The solar system consists of our sun and anything orbiting the sun. If you look at the solar system from the "big picture," all the material orbiting the sun (including the earth) is just fluff. The sun takes up over 99.8% of all the mass in the solar system, ... leaving only a fraction of the total mass for:
Planets and their satellites
Dwarf planets
Asteroids
Comets
I grew up learning that our solar system had 9 planets. That all changed in 2006 when Pluto got a demotion. Why? Look for my clues below. Our solar system has only 8 planets. Astronomers subdivide the planets into 2 main categories- Terrestrial and Jovian.
Terrestrial Planets - Mercury, Venus, Earth, and Mars are also often called the "inner planets" because they orbit close to the sun.
Jovian Planets - Jupiter, Saturn, Uranus, and Neptune are, therefore, also known as the "outer planets".
The reason astronomers created this subdivision is because there are extreme differences between each group, namely:
Terrestrial Planets
Jovian Planets
small and mostly rocky large and mostly gaseous high density (compactness) low density (Saturn would even float in water) low total mass high total mass very few moons and no rings many moons and all have rings rotate slowly on their axis rotate quickly on their axis
Pluto is small and icy .... clue #1
Interesting update: In 2018 astronomers found evidence that our solar system may have had another planet (between the size of Mercury and Mars) which formed and then later was destroyed by a collision. The evidence comes from "nanodiamonds" found in a meteor. According to the report, these nanodiamonds formed in the interior of a planet in an environment of 2.9 million psi. Read the story here.
Scale of the Solar System (diameters of the planets are not to
scale)
On this scale, Pluto would be about a yard to the right.
To give a sense of scale to the solar system, it is best is we use the Astronomical Unit (AU) as our basic unit. The following is a breakdown for the planets:
|
Object |
Diameter (relative to the Earth's size) |
Mean Distance to the Sun (AU) |
| Mercury | .38 | .39 |
| Venus | .95 | .72 |
| Earth | 1 | 1.0 |
| Mars | .53 | 1.52 |
| Jupiter | 11.2 | 5.20 |
| Saturn | 9.4 | 9.58 |
| Uranus | 4.0 | 19.2 |
| Neptune | 3.9 | 30.0 |
| Pluto (for old times sake) | .2 (clue #2) | 39.2 |
All the planets orbit the sun in the same direction and nearly on the same plane (except Pluto ... clue #3). For the most part, our solar system is flat.
A good way to envision the scale of the solar system is to imagine we shrink the entire system down so the sun is reduced to the size of a baseball (standard hardball) and we place the baseball (sun) on home plate at Miller Park (home of the Brewers). On that scale, the earth would be half way to the pitchers mound and only the size of the period at the end of this sentence. Jupiter (the largest planet and by far the most massive) would be found half way to the left field fence and only the size of a ladybug. Pluto would be around the sausage house (well past the outfield, bleachers, and into the parking lot) and would be the size of a dust mite. This should convince you that the planets are just "fluff" compared to the sun.
Poor moons! They generally get second billing to the "things" they orbit. It all comes down to mass. The big dog gets top billing ... whether the moon orbits a planet, dwarf planet, or even an asteroid. If you just happen to have a natural satellite gravitationally bound to it, it is just a moon. But if you ask me, all the moons of the Jovian planets are MUCH more interesting than the giant planets they orbit. Why? The Jovian planets are just giant, turbulent balls of gas with no discernable solid surface, but the satellites that orbit them are solids with some very interesting features. Our own moon is bigger than dwarf planet Pluto. Heck, some moons (Jupiter's Ganymede and Saturn's Titan) are bigger and more massive than the planet Mercury!
A small body orbiting the sun was discovered in 1801 and named Ceres. Soon afterwards, several more small bodies were found. Each one was found to be much smaller than any known planet. Often astronomers would refer to them as "minor planets" but today we refer to them as asteroids. Asteroids are small, rocky, irregular shaped objects which orbit mostly between the orbits of Mars and Jupiter. This is known as the asteroid belt. Today several thousand asteroids have been found. Most are found near the ecliptic plane (plane of the solar system) about 2-3 AU from the sun. Using our baseball model, that would place the asteroid belt around each of the bases (remember, the sun is a baseball at home plate).
Most asteroids are found between the orbit of Mars and Jupiter
The newest member of our solar system (latest discovery) is known as the Kuiper Belt. In 1992, a small, icy body was discovered just beyond the orbit of Neptune. Named 1992 QB1, this discovery was quickly followed up several more similar discoveries. It was soon apparent that astronomers needed to extend the family of objects in the solar system to include this group, which extends from just beyond Neptune to about 400 AU from the sun. Many Kuiper objects have been found with orbits up to around 40 degrees from the ecliptic plane. That is, the Kuiper belt is a doughnut shaped region surrounding the sun, ... starting at Neptune. Several of these objects were given exotic names such as Haumea, Makemake, and Eris (which was first thought to be bigger than Pluto ... clue #4). Using the baseball model, the Kuiper belt would start in the parking lot of Miller Park and extend all the way to Lake Michigan (a few miles away).
