Unit 3 : Activity 3 : Inventory of the Solar System : Figuring Out Solar System Distances : Unit Exam

Inventory of the Solar System

There are eight major planets in our solar system, of which the Earth is one. Starting closest to the Sun moving outward, the planets are:

Terrestrial Planets Mercury
Jovian Planets Jupiter

The inner planets are called Terrestrial planets because they most resemble Earth in terms of composition and internal structure. That does not mean that conditions on the surfaces of Mercury, Venus, and Mars are like the Earth. On the contrary, the Earth is a rare gem with vast amounts of liquid water and an oxygen-rich atmosphere. A human exposed to the atmospheres and temperatures on any of the other Terrestrial planets would only survive a few minutes or less.

The Jovian planets are so named since they resemble Jupiter. All four Jovians are gas giants. They are truly giants: the biggest, Jupiter, has a diameter 11.2 times that of the Earth.

And then there is Pluto, considered a planet from its discovery until August 2006 when it was reclassified as a "dwarf planet". Several things contributed to this decision, and it was not the first time a planet was "demoted". When the asteroid Ceres was discovered, it was thought to be a planet; however, when other asteroids were found between Mars and Jupiter Ceres was reclassified as an asteroid or minor planet. (It is now eligible for the category "dwarf planet" which requires it to be round, held together by gravity, but not necessarily to dominate its neighborhood.) When Pluto was discovered, it was thought that it affected the orbit of Neptune; that was later shown to be incorrect. It was thought to be bigger than its actual size for quite a while, until the discovery of Charon made it apparent some of the light used to estimate its size was actually coming from its moon. Finally, a number of slightly smaller but very similar objects have been found, and one body, Eris, that is larger than Pluto and also has a moon of its own, Dysnomia.

The definition for "planet" that was adopted by the International Astronomical Union in 2006 states that a planet is round, orbits the Sun (and is not a satellite), and has cleared its neighborhood of other debris. Another way to state the final condition is that it clearly dominates the neighborhood. The four terrestrial planets satisfy that condition, as do the four Jovian planets, but Ceres and Pluto do not.

To qualify as a dwarf planet, according to the 2006 IAU definition, a body must be orbiting the Sun, not be a satellite or moon, and be round. A body that is held together by gravity will generally be round; one that is held together by the strength of the material need not be. So, the Earth is held together by gravity, as is our moon, the Galilean moons of Jupiter, Pluto, Charon, Ceres, and Eris. If something held together by gravity alone gets too near another bigger body, like a comet flying too close to Jupiter (which did happen to comet Shoemaker-Levy), it will be broken up into bits that are small enough to be held together by the strength of the material. Close to a planet - for example, on the surface of Earth - you find only bodies held together by the strength of the material - you, a table or a chair. As of this writing (September 2006), there is not an official list of all the known dwarf planets in our solar system; there are still some objects being discovered beyond Neptune, and the details of the "roundness" rule are still being worked out.

Image by NSSDC Photo Gallery
Pluto and its moon Charon

One interesting note about Pluto is that for about 230 out of its 250 year orbital period (the time is takes to orbit the Sun or a Pluto "year") Pluto is farther from the Sun than any of the major planets. However, for 20 Earth years of Pluto's orbit, Pluto is actually inside the orbit of Neptune. Neptune was the farthest planet from the Sun from 1979 to 1999. Only that recently did Pluto pass outside the orbit of Neptune and reclaim its distinction of being the farthest planet from the Sun -- only to lose its status as a planet a short seven years later.

How big are the planets?

We have mentioned that Jupiter, the largest planet, has a diameter of slightly over 11 times that of the Earth. But how big is that? And, how does the Sun stack up?

The graphic below show the relative size of the major planets in the solar system.

Image by The Space Place

To give you an idea of how these sizes compare, imagine that the Sun is represented by a basketball. Jupiter would then be approximately the same size as a pingpong ball and the Earth would be the size of a "BB," the round end of a stick pin, or a small ball bearing. Pluto and Eris, the largest dwarf planets, would be represented by grains of sand.

How far apart are the planets?

Most graphics you see of the solar system show the planets close together and fairly large so you can recognize each one. It is important to realize how drastically exaggerated these drawings are.

Take our example above where the basketball represents the size of the Sun. The give the correct proportions for distances in the solar system, imagine that we place the basketball at one end of a 100 yard American football field. The BB representing the Earth would have to be placed on the 36 yard line, approximately 110 feet away. The pingpong ball representing Jupiter could not even be placed on the football field. In fact, the Jupiter pingpong ball would be two football fields (200 yards or 600 feet) away. Finally, the grain of sand representing Pluto would have to be 4,220 feet away, or 0.8 miles, from the basketball!