Mercury     Venus  Earth     Mars   Jupiter   Saturn       Uranus            Neptune     Pluto                

              0.4              0.7         1           1.5            5               10                   20                            30              40                   

My Very Educated Mother Just Served Us Nachos

Of course the major elements of our solar system are the 8 major planets, listed above.  It was 9 planets but Pluto is no longer considered a full fledged Planet, rather it has been downgraded to a new classification.....a Dwarf Planet.  More on that later.  For now it is important to know that the planets, which we are so familiar with, are only part of the story.  In addition to the Planets, there are three other categories of objects in our Solar System.....moons, Dwarf Planets, and Small Solar System Bodies. 

Planets

Initially, all lights in the sky were thought to be from a similar source, until early astronomers began to notice that a few of the lights behaved differently than the others.  As these lights were discovered and studied they were named Planets.  Today we know there are other kinds of bodies in our solar system and some are similar to the 8 major planets.  In 2006, the International Astronomical Union (IAU) defined a Planet as: (1) in orbit around the sun, and (2) enough mass for gravity to have drawn it into a round shape, and (3) enough mass to have cleared out everything else in it's orbital path.  Because of this new definition, Pluto was no longer a Planet since it did not clear out everything else in its orbit.  However, the term Dwarf Planet was given to Pluto (see below).  However, this did not end the discussion...the debate continues, but officially Pluto is a Dwarf Planet for now.  There are 8 Planets and there will not be any new planets in our Solar System simply because we've looked and there aren't any new objects that meet the criteria above.

The diagram at the top of this page is not to scale in regard to distance from the Sun.  The Planets are actually spread out much more than indicated.  Note the numbers below the Planet names...these numbers represent the distance from the sun, in terms of the Earths distance.  For example. Mars is one and half times further from the sun than the Earth.  A couple of things can be noticed right away:

1. Mercury, Venus, Earth, and Mars are tucked in pretty close to the Sun (all within 1½ earth distances), compared to Jupiter, Saturn, Uranus, and Neptune .  For this reason, these planets are referred to as the Inner Planets, and the latter Planets are referred to as the Outer Planets.

2. If we had a scale model of the Solar system on our desk and the earth was positioned 1 inch from the sun, the inner planets would be within the first 1½ inches.....and Neptune would be 30 inches away!  If you think about this for a minute you soon realize that the planets are well spread out.

3. There's another thing to notice that has some astronomers thinking.....each Planet seems to be about 1 to 1½ times further from the Sun then the previous Planet....except for Jupiter.  There seems to be a missing planet between Mars and Jupiter.  Now this on it's own might seem to be a bit academic however there is one other curious thing....right about where this missing Planet might be expected, there's a ring of debris!....the asteroid belt!  See the discussion below...The Exploding Planet Theory.

Planet.....a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.

Satellites (moons)

Any of the above objects can have a satellite, which is basically defined as an object circling a celestial body.  Satellites around objects other than stars are also commonly called moons, however strictly speaking the Moon is actually the name of Earth's satellite.   But for ease of use we'll call all the satellites moons, which is common anyway.  Several of the planets have moons.   There are at least 144 moons circling 6 planets in our Solar System...6 planets because Mercury and Venus don't have moons (surprised that there 144 moons?....see the Inner planets and the Outer Planets).  However, as noted above, moons are not limited to the Planets alone.  The Dwarf Planets have moons, as well as some of the larger asteroids.   

Moons have become especially important today because of their diverse character and make-up.  Many are very different in many ways, for example, some have volcanoes and are active geologically, some have substantial atmospheres, some have hydrocarbon lakes, some are heavily cratered, some are icy, and some rotate very slowly.  A few seem to have the conditions that might support life of some elementary form

Dwarf Planets

The Dwarf Planet is a relatively new classification of objects in our Solar System. It was defined in 2006 by the International Astronomical Union (IAU) as those objects which are large enough to be round but are not one of the current 8 Planets.  One of the characteristics is that the object must be large enough to become round.  How many round objects are there in the Solar System that are not planets or moons?  Maybe in the order of 200 when current surveys are completed.  However, officially the IAU has only identified 5 objects as Dwarf Planets, as of 2008.....Ceres (an asteroid in the Asteroid Belt); Pluto (recently demoted); Makemake, Haumea, and Eris.