Pluto orbits 17 degrees off the plane of the ecliptic (clue #5)
Did the clues work? Since the discovery of this new group of objects, astronomers found a way of understanding the existence and properties of Pluto. Ever since it was first discovered in 1930, Pluto never fit any of the physical characteristics of the other planets. Pluto is small and icy. It has an orbit which was inclined 17 degrees to the ecliptic (plane of the solar system). But now, astronomers see Pluto as a large (and possibly not even the largest) object found in the Kuiper Belt. Does this mean Pluto should lose its status as a planet?
On August 24, 2006 astronomers demoted Pluto from the list of planets ... it is now just a "dwarf planet" in name. Maybe because a group found a Kuiper belt object which, at first, was thought to be larger than Pluto. Originally unceremoniously named 2003 UB313 and later renamed Eris , this icy world made an even bigger case for dropping Pluto off the list of planets.
Dwarf Planets (a new term invented in 2010) - Ok, the debate is over. Pluto has been yanked off the list of planets and officially labeled a Dwarf Planet. According to the IAU , there are now 5 official dwarf planets - Ceres, Pluto, Haumea, Makemake, and Eris. You can bet the farm that this list will grow. For example, there are several objects (Sedna, 2012 VP113, and others) just waiting to become "official". It really comes down to defining what a planet is. A planet (now) has to meet these criteria:
Orbit the sun (which eliminates any moons)
Be round (pretty much) due to its own gravity
Have "cleared out" most of the debris in its neighborhood. That is, no nearby litter.
My feeling is they had to "fudge" the definition to avoid the headache of dealing with the multitude of large, distant, icy worlds that have been or will surely be discovered in the near future. This "fudge" means Ceres was promoted from the largest asteroid to a dwarf planet and Pluto is demoted to a dwarf planet (from its reign as a planet). Both orbit the sun and are round, but neither one of these objects have a neighborhood which is litter free. So here is the problem now. There have been so many objects found in the Kuiper belt and it is very hard to determine if they are round (due to its own gravity). The IAU let Haumea in but it is "roundish" ..... so where do you draw the lines since many, many more sort-of roundish objects have been discovered in the Kuiper belt? Are they dwarf planets too? The IAU makes the call but they have a dilemma. First it is very difficult to know the exact shape of these distant objects. The only way is by timing transits when they block the light from more distant sources and that just gives you a general idea of the shape (they are too distant to see directly). Second, there are simply too many things out there that have been discovered that are very close to the existing definitions to be called dwarf planets. Look again at this list and you will see the problem.
In 2015, the New Horizons spacecraft flew by Pluto and, for the first time, gave us a close-up view of our beloved former planet. It discovered that Pluto is officially the largest dwarf planet with a diameter of 1,473 miles ... 50 miles bigger than once thought.
Pluto and its largest moon Charon (which is certainly round but it orbits Pluto so no dwarf planet status for you! )
Pluto's ice mountains and surprisingly smooth young surface (why no crater impact sites???) Courtesy: http://pluto.jhuapl.edu/Multimedia/index.php
Planet 9 ??? ... from outer space (sorry ... I could not resist)
Ok, it has not been found ... yet. However, astronomers announced (on 1/20/2016) that an unseen planet could be the reason several Kuiper objects are "clustering" in an unexplainable manner. Computer models indicate that the gravitational tug of a surmised "Planet 9" could solve this mystery. Planet 9 would have to be very massive (about 10 time the mass of the earth) and very far away (600 AU). Observing such a planet is currently beyond our technology. It would be far too dim in the visible spectrum to be seen (since it reflects sunlight). Perhaps it could be discovered by its own "heat signature" in the far infrared. Anyway, there is nothing official ... so we are still holding at 8 planets in our solar system. Stay tuned.
Comets are also small objects (most are about the size of a mountain), but unlike asteroids (which are mostly rock), comets are mostly ice. About two hundred comets have highly eccentric orbits and often orbit far from the ecliptic plane.
These "short period" comets enter the inner solar system on a regular basis. For example, Halley's Comet enters the inner solar system every 76 years. Most comets, however, enter our solar system from much greater distances and from all possible directions. As a result, they do not re-appear in the inner solar system for thousands of years (or even longer time periods). Their orbits indicates that their place of origin is a very great distance from the sun, ... perhaps 50,000 - 100,000 AU away from the sun, in a spherical zone known as the Oort Cloud. This would make comets the most distant objects in our solar system (approaching interstellar distances). Using the baseball model, the Oort Comet Cloud would extend all the way to Canada (about 500 miles from Miller Park) and extends in all directions (in 3 dimensions).
We now take a look at some of the individual members of our solar system, starting with my favorite planet - Earth.
ŠJim Mihal 2004, 2014, 2019- all rights reserved