Section 1: Planet Motion

• Models of the Solar System

  • Geocentric Model: Earth is considered the center and every- thing else revolves around it.

  • Ptolemy presented his geocentric model of the solar system in 140 A.D.

  • Although Ptolemy’s model of the solar system was accepted and used for centuries, there were many problems with it.

  • One problem was the fact that planets periodically appear to move in a retrograde, or backward, direction when viewed against a background of stars.

  • Heliocentric Model: Sun-centered model of the solar system

  • Galileo concluded that Earth and Venus revolve around the Sun and that the Sun is the center of the solar system.

• Understanding the Solar System

  • The heliocentric model of the solar system is now known to be true.

  • Astronomical Unit: Used to measure the large distances within the solar system; equals the average distance from Earth to the Sun, about 150 million km.

  • The planets can be classified in several ways.

    • One system uses size and other characteristics.

      • Those similar to Earth are called terrestrial planets.

    • The system used most often classifies planets whose orbits are between the Sun and the asteroid belt as inner planets and those beyond the asteroid belt as outer planets.

    • The other system classifies planets whose orbits are between Earth’s orbit and the Sun as inferior planets, and those whose orbits are beyond Earth’s orbit as superior planets.

  • Evidence suggests that the solar system probably formed as part of a group of stars.

• Other Solar Systems

  • Now we know that many other stars also have planets around them.

  • Extrasolar Planets: planets in orbit around other stars

  • Astronomers have devised new techniques and instruments to find planets around other stars.

  • So far, they have found over 200 stars that have planetary systems.

Section 2: The Inner Planets

• Mercury: The smallest and closest planet to the Sun

  • Mariner 10 was the first American space probe sent to Mercury

  • Mercury is covered by craters, some of which have double rings

  • The magnetic field around Mercury, discovered by Mariner 10, indicates that Mercury has a much larger iron core than would be expected and is missing some lighter materials you would expect to find in its mantle.

  • Mercury’s relatively large core and thin outer layers resulted in some extreme differences in its surface terrain.

  • Mercury is small and has a relatively weak gravitational pull.

  • Its surface experiences extremes in temperature.

  • The planet has no atmosphere.

• Venus: the second planet from the Sun.

  • The entire surface of Venus is blanketed by a dense atmosphere.

  • The atmosphere of Venus, which has 92 times the surface pressure of Earth’s at sea level, is mostly carbon dioxide.

  • The clouds in the atmosphere also contain droplets of sulfuric acid, which gives them a slightly yellow color.

• Earth: The third planet from the Sun

  • Surface temperatures on Earth allow water to exist as a solid, a liquid, and a gas.

  • Ozone in Earth’s atmosphere also protects life from the Sun’s intense radiation. Life exists all over the planet.

• Mars: The fourth planet from the Sun; red planet

  • Mars is called the red planet because iron oxide in some of the weathered rocks on its surface gives it a reddish color.

  • Mars is tilted 25° on its axis, which is very close to Earth’s tilt.

  • As the seasons change during the Martian year, winds blow the dust around on the planet’s surface. When the wind blows dust off one area, it may appear darker.

  • The Martian atmosphere is much thinner than Earth’s and is composed mostly of carbon dioxide, with some nitrogen and argon.

  • Mars has two small, heavily cratered moons called Phobos and Deimos.

  • The Spirit and Opportunity probes sent to Mars seem to indicate that liquid water was once on the planet and may still be there.

• NASA on Mars

  • Mars has been a priority of NASA planetary exploration for several decades.

  • One of the earliest missions to Mars was the Mariner 9 space probe that orbited Mars in 1971–1972. It revealed long channels on the planet that may have been carved by flowing water.

  • Until 2003, most of the information about Mars came from the Mariner 9, Viking probes, Mars Global Surveyor, Mars Pathfinder, and Odyssey.

  • The Viking 1 and Viking 2 orbiters photographed the entire surface of Mars from orbit, while their landers conducted meteorological, chemical, and biological experiments on the planet’s surface.

  • Space probes sent to Mars are not the only way we have learned about our planetary neighbor.

Section 3: The Outer Planets

• Jupiter: The fifth planet from the Sun

  • Jupiter is the largest planet and contains more than twice the mass as all other planets combined.

