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Earth Space Science : 08 Our : 08.04 Objects in Our Solar System

Objectives

What can objects in our solar system tell us about its formation and future? At the end of this lesson, you will be able to:

• compare characteristics of inner and outer • define the role of “other” solar system objects in our solar system • discuss how knowledge of our own solar system can be applied to future planetary discoveries

Objects in Our Solar System My Very Excellent Mother Just Served Us Nine Pizzas Taking the first letter of each word in the above sentence gives the following: M V E M J S U N P This is the order of the planets in our solar system. You can remember the planets using this simple sentence or one like it.

• • • • • • • • •

But wait! How did scientists determine the number of planets in the solar system, anyway? What about Pluto? Do scientists still consider it a ? When Pluto was discovered in 1930, astronomers listed it as the ninth planet in the solar system. In 2005, astronomers detected yet another large body, called , beyond Pluto's orbit. A debate erupted among scientists over the characteristics a heavenly body must have to be considered a planet. To resolve the issue, in 2006, astronomers downgraded Pluto from planet to . (Eris is also a dwarf planet.) Why was there so much debate about how to define a planet? As with any discovery in science, new information leads to new ideas. Use the following activity to investigate the difference between a planet and a dwarf planet.

Differences between Planet and Dwarf Planet Interactive

Characteristics of planets and dwarf planets:

• A planet o is in orbit around the o has sufficient to assume a nearly round shape o is not a satellite or of another object o has enough to clear away smaller objects near its orbit; when a planet interacts with smaller objects, it either consumes them or slings them away with its gravity

• A dwarf planet o is in orbit around the sun o has sufficient mass to assume a nearly round shape o is not a satellite or moon of another object o does not have enough gravity to clear away smaller objects near its orbit, so it exists in a zone of other, similarly sized objects

The International Astronomical Union changed the definition of a planet so that all astronomers worldwide could classify new objects such as Eris, found beyond Pluto's orbit. However, astronomers today still debate the definition of a planet. In this lesson, you will be reviewing the characteristics of the inner and outer planets in the solar system. You will also investigate other objects in the solar system that orbit the sun. The debate over the definition of a planet illustrates that scientific knowledge constantly changes as scientists discover new evidence and make new observations. Often, scientists must rethink their existing hypotheses and present new ones to better explain the new information. The reclassification of Pluto as a dwarf planet is a good example of how scientists use new information to improve classification systems. In our solar system, astronomers classify planets as inner or outer. Inner and outer planets differ according to the criteria in the table below.

Inner Planets Outer Planets

Examples Mercury, Venus, Earth, Mars Jupiter, Saturn, Uranus, Neptune

Number of Few or no moons Many moons

Distance from Sun Closest to the sun Farthest from sun

Composition Rocky surfaces Gaseous

Presence of Rings No rings Has a Inner Planets Outer Planets

Size Smaller size Larger size

In the activities below, you will investigate each of the planets. As you investigate each planet, record detailed information in your notes. Keep the following points in mind:

• Each of the planets follows an elliptical orbit around the sun. An ellipse has a shape like a flattened circle. The following diagram approximates the orbit of each planet around the sun. Note that some orbits are more elliptical than others.

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• Scientists have investigated some planets more than others. The activities below do not list every mission that has been launched to explore each planet. • There are still poorly understood and undiscovered objects in the solar system. Scientists are constantly discovering new information about the planets and other solar system objects. Inner and Outer Planets Interactive

