The Solar

Have you ever looked up at the and wondered what was out there? You may see , , or even , but can you define each of these objects? What is the difference between a and a ? How are they different from , , , and the other kinds of objects in the system?

Discover : Geocentric vs. Heliocentric

For many centuries, most people assumed the and other stars orbited—or moved around—our planet, . This is called the of the . (Geo- means

geocentric “Earth,” so means “Earth-centered.”) Ancient peoples saw the Sun rise in the and set in the with the various bright of also moving across the . This gave the false impression that celestial objects moved around Earth.

The invention of the in the early 1600s gave scientists a much more accurate view of from Earth. Using measurements made while looking through , scientists such as Galilei demonstrated the truth of the heliocentric model. The Sun is the center of the solar system, with the planets orbiting the Sun and the stars remaining stationary (not moving).

What is in the solar system? To ancient peoples, only five planets (, , , , and ) and the Moon were visible to the naked . Unmanned such as the Voyager on the left have traveled beyond the edges of our solar system, revealing new views of old and surprising views of newly discovered worlds. Today we know that our

solar system consists of our Sun, planets, dwarf planets (smaller planets), moons, asteroids, comets, meteoroids, and a host of frozen worlds beyond . We also know that Earth and its solar system are part of the , which is one of many in the .

1 The Solar System

The Sun The Sun has enough that its gravitational pull holds the planets and other objects in the solar system in . The Sun rotates, or spins, about its axis like all the other stars located in the Milky Way galaxy. A is an extremely hot, dense mass of gases. As these gases burn, the star gives off visible as well as other forms of and charged . Most of the energy that reaches our planet, including light and , comes from the Sun. The Sun is actually medium-sized compared to other stars. Still, it is the most massive object in the solar system.

The gravitational attraction between celestial objects decreases with smaller and greater distances and increases with greater size and smaller distances. The Sun, in turn, is pulled by the massive gravitational of the center of the Milky Way, which cause stars and stellar to orbit around the center of the galaxy.

The Sun is even larger when compared to the planets. In fact, the Sun’s is more than 100 greater than Earth’s. Find a spherical object, such as an , and measure its diameter. (You can do this by cutting it in half.) If the orange represents Earth, what size of object would represent the Sun?

Planets Inner Planets Within our solar system are eight major planets grouped by proximity to the Sun. The inner planets closest to the Sun are small and rocky: Mercury, Venus, Earth, and Mars. The outer planets are the gas : Jupiter, Saturn, , and Neptune.

2 The Solar System

The Inner Planets Mercury, the smallest planet, has a cratered that looks like the Moon. Named for the swift of the gods, Mercury is the fastest planet to orbit the Sun, with a that is only 88 days long. Venus, the second planet from the Sun and similar in size to Earth, is covered in a thick of dioxide with that make Venus the hottest planet. Known for the longest in the solar system, Venus spins so slowly that by the it has orbited the Sun, it has spun almost completely on its axis making its day longer than its year. Earth, the third planet from the Sun, is noted for its , covering three-quarters of its surface, and its one moon. Mars, the fourth planet from the Sun, is called the planet, due to (rust) in the . Mars is a , lifeless, rocky where water once flowed in ancient times. Mars is known for its caps, having the largest and the longest in the solar system, and its two moons. You will find the belt (large space rocks) between Mars and Jupiter.

The Outer Planets Jupiter, Saturn, Uranus, and Neptune are the four gas giants that make up the outer planets. They are characterized by freezing temperatures, many moons, and systems. Jupiter, the largest of the planets, has the most moons (63 and counting) and the giant red spot, a famous that has lasted over 400 . Saturn is known for having the largest and has close to 60 moons. Uranus, a , has 27 moons and is tipped on its side in its orbit around the Sun, the result of an ancient impact. The last major planet is a blue gas giant named aptly after Neptune, the Roman god of the , and having 13 moons. All the gas giants have ring systems, although Saturn’s is the most prominent. The Solar System

Planets to Scale How do you think the planets compare in size? The largest planets are the gas giants in the outer solar system. Jupiter is the largest, followed by Saturn, Uranus, and Neptune. Earth is the largest of the inner planets, followed by Venus, which is almost the same diameter as Earth. Mars is next, at about half that diameter. The smallest planet is Mercury, at about one-third the diameter of Earth.

Pluto is no longer a major planet. once identified nine planets in the solar system. In addition to Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, astronomers considered tiny to be a planet. However, in 2006, the International Astronomical Union (IAU) officially defined planet as a celestial object that meets all three of the following criteria:

Pluto does not meet the third part of this definition, due to the large number of other frozen worlds that share its orbit. Because Pluto is so small, its is not strong enough to clear smaller objects from its orbit. Since 2006, astronomers have considered Pluto a . Pluto is part of a vast grouping of frozen worlds beyond Neptune called the . Astronomers have named several other dwarf planets in the solar system, including , which is nearly as large as Pluto. The largest asteroid in the , , has also been reclassified as a dwarf planet due to its large size. There may be dozens more dwarf planets awaiting discovery!