There is another term, Minor Planets, which has been used in the past to refer to asteroids in the Asteroid Belt.  This term has never been fully accepted by the astronomical community.  It often is used interchangeably with the term asteroid, although usually applied to the larger asteroids.  I believe its use has now been largely discontinued as a result of the new definition of Dwarf planet, however you may still find it in some of the literature.  Don't get too excited about it...I'd just ignore it's use.

Dwarf Planet....A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has not cleared the neighborhood around its orbit.

Within the category of Dwarf Planet, there is a classification known as Plutoid.  A Plutoid is basically a Dwarf Planet beyond the orbit of Neptune.   Currently there are 4 Plutoids.  The first was named in June of 2007 and is, oddly enough, Pluto.  The others are Makemake, Haumea, and Eris.  All of these are located in the Trans-Neptunian region where Pluto orbits.  What about Dwarf Planets that are located within the orbit of Neptune?  Do they have a separate classification?  No.  There is currently only one Dwarf Planet in this area, Ceres, and it is located in the Asteroid Belt.  It is possible that a future classification will be named if more Dwarf Planets are found within the orbit of Neptune.  But for now there is no further classification, Ceres is  simply called a Dwarf Planet.

Small Solar System Bodies

Small Solar System Bodies are all the other objects not in the Planet or Dwarf Planet category.  They include asteroids (between Mars and Jupiter), comets, near Earth Objects,  and Kupier Belt Objects/ Oort Cloud objects  (in and beyond the area of Pluto's orbit).

Asteroids

Asteroids are  small rocky and icy objects of irregular shape.  They can be a few meters in size to several thousand meters in size.

Between Mars and Jupiter there is the Asteroid Belt, in which there are hundreds of thousands of asteroids ranging from less than 1km in diameter to over 200km in diameter.  There are 26 asteroids over 200km in diameter.  Nearly all of the asteroids in excess of 100 km have been cataloged.  Over half of the asteroids between 10km and 100km have been cataloged.  Why are these asteroids here....in a nice orbit around the sun, between Mars and Jupiter?  Actually, there is some conjecture that this Asteroid Belt is the remnants of a previously existing Planet which exploded! This mystery Planet has been given the name Planet V to indicate it would have been  the fifth planet from the sun.

......did you say exploding planet??

The Exploding Planet Theory.  Yes, there is sufficient evidence to suggest that a planet did explode.  It is not understood how a planet can explode, i.e.. the process by which that could happen has not been proposed, but there are conditions in our solar system today which make the exploding planet idea plausible..... 

In any event, the origins of the asteroids will continue to be debated until further evidence points the way.  This is a good example of observations and measurements from which scientists develop theories to fit the measurements.  As additional information is discovered, the theories are refined...or dropped as the case may be.

Comets

Comets are basically asteroids which are in highly elliptical orbits around the sun.  Because their orbits bring them close to the sun they heat up and give off gases which result in long spectacular tails as they approach the sun.

In contrast to the Exploding Planet Theory for the source of comets, the conventional thinking is that there is a cloud of material a great distance beyond Pluto (several thousand times the distance from the Sun - this is a huge distance away!) which is the source of the comets.  This is the Oort Cloud.  If disturbed for some reason one of these objects could be set off towards the Sun and become what we see as Comets.  These comets from the Oort Cloud have extremely long orbits so that any given generation here on earth would only see them once.  The theory actually considers that 1000's of these comets have been  setoff on their journeys but we see only a few. 

In addition to the Oort Cloud there is another source for comets known as the Kuiper Belt, (pronounced Ki-per).  The Kuiper Belt is located just beyond the orbit of Pluto.  The Kuiper Belt is populated with thousands of icy objects and approximately 1000 have been identified.  These objects are called Kupier Belt Objects (KBO) or Trans-Neptunian Objects (TNO).  Because they are so much closer than the Oort Cloud Objects, once disturbed to begin their orbit of the sun their orbital periods are much shorter and therefore a generation here on earth may see them repeatedly.  

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