  • Its atmosphere has bands of white, red, tan, and brown clouds.

  • Continuous storms of swirling, high- pressure gas have been observed on Jupiter. The Great Red Spot is the most spectacular of these storms.

  • Jupiter has more than 60 moons.

  • Galileo was the first to discover these moons when he looked through his telescope and he drew a picture of what he saw.

• Saturn: the sixth planet from the Sun.

  • It is 9.6 AU from the Sun and is about nine times larger than Earth.

  • It is not the only planet with rings, but it has the largest and most complex ringed system.

  • Saturn is the second-largest planet in the solar system, but it has only about one-third as much mass as Jupiter.

  • It has the lowest density of any planet and, in fact, is less dense than water.

  • Saturn is composed almost entirely of hydrogen and helium, with traces of methane, ammonia, and water vapor.

  • Saturn’s rings are made of billions of ice and rock particles.

  • The structure of the rings is caused by the gravitational inter- action between the ring particles and some of Saturn’s moons.

• Uranus: the seventh planet from the Sun.

  • It is about 19.2 AU from the Sun and is about four times as large as Earth.

  • Uranus has at least 27 moons and also has a thin ring system made of eleven separate rings.

  • Unlike Earth and the other planets, Uranus’s axis of rotation is tilted so that it is nearly parallel to the plane of the planet’s orbit.

  • Uranus’s atmosphere contains about 83 percent hydrogen, 15 percent helium, 2 percent methane, and traces of other gases

• Neptune: the eighth planet from the Sun.

  • Neptune was discovered in 1846 after studies of Uranus’s orbit indicated that the orbit was being affected by gravity of another unseen planet.

  • Neptune has at least 13 moons.

  • Voyager detected methane geysers erupting on Triton.

• Dwarf Planets

  • Pluto, Eris, and Ceres are now known as dwarf planets.

  • Discovered in 1930, Pluto, has a diameter of 2,300 km, a thin atmosphere, and a surface of solid, icy rock.

    • Pluto’s three moons are Charon, Nix, Hydra.

  • Eris is about 2,400 km in diameter and has one moon.

  • Ceres, discovered in 1801, has a diameter of 940 km.

Comets and Other Objects

• Comet: composed of dust and rock particles mixed with frozen water, methane, and ammonia.

  • Most comets come from a vast disk of icy comets called the Kuiper Belt near Neptune’s orbit, or from the Oort Cloud that surrounds the solar system.

  • Comets often have two tails—one made of dust and one made of ions.

• Asteroids: Rocky objects formed from material similar to that of the planets

  • Most asteroids are found in a belt between the orbits of Mars and Jupiter.

• Meteoroids: Other rocky objects orbiting within the solar system

  • They may have formed when asteroids collided or when comets passed close to the Sun, leaving a trail of debris.

• Another object in the solar system that has puzzled astronomers is Sedna

  • Sedna has been labeled a distant planetoid and with a diameter of 1,200 to 1,700 km, it is smaller than Pluto, but larger than comets in the Kuiper Belt.

  • It has a very elliptical orbit. Sedna comes as close as 76 AU from the Sun, but travels to a distance of 950 AU—far beyond the Kuiper Belt, but much closer than the Oort Cloud.

  • Another puzzle is Sedna’s apparent rate of rotation of 40 days.

Section 4: Life in the Solar System

• Life As We Know It

  • We know life exists in our solar system.

  • Life-forms found included crabs, clams, and tube-worms. They were living and thriving in a dark environment under tremendous pressures

  • Alvin also discovered colonies of bacteria living off the extremely hot material spewing from volcanic vents.

  • A surprising amount of life has been found elsewhere on Earth in extreme conditions—scalding heat, darkness.

• Can Life Exist on Other Worlds?

  • Extraterrestrial Life: life on other worlds.

  • If life existed and left evidence, future astronauts will find it

  • Searches for life on Mars already have been done.

  • Data from the Galileo probe of Jupiter and its moons indicate that three Galilean moons have ice-covered oceans.

  • Europa’s ocean could hold more than twice the amount of water that Earth’s oceans hold.

  • Certain features on Titan’s surface indicate this moon has a long-term, interesting geologic history.

  • Certain features on Titan’s surface indicate this moon has a long-term, interesting geologic history.