SOLAR SYSTEM SURVEY

Key Concepts

The solar system contains the Sun, the Jovian planets, the terrestrial planets, moons, asteroids, comets, etc.
Terrestrial planets are closer to the Sun; Jovian planets are farther away.
The orbits of the planets lie in nearly the same plane; orbital motion is all in the same direction.

(1) The solar system contains the Sun, the Jovian planets, the terrestrial planets, moons, asteroids, comets, etc.

If the solar system, like a box of cereal, were labeled with its contents in order of their mass, the list would look something like this (the number in parentheses is the mass of each object, in units of the Earth's mass):

1) Sun (330,000)
2) Jupiter (320)
3) Saturn (95)
4) Neptune (17)
5) Uranus (15)
6) Earth (1)
7) Venus (0.82)
8) Mars (0.11)
9) Mercury (0.055)
10) Ganymede (0.025)
11) Titan (0.023)
12) Callisto (0.018)
13) Io (0.015)
14) Moon (0.012)
15) Europa (0.008)
16) Triton (0.004)
17) Pluto (0.002)

Listed are the 17 biggest objects in the solar system to show how they fall naturally into five classes:

First: the Sun
Second: the Jovian planets
Third: the terrestrial planets
Fourth: the giant moons (or satellites)
Fifth: Pluto, the odd planet

Let's examine each of these classes in turn.

The Sun

The Sun is a star: a large sphere of gas (mostly hydrogen and helium) which emits light. The Sun is the dominant source of gravity in the solar system; it contains 99.8% of the total mass of the solar system.

The Sun is also the dominant source of electromagnetic radiation in the solar system. The Sun shines because it is hot. The surface of the Sun is at roughly 5800 degrees Kelvin, and has a wavelength of maximum radiation at 500 nanometers (in the middle of the visible range of the electromagnetic spectrum). The Sun has a much higher temperature than any of the planets in the solar system, so it radiates much, much more energy per unit area. The Sun also has a larger surface area than any of the planets in the solar system, so the total energy it radiates is much, much, much more than that radiated by any planet. When we see the planets with our naked eyes, we are seeing reflected sunlight.

The Sun stays hot because it is powered by nuclear fusion. At the Sun's center, conditions are hot and dense enough to permit four hydrogen nuclei to fuse together to form a single helium nucleus. The mass of a helium nucleus is slightly less than that of four individual hydrogen nuclei; the mass lost is converted to energy.
(E = mc², and all that).

The Jovian Planets

The Jovian planets are Jupiter, Saturn, Uranus, and Neptune. They are called ``Jovian'' planets after the largest member of the class, Jupiter (otherwise known as Jove in Roman mythology). The Jovian planets are the most massive planets: at least 15 times the mass of the Earth. The Jovian planets are also relatively low in density: at most 1700 kilograms/meter³. (By comparison, water has a density of 1000 kg/m³. The lowest density planet, Saturn, is actually less dense than water.)

The Terrestrial Planets

The terrestrial planets are Mercury, Venus, Earth, and Mars. They are called ``terrestrial'' planets after the largest member of the class, Earth (otherwise known as Terra in Latin). The terrestrial planets are lower in mass than the Jovian planets: Earth is the most massive terrestrial planet. The terrestrial planets are also relatively high in density: at least 3900 kg/m³, consistent with their being made of a mixture of rock and metal.

The Giant Moons

The next seven objects on our list are not objects which orbit the Sun directly. They are moons, or satellites, which orbit planets. (Clarification: I use ``moon'' in lower case to refer to any natural object which orbits any planet. I use ``Moon'' in upper case to refer to the Earth's moon. The word ``satellite'' is a synonym for ``moon''.) The giant moons of the solar system are typically one or two percent as massive as Earth. Let's list the giant moons of the solar system according to the planets which they orbit:

Earth: Moon
Jupiter: Io, Europa, Ganymede, Callisto (the Galilean moons, first seen by Galileo)
Saturn: Titan
Neptune: Triton

Note that the Earth's Moon is the only giant moon which orbits a terrestrial planet; Mercury and Venus has no moon, and Mars only has the relatively tiny moons Phobos and Deimos. If the giant moons orbited the Sun, we would not hesitate to call them planets, since they are all more massive than the smallest of the planets, Pluto.

Pluto, the Odd Planet

Pluto is less massive than the other planets, and has a density of about 2 grams/cm³, intermediate between the Jovian and terrestrial densities. Its orbit is more eccentric than that of the other planets Its orbit is tipped at 17 degrees relative to the Earth's orbit (a greater tilt than that of the other planets' orbits). Pluto is really an oddball planet, which doesn't fit neatly into the Jovian/terrestrial classification scheme.

The Leftovers

There are also many objects in the solar system that are smaller than Pluto. Small moons are made of rock, metal, and ice, and orbit planets. Asteroids are made of rock or metal, and orbit the Sun. Comets are made of dirty ice, and orbit the Sun. Meteoroids are made of rock or metal, are smaller than asteroids, and orbit the Sun. Dust grains are made of rock or metal and are smaller than meteoroids. (Note that planets, asteroids, meteoroids, and dust grains are distinguished from each other mainly by their size. However, objects orbiting the Sun form a continuum of sizes; the classification of objects as ``planets'', ``asteroids'', ``meteoroids'', or ``dust grains'' is purely for the convenience of astronomers.)

(2) Terrestrial planets are closer to the Sun; Jovian planets are farther away.

In addition to being lower in mass and higher in density than the Jovian planets, the terrestrial planets are also distinguished by being closer to the Sun. The terrestrial planets have semimajor axis lengths ranging from 0.4 astronomical units (for Mercury) to 1.5 astronomical units (for Mars). By contrast, the Jovian planets have semimajor axes ranging from 5 astronomical units (for Jupiter) to 30 astronomical units (for Neptune). The gap between Mars and Jupiter contains no planet, but is full of asteroids. It's probably no coincidence that the terrestrial planets and the Jovian planets are segregated in this way; it is an important clue for deducing how the solar system formed.

(3) The orbits of the planets lie in nearly the same plane; orbital motion is all in the same direction.

Except for Pluto, the oddball planet, all the planets have orbits which lie within seven degrees of the ecliptic plane. (Remember, the ecliptic plane is the plane determined by the Earth's orbit around the Sun.) The solar system, as defined by the orbits of the planets, is very flat. Why should the solar system be shaped like a Frisbee (or disk) rather than a beach ball (or sphere)? This is another important clue which will enable us to deduce how the solar system formed.

In addition, all of the planets (even Pluto) orbit the Sun in the same direction. If we viewed the solar system from a vantage point far above the Earth's north pole, we would see all the planets revolving in a counterclockwise direction around the Sun. You guessed it - this uniformity of direction is another clue for deducing how the solar system formed.

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