By Chris Fellows, Serenity Mobile Observatory
(click any image to enlarge – except Chris’s 😉 )
In my travels and conversations with folks during star parties and coffee table discussions I have noticed that there is some confusion about the general structure and scale of this wonderful universe we live in. I am going to cover this in a few of installments because there is too much information to squeeze into a single article. Most people know the labels given to structures and objects but don’t know or are confused about how the pieces of this largest jigsaw puzzle fit together. In this article I am going to attempt to clear up some of this confusion and see if we can get a glimpse of the larger universe. Let’s start small and work our way up.
At the smallest level, on a universal perspective at least, we have a planetary system. A planetary system consists of a parent star and all the stuff gravitationally bound to it. This includes many objects, and we will deal with some of them here. Remember that the only system we have a lot of information about is our own; we believe this is a typical system and extrapolate to the larger universe based on that information. We gain more and more information every year and so far this assumption has been born out, but as critical thinkers we should remain open to new information that could radically change our views in the future. With that said, let’s talk about what we know for sure.
A planet is now defined as any object that meets the following requirements: (1) It orbits its parent star; (2) It is of sufficient mass to settle into a basically spherical shape; and finally (3) It has mostly cleared its orbit of other objects through gravitational interactions.
This final criteria is the deal breaker for Pluto. Pluto orbits the sun, is nearly spherical, but resides in the Kuiper belt that has many other objects, such as Quaoar, Sedna and Eris, that are comparable in size and are in the same solar orbit as Pluto. In other words, if we count Pluto as a planet we would very quickly move from having eight planets in our solar system to possibly dozens or even more. Sorry folks, Pluto has been downgraded to a dwarf planet in the minor planet category in the trans-Neptunian Kuiper belt grouping of objects.
Many planets have natural satellites we call moons. These objects don’t orbit the parent star directly but orbit a planet. Most, but not all, planets in our solar system have moons. The gas giants Jupiter and Saturn have many moons – in fact, our solar system has more than 175 identified moons: Jupiter having 79 discovered, Saturn has 53 named moons, Neptune has 14 and Uranus with 27. That accounts for 173 moons in our solar system. Mars has two small moons and of course Earth has one very large moon. Although Pluto has five objects around it – Charon, Styx, Nix, Kerberos and Hydra – they are all gravitationally in more of a swarm than direct orbits of Pluto.
In addition, our solar system at least has a few other structures that need to be mentioned. First is the Asteroid Belt. This is a collection of rocky and metallic detritus in between the orbits of Mars and Jupiter. It contains an estimated 1 to 2 million rocks larger than 1 kilometer in diameter and millions of smaller objects. This sounds like a lot, but it isn’t really. The total mass of the Asteroid Belt is approximately 4% that of the Moon, or 22% that of Pluto, and roughly twice that of Pluto’s moon Charon. Hollywood would have you think that it is a crowded minefield of wildly swarming rocks. In fact, if you were standing on any given asteroid you probably wouldn’t be able to see any other asteroids. Movies are fiction, folks.
Heading outwards we will run into two more structures in our solar system. First is the above mentioned Kuiper Belt –a large, flat disk of solar system formation leftovers like the Asteroid Belt but much larger. It is 20 times as wide and 20 to 200 times as massive. Unlike the Asteroid Belt, Kuiper Belt objects are (mostly) not rocky but are made up of volatiles like methane and water ice. It is estimated that there are around 35,000 Kuiper Belt objects that are larger than 100 km in diameter. There may be as many as 100 million small and faint objects in the belt, with a diameter of 20 km or less.
Finally, still heading outwards, we have a theoretical object called the Oort cloud. It is a so-far not directly observed cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 50,000 to 200,000 astronomical units (AU), or 0.8 to 3.2 light years.
Lastly but arguably the most important objects in our solar system are the star we call the sun and the planets. These deserve articles on their own so you will have to wait till the next installment to get that scoop. Have I missed anything? Let me know in the comments.
Till next time!
Chris Fellows, Serenity Mobile Observatory
Find Chris on Facebook (or, if you’re lucky, at your campground). (Editor: Check out his amazing photos on his Facebook page!)