A large, two-tabbed interactive divided into inner and outer planets. Each planet exists as a trading card with a front and back, as previously described The Inner Planets Card 1: Mercury Location: 47–70 million kilometers from the sun. Characteristics: Completes one orbit around the sun every 88 days. Completes one rotation on its axis every 58.65 Earth days. Diameter is 4,880 kilometers. Satellites: None. Surface: Covered with rocky cliffs and valleys with many craters from impacts by other objects during the formation of the solar system. May contain water at the North and South Poles. Atmosphere: Because there is virtually no atmosphere on the planet to trap heat, temperatures range from 430 degrees Celsius in the day to −180 degrees Celsius at night, which is the largest day and night temperature differential in the solar system. Interior: Contains a liquid metallic core. Exploration: In 1974 and 1975, the Mariner 10 mission photographed Mercury for the first time. In 2008 and 2009, the Messenger mission studied the planet in even greater detail. Fun Facts: Mercury rotates only three times for every two orbits it completes around the sun. That means Mercury’s year is only slightly longer than its day. Card 2: Venus Location: 108.5–108.9 million kilometers from sun. It has a nearly circular orbit. Characteristics: Completes one orbit every 225 days. Completes one rotation on its axis every 243 days. Diameter is 12,100 kilometers. Satellites: None. Surface: The surface is covered with volcanoes. Temperatures are hot enough to melt lead. Atmosphere: The atmosphere comprises thick clouds of dioxide and sulfuric acid, resulting in a powerful greenhouse effect. The atmosphere moves with winds at speeds of 360 kilometers per hour. Temperatures are a constant 470 degrees Celsius. Interior: Contains an core but does not have a magnetic field because of its slow rotation. Exploration: In 1962, the Mariner 2 became the first to send back information about Venus, which is the first time humans had explored another planet in the solar system. Beginning in 1990, the Magellan spacecraft photographed 98 percent of the planet’s surface. Fun Facts: Venus is the brightest planet in the night sky. If you could observe sunrise on Venus, the sun would rise in the west and set in the east because Venus rotates in the opposite direction as Earth. Card 3: Earth Location: 146–152 million kilometers from the sun. Characteristics: Completes one orbit around the sun every 365.26 days. Completes one rotation on its axis every 23.93 hours. Diameter is 12,756 kilometers. Satellites: One moon, named the Moon (with a capital M). Surface: Water covers 70 percent of Earth’s surface. The land surfaces are covered by a rocky terrain of mountains, valleys, deserts, and glacial regions. Atmosphere: Atmosphere consists mostly of nitrogen (78 percent) and (21 percent). Temperatures range from 58 degrees Celsius to −88 degrees Celsius. Interior: Contains a molten metallic core made of and iron. Exploration: Scientists are constantly seeking to understand our home planet with missions to the upper atmosphere and the deepest depths of the . Fun Facts: Earth is tilted on its axis approximately 23.5 degrees from the vertical, which causes changing seasons. Card 4: Mars Location: 205–249 million kilometers from the sun. Characteristics: Completes one orbit around the sun every 687 days. Completes one rotation on its axis every 24.6 hours. Diameter is 6,785 kilometers. Satellites: Two moons, and , discovered in 1877. Surface: The rocky surface contains many mountains, canyons, volcanoes, ice, and craters. The surface appears red as a result of iron oxide (better known as rust) in the Martian soil. Atmosphere: The atmosphere is composed of carbon dioxide, nitrogen, and argon. Temperatures range from 20 degrees Celsius to −125 degrees Celsius [Source: NASA] Interior: A layer of water ice is thought to exist just under the surface of Mars. The core is likely to be made of iron. Exploration: The Mars rovers, named Spirit and Opportunity, found evidence that liquid water may have existed on the planet billions of years ago or perhaps more recently. The Phoenix rover landed in 2008 to further investigate the existence of water. It confirmed that liquid water exists in the regions. Fun Facts: Olympus Mons, located on Mars, is the largest volcano in the solar system, rising 23 kilometers above the planet’s surface. The darkened line across the equatorial region is the Valles Marineris. The canyon system is more than four times deeper than the Grand Canyon. The Outer Planets Card 5: Jupiter Location: 741–817 million kilometers from the sun. Characteristics: Completes one orbit around the sun every 12 years. Completes one rotation on its axis every 9.9 hours. Diameter is 142,800 kilometers. Satellites: Jupiter has at least 63 known moons. The four largest moons—, , , and —are called the after their discovery by in 1610. There are at least three ring systems around the planet, although these were discovered only recently by the Voyager I spacecraft in 1979. Surface: Beneath the atmosphere, the gases are under so much pressure that they form a liquid. There is no land-atmosphere interface. Atmosphere: Mostly hydrogen and helium, similar to the sun. If Jupiter were more massive, it could have become a star. Interior: The center of the planet may contain a small rocky core about the size of Earth. Exploration: In 1973, the Pioneer 10 spacecraft made the first mission beyond the planet Jupiter. The Galileo spacecraft dropped a probe through Jupiter’s atmosphere in 1995. Fun Facts: The giant red spot on Jupiter is a storm similar to a hurricane. The moon Europa is of particular interest to scientists because it may contain liquid water beneath its of ice. Card 6: Saturn Location: 1.35–1.5 billion kilometers from the sun. Characteristics: Completes one orbit around the sun every 29.5 years. Completes one rotation on its axis every 10.7 hours. Diameter is 119,871 kilometers. Satellites: Saturn has at least 62 known moons. The second largest moon in the solar system, , is one of the only moons in the solar system to have a significant atmosphere. Surface: Although there is no land-atmosphere interface, the middle portion of the planet is likely a liquid hydrogen and helium mix. Atmosphere: Mostly hydrogen and helium. The planet spins so fast that the winds can whip around the planet at 500 meters per second. Interior: Little is known about the interior of Saturn, but it may have a central core made of . Exploration: The Cassini-Huygens mission was named for the seventeenth-century discoverers of the planet’s ring system. Cassini began an orbit around Saturn in 2004. It later dropped the Huygens spacecraft into Titan’s atmosphere, landing a probe on the moon’s surface. Fun Facts: Saturn’s rings extend outward thousands of kilometers but are only about 10 meters thick on average. Some parts of the rings are as thick as 3 kilometers. Card 7: Uranus Location: 2.7–3 billion kilometers from the sun. Characteristics: Completes one orbit around the sun every 84 years. Completes one rotation on its axis every 17.24 hours. Diameter is 51,488 kilometers. Satellites: Uranus has 27 known moons and two sets of rings. Surface: There is no land-atmosphere interface but rather a gradual thickening of the hydrogen, helium, methane, and ammonia present on the planet until the mixture becomes solid. Atmosphere: The atmosphere is mostly hydrogen and helium, with traces of methane, which is why the planet is blue in color. Temperatures are below −200 degrees Celsius. Interior: Methane ice is the primary component of the interior of Uranus. Exploration: The Voyager II spacecraft flew past the planet in 1986, taking detailed pictures. Fun Facts: The planet rotates almost completely on its side. The planet may have a water ocean beneath its clouds. Card 8: Neptune Location: 4.46–4.54 billion kilometers. Characteristics: Completes one orbit around the sun every 165 years. Completes one rotation on its axis every 17.24 hours. Diameter is 49,493 kilometers. Satellites: Neptune has 13 known moons and at least six rings. , the largest of the moons, has ice volcanoes that spit out icy material. Surface: Similar to Uranus, the planet’s surface is a gradual thickening of the hydrogen, helium, and methane atmosphere until the mixture becomes solid. Interior: A solid, Earth-sized core is thought to exist inside the planet. Exploration: The Voyager II flew by the planet in 1989, discovering six new moons around the planet. Fun Facts: A large spot, known as the Great Dark Spot, is likely a “hole” in the atmosphere of the planet. It appears and disappears periodically. Since its discovery in 1846, the planet has completed only one orbit around the sun, in 2001. The next time it completes an orbit will be in the year 2166. Belt The acts as a natural boundary between the inner and outer planets. An asteroid is a rocky body too small to be considered a planet or dwarf planet. The asteroid belt contains as many as 1 million objects, ranging in size from 1 kilometer to nearly 1,000 kilometers across. Even with a large number of , the total mass of all the asteroids together would not equal the mass of our Moon. Let’s learn more about asteroids. Take notes of key concepts as you view the video below. Asteroids Asteroids are objects made of rock and metal that orbit the sun but are too small to be considered planets. They are usually irregular in shape, and they can be as small as one kilometer in diameter or as large as 940 kilometers, which is the size of , the first asteroid discovered. The solar system contains hundreds of thousands of asteroids. Most of them revolve around the sun in an orbit between those of Mars and Jupiter. They form a wide band called the Asteroid Belt. Other asteroids have orbits that cross Earth's orbit. These asteroids are called "near Earth objects." In the past, some "near Earth objects" have collided with Earth, leaving huge impact craters. Asteroids probably consist of matter that never agglomerated into a planet when the solar system was forming. The study of these ancient rocks by means of satellites should tell us more about how our solar system was formed.

