It’s a good question; and a very long-standing one.
You and your classmates may have seen the spectacular show being put on by Mars, which is closer now to our Earth than it will be for quite some time to come. Mars is fourth from the Sun (Earth is third), then comes Jupiter, and then sixth from the Sun, is Saturn. It’s a truly beautiful planet made particularly so by its ring system. This was first seen by Galileo in 1610, though his early telescope was not quite up to the task (he thought the rings were actually a couple of big moons on either side of the planet). The first really good photograph of Saturn, clearly showing details of the rings, was not taken until 1883 (that’s more than 2 and 1/2 centuries after Galileo’s first observation).
Actually even before this, careful observations had indicated that you could sometimes see the outline of the planet itself THROUGH some of its rings. It would therefore be pretty hard to say that the rings were one solid object, if we could see through them! So, it seems that they are not like the sort of thick-circle you might get by cutting a solid ring-like section out of a frisbee, and having it rotate around a ball at the center which would represent the planet itself. The alternative, a “non-solid rings” viewpoint, was really pinned down in 1895 when astronomers observed that the inner parts of the rings went around the planet faster than did the outer parts; this would be hard to reconcile with a single solid object, wouldn’t it?
As early as forty years before these ring-speed observations it had been proposed that the rings simply couldn’t be one solid object, but had to be made up of a vast number of small objects of varying sizes. Imagine if we took our moon, broke it up into a huge number of small pieces, and then spread these into a ring system. The pieces all “connect” with each other, rather weakly, making a mobile fluid-like disk (actually Saturn has several identifiable ring systems). You can probably see that your question is leading us to other important questions, such as, what are the pieces actually made of, how thick overall are the rings, and why are the rings flat anyway?
The reason for mentioning Saturn’s position in our planetary system is that along with Jupiter we have a pair of planets that between them account for around 90% of all the matter we have in all ten of our planets. Jupiter and Saturn are often called “Gas Giants” simply because they are largely made from the most abundant element in the Universe. This is hydrogen, but it is in very highly compressed form in these planets. (You may have noticed that it’s much in the news these days as a possible energy source.) Well, this doesn’t necessarily say that the rings are made of chunks of cold, solidified hydrogen, but it’s interesting that we now know their composition to be mainly ice, that is, solid water; and as you may have learned by now water (or H2O) is two-thirds hydrogen.
So ice-particles (some are big, around a few meters in size) are what mostly form the rings in which small quantities of other materials are also present. Compared with the size of the planet we know the rings must be very, very thin because Saturn (and its rings) rotate and occasionally the rings are presented to us ‘edge on’ and they are very hard to see (imagine how this must have puzzled Galileo when he looked back later and could not see the striking features that made Saturn so prominent to him in the first place). Estimates are that the ring system could be as thin as a few meters in some places.
Why so thin? Why are the ice particles not spread out all around the planet? This is a longer story and it has a lot to do with the fact that the ring system is rotating, as we said. But you can probably see that if the disk had begun to form, with many particles in it, but others were in orbits slightly tilted from it, then over the ages these tilted orbit particles would constantly be passing through the disk, once on the way up and once on the way down, so to speak. But this passage is a dangerous process for these tilted orbit particles; they have a good chance of colliding with the disk particles, and even particles in orbits tilted the other way. These collisions tend to gradually reduce the tilt and a “settling into the disk” process is the result, and what we see today. Saturn also has 22 moons and they are also known to assist in keeping the ring system stable. By the way, though they are a lot more difficult to see, Uranus, Neptune and Jupiter also have rings; Cornell’s astronomers had quite a hand in the discovery of Jupiter’s ring system.
Astronomy is our oldest science, and the heavens are an almost unending source of questions for us all. If you or your classmates are curious, and you want to find out more about the planets, our moon, the stars and the galaxies, the comets, and so on, by all means ask your teacher to help you gain access to the internet and to help you look up the web site, curious.astro.cornell.edu, maintained by Cornell University’s Astronomy Department, and its “Ask an Astronomer” program. Finally, astronomers still debate how (and when!) the ring system on Saturn got started in the first place. This means that after some time you might want to ask your question again, and even again!
Neil W. Ashcroft
Ph.D. Cambridge University
Theoretical condensed matter Physics
Wife, Judith Ashcroft; sons, Dr. R.N. Ashcroft, Major I.R. Ashcroft, and a new granddaughter, Laney Ashcroft