7th Grade Science: The Solar System

• Section 1: An Overview of The Solar System

• Section 2: The 9 Planets of The Solar System

What’s in Our Solar System?

• Our Solar System consists of a central star (the Sun), the nine planets orbiting the sun, moons, asteroids, comets, meteors, interplanetary gas, dust, and all the “space” in between them.

• The nine planets of the Solar System are named for Greek and Roman Gods and Goddesses.

The Sun

• The Sun’s age is about 5 billion years. • Its energy comes from nuclear fusion (where hydrogen is

converted to helium) within its core. This energy is released from the Sun in the form of heat and light.

• Stars are the only solar bodies that generate their own light. Very bright planets, such as Venus and our moon, appear bright because they are reflecting sunlight. Remember: Stars produce light. Planets reflect light.

• Our sun is classified as a yellow main sequence star. A star’s temperature determines its “color.” The coldest stars are red. The hottest stars are blue.

Geocentric vs. Heliocentric Theory

• Geocentric Theory (or “Ptolemaic” Theory – pronounced “Tole-ah-may-ic”) is an ancient view of the universe based primarily on religion, philosophy, and mathematical ideals.

• Heliocentric Theory (or “Copernican” Theory) is a revised view of the universe based on the studies of Nicholas Copernicus, who was a mathematician in the 1500’s.

Geocentric Theory: Basic Beliefs

1. The cosmos (or universe)1. Is geocentric: has the earth as its center with

the sun and other planets revolving around the earth

2. Is geostatic: the earth itself does not move2. Celestial bodies move in uniform, circular

motion around a central point.3. Celestial bodies are composed of a fifth element:

the “quintessence.”4. The cosmos is “finite,” which means that it does

not go on forever.

Heliocentric Theory: Basic Beliefs

1. The earth rotates daily on its axis.

2. The earth and the planets revolve around the sun (a heliocentric model).

3. The earth’s orientation is fixed in space because of the conical axial motion of the earth.

The 9 Planets of the Solar System

• Planets are categorized according to composition and size. There are two main categories of planets:

– small rocky planets (Mercury, Venus, Earth, Mars, and Pluto)

– gas giants (Jupiter, Saturn, Uranus, and Neptune)

The Relative Size of the Planets in the Solar System

Characteristics of Small Rocky Planets

• They are made up mostly of rock and metal.

• They are very heavy.

• They move slowly in space.

• They have no rings and few moons (if any).

• They have a diameter of less than 13,000 km

Mercury

• Mercury has a revolution period of 88 days. A revolution period is the time it takes for a planet to complete one full orbit around the sun. This is also called a year.

• Mercury has extreme temperature fluctuations, ranging from 800F (daytime) to -270F (nighttime).

• Even though it is the closest planet to the sun, recent radar info suggests there is ICE on Mercury! Scientists believe the ice is protected from the sun’s heat by crater shadows. It is covered with meteorite impacts and barely has a trace of an atmosphere.

Venus• Venus is the brightest object in the sky after the

sun and moon because its atmosphere reflects sunlight so well. People often mistake it for a star.

• The atmosphere of Venus has a large amount of carbon dioxide gas. Carbon dioxide traps heat in Venus’s atmosphere, causing the surface temperatures to increase greatly. We call this effect the Greenhouse Effect. Venus’s Greenhouse Effect is so strong that its maximum surface temperature may reach 900F.

• Venus has no moons. It has thick yellowish clouds composed of sulfuric acid driven by fast winds. Its surface is dry and dusty with craters, mountains, and volcanoes.

Earth• Earth is the only planet known to support living

organisms.• Earth’s surface is composed of 71% water.

– Water is necessary for life on Earth.– The oceans help maintain Earth’s stable temperatures.– Water is a primary cause of Earth’s surface weathering.

• Earth has a distinct atmosphere.– It helps screen harmful radiation from the sun.– It prevents meteorites from reaching Earth’s surface.– It traps heat to help maintain Earth’s stable

temperatures.

• Earth has one moon.

Earth’s Moon• It takes the same amount of time for the moon to rotate once

on its axis as it does for it to orbit the earth (27.3 days). Thus, the same side of the moon always faces us.

• The moon’s surface is covered in dust and rocky debris from meteor impacts. It has no water or atmosphere. The dark areas of the moon are called maria (Maria is a Latin word meaning “seas”). Maria are large craters which have been filled by solidified lava.

• The moon reflects light from the sun onto the earth’s surface. Sometimes the moon may appear reddish-brown in color as the sunlight is deflected through dust in the earth’s atmosphere.

• The moon’s gravitational effects on the earth are most apparent in the “coming” and “going” of the tides

• Mars has permanent ice caps at its poles. The ice caps are composed mostly of frozen carbon dioxide (what we call “dry ice”).

• Mars has the largest volcano in our solar system: Olympus Mons. Olympus Mons is approximately 24 km high (15 miles).

• Atmospheric dust makes Mars’s sky pink in color.• Mars has two moons: Phobos and Deimos

Mars

Pluto• Pluto is the only planet not visited by spacecraft.

We are not sure of its composition. Based on current data, scientists believe it is a small, rocky planet.

• Pluto seems to lie on its side: its equator points straight up, and one of its poles points directly at the sun.

• Pluto has only one moon (Charon). Pluto’s moon is half the size of Pluto itself, which is unusually large for a moon. Because they are so close in size, sometimes Pluto and its moon are considered to be a double planet system.

• Pluto was located and named in 1930.

Characteristics of Gas Giants

• They are made up mostly of gases (primarily hydrogen & helium).

• They are very light for their size.

• They move quickly in space.

• They have rings and many moons.

• They have a diameter of less than 48,000 km

Jupiter• Jupiter is the largest and most massive planet: it’s diameter

is 11 times bigger than that of the Earth’s. Overall, Jupiter is about 318 times the size of Earth. Jupiter is composed almost entirely of hydrogen and helium.

• Because Jupiter rotates so quickly, it’s clouds form belts (low-lying, relatively warm cloud layers) and zones (bright, high-altitude, cooler cloud layers) that encircle the planet.

• Jupiter’s characteristic Great Red Spot is an enormous storm, consisting of a spiraling column of clouds big enough to contain three Earths. The clouds are colder than the surrounding areas, and so the Great Red Spot sits about five miles above the upper cloud layer.

• Jupiter has 16 known moons.

Saturn

• Saturn, like Jupiter, is composed almost entirely of hydrogen and helium. Saturn is the least dense of all the planets. In fact, Saturn’s density is less than that of water.

• Saturn has many rings composed primarily of ice with some ice-coated rocky particles. Saturn’s rings are very wide (they extend outward to about 260,000 miles from the surface) but very thin (less than 1 mile thick).

• Saturn has 18 known moons, some of which orbit inside the rings!

Uranus

• Uranus is tilted on its axis at 98. Because of its strongly tilted axis of rotation, Uranus essentially spins on its side as it orbits the sun.

• Uranus is blue in color due to methane gas in its atmosphere.

• Uranus has 11 dark rings surrounding it. These rings contain some of the darkest matter in our solar system.

• Uranus has 15 known moons, and scientists suspect more lurk within its rings.

Neptune

• Neptune has the fastest winds on the solar system: up to 2000 km/hr.

• Neptune is also blue in color due to methane gas in its atmosphere.

• Neptune has a Great Dark Spot in its atmosphere. The Great Dark Spot is a huge storm the size of Earth.