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Asteroids are not crowded together as often depicted in action movies set in space. They are more closely spaced than other solar system objects, but a could fly for years through the asteroid belt without colliding with an asteroid. Because of Jupiter's gravitational pull, asteroids can be pulled out of the asteroid belt. From there, they can wander around the solar system. In history, they have slammed into other planets, often with devastating effects. An asteroid impact likely caused the extinction of the dinosaurs.

Meteors

Often called shooting stars, meteors are not really stars at all. A meteor is a streak of light formed when particles of , called , burn up in Earth's atmosphere. The friction on the as it hits the atmosphere causes it to burn up, leaving a streak or tail of light behind it as it falls. When a large number of meteoroids burn up in the atmosphere, it is called a meteor shower, which can be experienced as many streaks of light passing overhead. Whether a meteoroid burns up entirely or hits Earth depends on its speed and size. Once a meteor hits Earth's surface, it is called a .

Meteorites A meteorite is a fragment of spatial matter that falls to the surface of a planet. Most that fall to Earth come from a part of the Solar system located between the orbits of Mars and Jupiter, the Asteroid Belt. Attracted by Earth's mass, the rocky or metallic fragments enter Earth's atmosphere at very high speeds. They become so hot that they burn up leaving behind a tail of incandescent debris, commonly called a shooting star. The smallest fragments burn up entirely in the atmosphere while the larger ones reach the ground. They then become meteorites in the true sense of the word. When it touches the ground, a very large meteorite causes an explosion proportionate to its size. With the impact, rocky debris is sometimes dispersed over several kilometers around the impact crater. Earth bears traces of more than a hundred major impacts. One of the oldest craters, the Manicouagan crater in Canada, forms a ring 100 kilometers in diameter. It is the result of a large meteorite crashing into Earth more than 210 million years ago. Most meteors never reach Earth's surface; when they do, however, the results can be catastrophic. Deep in the desert regions of Arizona is a crater called the Barringer Crater, shown below.

