2.  What makes up our solar system? The sun, planets, their moons, and smaller objects.  What is at the center of the solar system? The sun.  How do you describe a model of the solar system with the sun at the center? Heliocentric.  What holds the solar system together? Gravity  What unit is used to measure large distances in space? Astronomical units.  How large is one astronomical unit? 150,000,000 km.  Students page 137 do the math.

3.  Which two kinds of objects are planetary? Planets and dwarf planets.  What are the characteristics of a planet? It is round, it orbits the sun, and has cleared out other objects from the area around its orbit.  What are the eight planets? Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.  There are five dwarf planets in our solar system.  Similarities and differences of planets and dwarf planets? Both orbit the sun, but dwarf planets have less gravity- not enough to clear their orbital area of other objects.

4.  Every planet in the solar system has at least one natural satellite, or moon.  Which planets do not have satellites? Mercury and Venus.  Which planets have the most satellites? Jupiter and Saturn.  Earth has the fewest moons, with just one.  What kinds of objects are in the category of smaller objects? Asteroids and comets.  Asteroids are small rocky bodies. They are found between the orbits of Mars and Jupiter.  Comets are loose balls of ice and rock that they have very long narrow orbits.  Like Earth each planet has a day and a year. Day is the time it takes to rotate on its axis. Year is the time it takes to orbit the sun.  Students page 139 assess your understanding 1a,1b,

5.  Scientists think the solar system formed about 4.6 billion years ago from a cloud of hydrogen, helium, rock, ice, and other materials pulled together by gravity.  Students look at figure 2 on page 140. The images show the formation of the Sun and planets from a cloud pulled together by gravity.

6.  Planet formation : away from the Sun, planets began to form as gravity pulled rock, ice, and gas together.  The rock and ice formed small bodies called planetesimals. Over time planetesimals collided and stuck together, combining to form all the other objects in the solar system.  Inner planets: close to the sun the solar system was very hot. Most water evaporated. Gravity was too weak to hold onto light gases such as hydrogen and helium. This is why inner planets are small and rocky.  Outer planets: at greater distances from the Sun temperatures were cooler. Ice formed adding mass to the planets. The gravity was strong enough to hold hydrogen and helium forming the gas giant planets. Beyond the gas giants temperatures were even lower. Ice and other materials produced comets and dwarf planets.

7.  Summary of planet formation:  One of the defining characteristics of the planet is that it has cleared out the area around the path of each orbit.  What are planetesimals? Small bodies made of rock and ice.  How did planetesimals form planets? They collided and stuck together.  How do you think this is related to the definition of planet? By colliding with and sticking to other matter in their path, planetesimals cleared out the area around their orbits. They also became larger in the process.  Planetesimal: the ending of the word- tesimal- means “much much less”. So an individual planetesimal has much much less mass and is much much smaller in size than a planet.

8.  Students page 141 assess your understanding 2a,2b.  Figure 3 solve the solar system; solve the crossword puzzle.  Answer the compare and contrast question under the crossword puzzle.

9.  Homework:  Create a similar puzzle about objects in the solar system like the puzzle in page 141. Use the puzzle in the text as a model and information in the text for clues. Solve the puzzle.