Our solar system consists of one star, which we call the Sun, eight planets and all their moons, several thousand minor planets called asteroids, and an equally large number of comets.
Our Sun is classified as a G2V star, or yellow dwarf. G2 stars are approximately 6,000°C at the surface, are yellow, and contain many neutrally charged metals such as iron, magnesium, and calcium. The “V” indicates that the Sun is a dwarf star, or fairly small by the standards of stars. The Sun’s age is calculated to be around 4.7 billion years, which is only slightly older than the Earth itself.
Although a dwarf in comparison to other stars, the Sun contains almost 99.9 percent of the mass of our solar system. Like all stars, the Sun is a gigantic ball of superheated plasma, kept hot by atomic reactions emanating from its center. The temperature at the core of our Sun is thought to be about 15,000,000°C; temperatures at the surface range between 4,000° and 15,000°C. The diameter of the Sun is about 1.4 million kilometers, or more than 100 times that of Earth, and its surface area is approximately 12,000 times that of Earth.
The four planets closest to the Sun—Mercury, Venus, Earth, and Mars—are called terrestrial planets, meaning they are similar to our own planet in composition, with inner metal cores and surfaces of rock. Earth and Mars are the only terrestrial planets that have moons of their own, although the moons of Mars are much smaller than our own Moon. Earth is the largest of the terrestrial planets.
The four planets beyond Mars—Jupiter, Saturn, Uranus, and Neptune—are referred to as the outer planets. They also all have rings, most notably so on Saturn. The rings of Saturn, and most likely the other planets as well, are composed mostly of ice crystals. Pluto, once considered the ninth planet, is no longer categorized by scientists as a true planet. You may have learned a mnemonic (or memory trick) for the planets in school; here’s one that allows for the omission of Pluto: My Very Educated Mother Just Served Us Nachos. (In the diagram below, Pluto is included, but remember that it is no longer considered a planet. Also, note that the diagram below is not drawn to scale.)
The Solar System
In addition, the solar system contains thousands of small bodies such as asteroids and comets. Smaller fragments of asteroids and particles shed by comets are known as meteoroids, and when they fall into the Earth’s gravitational field, they are seen as “falling stars,” called meteors, as they burn up in the Earth’s mesosphere. Those meteoroids that make it to the Earth’s surface are called meteorites. A belt of asteroids lies between Mars and Jupiter. The Kuiper Belt, a much larger collection of asteroids and other objects left over from the formation of the solar system, lies beyond the known planets.
Comets are sometimes called “dirty snowballs” or “icy mudballs.” They are a mixture of ices (both water and frozen gases) and dust that for some reason didn’t get incorporated into planets when the solar system was formed. Comets are invisible except when they are near the Sun. When they are near the Sun and active, comets have highly visible tails, up to several hundred million kilometers long, composed of plasma and laced with rays and streamers caused by interactions with the solar wind.
By far the most important body in the solar system aside from the Sun, as it relates to life on Earth, is our own Moon. Because of the gravitational pull that exists between the Moon and Earth, we have tides. High tides occur twice a day, when the Moon is at the points closest to and farthest from the affected mass of water. It is suspected that life would never have evolved on land without the constant ebbing and flowing of the oceanic tides on coastal areas.
Earth orbits the Sun once per year. Because the Earth is somewhat tilted on its axis, the northern pole is tilted toward the Sun and the southern pole is tilted away from it for part of the year. That portion of the year is winter in the northern hemisphere and summer in the southern hemisphere. For part of the year that situation is reversed: that period is winter in the northern hemisphere and summer in the southern hemisphere. In between summer and winter, the Earth is tilted so that neither the northern pole nor the southern pole is pointing toward the Sun. These periods are spring and fall. Study the diagram below.
Earth’s Seasons
Imagine how terrified ancient humans must have been when their view of the Sun or the Moon became partially or completely obscured during an eclipse. Today we understand how eclipses occur and can predict them well in advance.
A solar eclipse occurs when the Moon passes directly between the Earth and the Sun during daylight hours. The relatively small shadow cast by the Moon onto the surface of the Earth can either partially or totally obscure the Sun. Because the Moon is moving in its orbit around the Earth and the Earth is rotating on its axis, solar eclipses are brief and move across the face of the Earth rapidly.
A lunar eclipse occurs when the Earth passes directly between the Sun and the Moon and casts a shadow on the Moon. Therefore, a lunar eclipse can be seen from any place on Earth where the Moon is above the horizon. Like solar eclipses, lunar eclipses can be either partial or total. Because the Earth has a much larger shadow than does the Moon, lunar eclipses last much longer than their solar counterparts.
Study this example of how an expert test taker would approach an ASVAB question about the solar system.
| Question | Analysis |
|---|---|
| Which of the following is a characteristic of terrestrial planets? | Step 1: The Earth is a terrestrial planet. What are its major characteristics? |
| Step 2: The Earth has a solid, rocky surface. There are many layers to it that include a solid inner core. | |
| Step 3: The answer choice will list one of those characteristics. | |
| (A) an inner core made of ice crystals (B) a surface temperature similar to that of Earth (C) an inner core of metal (D) a surface composed of gasses |
Step 4: Based on the prediction above, select answer choice (C). Terrestrial planets have an inner core of metal and a rocky surface. Outer planets have inner cores of ice crystals. |
Try this one on your own.
The correct answer is (A). A “falling star” is a meteor seen when a meteoroid is burning up in the Earth’s atmosphere. If the meteroid reaches the surface of the Earth, it is then called a meteorite.