Public Domain An impact crater forms when one object in space collides with another object. Every sizable object in the solar system, since its formation, has been hit by something else. Even asteroids have impact craters. The Barringer Crater was caused by a 100,000 ton meteorite slamming into Earth approximately 50,000 years ago. Over time, Earth's active atmosphere and moving tectonic plates eradicate evidence of meteorite impacts, so there are few visible craters on Earth today. For example, scientists had to use advanced technology to detect the , thought to be responsible for the extinction of the dinosaurs 65 million years ago. It is buried near the Yucatan Peninsula in Mexico, as shown in the below.

Kuiper Belt One of the reasons Pluto was downgraded to a dwarf planet was because it bore little resemblance to the large, gaseous outer planets. Pluto is small and rocky. Its moon, , is almost as big as Pluto itself. Pluto exists in a region known as the . The Kuiper belt is a region of debris left over from the formation of the solar system. As the sun began to burn, the solar pushed much of the material remaining from the formation of the planets to regions beyond the orbit of Neptune. The Kuiper belt is shown in the image below. Public Domain As you have learned, scientists know much about our solar system, but there are many questions left unanswered. As research and technology advance, our knowledge of the solar system will increase. Our knowledge can be used to investigate other solar systems close to our galaxy. For instance, understanding the nature of the Kuiper belt or features of the planets helps scientists theorize about solar systems around distant stars. You can think of a as a dirty snowball in space, but it's not a snowball made of water ice, as we make on Earth. A comet is made of frozen gases on the surface of a rocky core, known as the nucleus. A comet is a small, icy celestial body whose spectacular tail can be seen from Earth. A comet's luminous plume is visible only when it approaches the sun. The comet's core, composed of dust and ice, heats up and begins to melt. The matter surrounding the core is vaporized and forms a very bright halo of gases, and an enormous cloud of hydrogen envelopes the head of the comet. Tiny particles expelled from the core make up the dust's tail of the comet, which is several million kilometers long. An even longer ionic tail is formed of gases that are ionized, that is, electrically charged through interaction with the . Most comets orbit around the sun in very eccentric orbits that take them out to the edge of the solar system, beyond Pluto's orbit. They therefore appear in the sky only once a century or once a millennium. Others, like Halley's comet, do not go beyond Pluto's orbit. They therefore pass close to the sun more often—every 76 years in the case of Halley's comet, whose next passage is expected in 2061.

Once a comet approaches the sun, its frozen gases begin to melt, forming a coma around the nucleus. The coma is the atmosphere of gas caused by the melting ice. The solar wind pushes the coma material outward and away from the nucleus. The ejected gases appear as a long tail streaking behind the nucleus. The tail always points away from the sun. It consists of two parts: one part is dust, and the other part is ionized gases called plasma. The dual nature of a comet tail can be seen in the image.

8.04 Exam Question 1 Which of the following terms describes a piece of space debris that can enter Earth's atmosphere? a. Impact crater b. Meteor c. Meteorite d. Meteoroid Question 2 Which of the following is created when the sun pushes the ionized gases of a comet away from the central core? a. Coma b. Nucleus c. Plasma tail d. Solar wind Question 3 Which planets have no satellites? a. Mars and Earth b. Mercury and Mars c. Mercury and Venus d. Venus and Earth Question 4 Ganymede, the largest moon in the solar system, orbits which planet? a. Jupiter b. Mars c. Saturn d. Uranus

Question 5 Which planet is shown in the image below?

a. Mars b. Jupiter c. Mercury d. Venus Question 6 A new object, Object Z, was discovered outside our solar system. Object Z is made of frozen gases that surround a rocky core and it travels in a highly elliptical orbit around the sun. The characteristics of Object Z most closely resemble those of other a. asteroids. b. dwarf planets. c. comets. d. moons.

Question 7 Which of the following sets of characteristics describe what we know about the inner planets? a. They are closest to the sun, have many moons, and include ring systems. b. They are closest to the sun, have few or no moons, and include ring systems. c. They have rocky surfaces, no rings, and few or no moons. d. They have rocky surfaces, ring systems, and are larger in size. Question 8 The contains primarily which gas? a. Carbon dioxide b. Methane c. Nitrogen d. Oxygen Question 9 Which of the following pairs of planets have the most in common? a. Earth and Neptune b. Mercury and Venus c. Venus and Saturn d. Uranus and Mars Question 10 Which of the following is a characteristic of inner planets? a. Hydrogen and helium atmosphere b. Many moons c. No land-atmosphere interface d. Rocky surface