Planet, or Something Else?For hundreds of years, the word “planet” did not have a good scientific definition. Pluto was declared the ninth planet when it was discovered in 1930. However, it is different from the other planets.

• It is smaller than Earth’s Moon.

• It has a flattened orbit that crosses the orbit of Neptune so that sometimes it is closer to the Sun than Neptune (Figure 1).

• Its moon is almost as large as itself.

Then, in the late 20th century, other small, planet-like objects were discovered farther from the Sun. Some were larger than Pluto. Astronomers did not know what to call these objects. It was obvious that a clearer definition was needed for the term “planet.” In 2006, scientists decided on the three-part definition on page 293.

Pluto does not satisfy the third rule. It is not able to clear other objects out of its orbit. Thus, Pluto is no longer considered a planet. It is now considered to be a dwarf planet. A dwarf planet is a celestial object that orbits the Sun and has a spherical shape but which does not clear its own orbit.

There are five recognized dwarf planets: Ceres, Haumea, Makemake, Eris, and Pluto. However, astronomers estimate that there are at least 70 000 icy objects beyond Neptune that orbit the Sun. Some dwarf planets, such as Eris, are much larger than Pluto and take nearly twice as long to orbit the Sun.

dwarf planet: ▶ a celestial object that orbits the Sun and has a spherical or round shape, but does not clear its own orbit

Pluto

Uranus

Neptune

the inner planets

SaturnJupiter

Figure 1 Pluto’s tilted orbit crosses the path of Neptune’s orbit.

dig deeper

Did You Know?other Celestial objects

Astronomers estimate that there are at least 70 000 icy objects in orbit around the Sun beyond Neptune with about the same composition as Pluto. The dwarf planet Eris is 27 % larger than Pluto and takes nearly twice as long to orbit the Sun.

8.4After the Sun, the largest objects in the Solar System are planets. Astronomers recognize eight planets: the four inner terrestrial planets and the four outer gas giants.

NEL 8.4 Planets 305

Outer PlanetsThe outer planets are known as gas giants, because they are made up mostly of gas. Jupiter is the largest of all the planets, and it has more than 60 moons. Ganymede, Jupiter’s largest moon, is bigger than the planet Mercury. Saturn is famous for its rings, which are made up of ice crystals and can only be seen through a telescope (Figure 2). Jupiter, Neptune, and Uranus also have rings, but they are much more faint. Uranus is mostly gas, but it may have liquid water and a solid core. One of Neptune’s moons, Triton, may once have been a ball of ice and dust—a comet—that was captured by Neptune’s gravity when it passed too close to the planet.

The distances between the inner planets and the outer planets are too large to be shown to scale in a diagram. Figure 3 shows the relative sizes of the eight planets in the Solar System. Of the outer planets, only Jupiter and Saturn can be seen from Earth with the unaided eye.

the inner PlanetsMercury, Venus, Earth, and Mars are the planets closest to the Sun. The inner planets are called terrestrial planets because they have a solid surface made up of rock. Earth is the largest of the inner planets and the only planet known to support life. Mercury was named after the speedy Roman messenger god because it travels around the Sun very quickly. From Earth, Venus appears brighter than any planet or star (except the Sun). Mars may once have had liquid water, but now it is so cold that it has huge amounts of ice beneath its surface. Mercury, Mars, and Venus can all be seen from Earth with the unaided eye.

dig deeper

Did You Know?Europa

Europa is one of the 60 moons of Jupiter. Scientists think that there is water more than 100 km below Europa’s surface. It is possible that Europa’s liquid water and relatively warm temperatures might support life of some kind.

MarsEarth

Venus

Mercury

Sun

Jupiter

Saturn

UranusNeptuneNeptune

Figure 3 The planets in the Solar System—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. (Note: This diagram is not to scale.)

Figure 2 Saturn’s rings are composed of small pieces of ice and a bit of dust. This makes them reflect the Sun’s light very effectively.

NEL306 Chapter 8 • Touring the Night Sky

CheCk youR LEaRning

1. Which of the planets can be viewed without a telescope? K/U

2. What made scientists decide that a new definition of the term “planet” was needed? K/U

3. Only five objects are currently classified as dwarf planets. They are Ceres, Pluto, Haumea, Makemake, and Eris. Do you think other dwarf planets will be found? Why or why not? T/I

4. Imagine what it would be like to take a trip around the Solar System. What unique sights might you see on the other planets or moons compared to what you see on Earth? Write a brief postcard message to a friend or draw a comic strip illustrating such a journey. K/U C

5. Make a diagram showing the location of all the planets in relation to the Sun. Label each planet and include one significant fact that would help someone identify it. K/U C

8.4 Wrap Up • A dwarf planet orbits only the Sun and is shaped like a sphere, but (unlike a

planet) orbits among many other objects.

• The four inner terrestrial planets are made of solid matter.

• The four outer gas giant planets are huge and made mostly of gases around a solid core.

ReseARCh This What is it LiKE on othER PLanEts?

skills: Researching, Analyzing the Issue, Communicating, Evaluating

You are familiar with Earth’s weather, climate, and landforms. But, what are the weather conditions like on other planets?

1. Research more information about the conditions on each planet, such as the composition of the atmosphere, extreme temperatures, storm systems, surface features, and gravity.

A. Choose one planet and describe what it would be like to visit it. How would you deal with the conditions on the planet? Would living there be possible? Why or why not? T/I C

B. Choose a format to present your research, such as an illustrated poster, a written report, or a video presentation. T/I C

4.A., 4.B.

GO TO NELSON SCIENCE

NEL 8.4 Planets 307