Also remember that mass produces gravity. If the cloud is dense enough and gets a boost from something like a nearby explosion or a collision with another cloud, it can start to collapse as its own gravity drags it inward. It will begin to shrink. If there is any motion in the cloud (and there almost always will be), the cloud will begin to rotate as it shrinks. As it gets smaller it will spin faster. A spin axis will develop. Material along the equator of the cloud will stay in orbit while material near the poles falls toward the equatorial plane. The cloud collapses into a rotating disk.
This theory predicts that, with a powerful enough telescope in the right wavelength band, that dust disks should be visible around other stars. Dust disks have been detected around a number of stars, the bright star Vega being one.
Here are some links to photographs of real dust disks.
Although we cannot possibly watch a planet form (takes far too long) we can look for systems that may be forming planets. They might give more clues. So far, the dust disk idea is the only one that makes sense for our solar system. It would naturally produce a system with the planets in the same plane, orbiting in the same direction and rotating in the same direction.
We now have some evidence supporting this idea. The ALMA millimeter radio telescope array in Chile has imaged a young star with a set of planetary rings around it. Nothing beats finding an example of what your hypothesis predicted. Here are 20 ALMA images of protoplanetary disks. These are causing reevaluation of some of our ideas about planet formation. They are from the DSHARP campaign.
An extrasolar planet is, simply, a planet orbiting some other star. Astronomers have long believed that the Galaxy should contain a lot of planets. The fact that nobody could detect any was frustrating. If you look for extrasolar planets and don't find any, there are two possible reasons for the failure.
Remember that detecting something means that you have observed some effect that indicates the presence of the target although you cannot directly see it. Consider driving down the highway at night and smelling a skunk. You have detected the skunk but do not actually see it.
Astronomers at McDonald Observatory can measure velocites of less than 6 meters per second using Doppler shift. This is about 15 miles per hour! This capability stretches the size limit for planet detection downward, allowing smaller planets to be detected.
So far, most of the extrasolar planets are large, measured in "Jupiters." A planet that is "2 Jupiter" is twice the mass of Jupiter. The detection limit is steadily being pushed down. Detection of an Earth-sized (or a little larger) planet may be near. A Neptune-sized planet has been found recently, orbiting VERY close to its star.
In 2017 a rare event was observed: an object from interstellar passed through the Solar System and departed, never to return. It was an asteroid-like object that was named Oumuamua. Its orbit is hyperbolic, which means that it is an open orbit; the object will not return to the Solar System.