1. The object is in orbit around the Sun. 2. The object has a nearly spherical shape. 3. The object has cleared the neighborhood around its orbit of smaller objects. The Solar System

Do all planets take the same amount of time to rotate (spin on its axis) and revolve (orbit the Sun)? Like the Sun, each planet spins on an imaginary axis. Planets complete their in different amounts of time. Earth rotates once every 24 , or 1 day. In contrast, Jupiter takes only 10 hours to complete one . Venus takes 243 days to rotate once! Planets that are farther from the Sun take longer to complete one revolution. Neptune takes more than 160 times longer than Earth to orbit the Sun! Earth the Sun in about 365 Earth days, or 1 year. The orbital paths of the planets are not perfect . The planets’ orbits are elliptical, or oval shaped. This means the planets are sometimes closer to and sometimes farther from the Sun during their orbits.

(dwarf planet)

Not all solar system objects rotate in the way in which the Sun would appear to rise in the east and set in the west, as Earth does. Venus, for example, rotates backward, called retrograde rotation, so that the Sun rises in the west and sets in the east. Uranus spins on its side, the result of an ancient collision. Planets are generally in the same and thus appear in the night sky along a path called the . Sometimes the slower planets are passed by Earth and appear to move backward for a short while before progressing forward again. Planets do not shine like stars but instead reflect the to observers on Earth. observers thought these moving points of light that shifted among the fixed patterns were “wandering stars,” which they called planets.

5 The Solar System

Moons About 180 discovered moons orbit the planets of our solar system. Moons are smaller than the planet they orbit and are held in place by the pull of gravity.

The gas giants attract scores of moons. Jupiter, the largest planet in the solar system, not only has the most moons (more than 60), but also has some of the largest moons in the solar system: , , , and (pictured above). Ganymede is the largest moon in our solar system and is even bigger than the planet Mercury. These are known as the because they were discovered by the Italian scientist . The telescope was invented during Galileo’s lifetime, and he made many discoveries with it. Astronomers since Galileo have used improved telescopes and to gather information about the physical properties of moons. Some moons have large volcanoes. Others have major areas of frozen water. Some of the moons have layers similar to Earth’s layers: an inner core, a thick , and a thin outer .

Smaller Bodies The solar system contains other objects besides planets, moons, and the Sun. There are also asteroids, meteoroids, and comets. Asteroids are rocky objects that orbit the Sun. Asteroids are too small to be called planets, dwarf planets, or even moons. Asteroids are located in two different places in the solar system. The first and most well known is in the wide area between the orbits of Mars and Jupiter. The second is in the area past Pluto, known as the Kuiper Belt; it occupies a Most of the asteroids are located large of space. When an asteroid is knocked out of its in the asteroid belt between Mars orbit, it becomes a and a potential threat to and Jupiter. Pictured here is the planets, moons, spacecraft, and satellites. asteroid .

A meteoroid is another type of rocky object moving in space between the planets. Meteoroids are smaller than asteroids. Most meteoroids that enter Earth’s atmosphere are about the size of a pebble, yet they produce tremendous and heat as they through the air toward the ground. When we can see this heat in the sky as a glowing path, we call it a meteor. Most meteors burn up in Earth’s atmosphere before they reach the ground. A is a piece of a meteoroid (or an asteroid) that survives its passage through the atmosphere and strikes Earth’s surface. We sometimes see meteors streaking through the night sky.

6 The Solar System

A is a small mass of and ice that originates in an area beyond Pluto called the Oort . The gravity of the gas giants and the Sun pulls them inward in giant elliptical orbits much larger than those of the planets. This means that comets can get quite close to the Sun in some parts of their orbits. When comets are close to the Sun, solar and solar give them a visible and tail. The coma is a cloud of dust and gas around the comet. The tail is a trail of Many comets, including dust and gas that stretches behind the comet as it travels Hale-Bopp, take thousands of through space. In other parts of their orbits, comets may travel into the deepest reaches of the solar system. years to orbit the Sun. Astronomers believe comets are frozen left over from our solar system formation.

So, where exactly is our solar system? The universe has a vast collection of clusters of galaxies. In our part of the universe, the Milky Way galaxy is part of a of galaxies. Our solar system sits inside one of the spiral arms of the Milky Way.

7 The Solar System

Decide whether each characteristic or name listed below describes an outer planet, inner planet, moon, asteroid, meteoroid, or comet. Write each characteristic or name in the correct section of the Venn diagram.

Characteristics and Names of Celestial Objects

• Orbits the Sun • Neptune • Venus • Mars • Mercury • Callisto • Orbits a planet • Ganymede • Earth • Io • Smaller than planets • Saturn • Uranus • Europa • Jupiter