Pluto is no longer considered a planet. Ch. 29 – The Solar System The planets within the solar system have various sizes, surface conditions, internal structures. They all orbit the Sun in the same direction and similar planes. Pluto is no longer considered a planet. Early astronomers assumed that the Sun, planets, and stars orbited a stationary Earth in what is now known as a geocentric model. Planetary motion was difficult to explain with this geocentric model. Mars for example will undergo a retrograde motion, where it moves in the opposite direction across the sky. Fig. 29-1 (pg. 776)

Nicolaus Copernicus was the first to suggest that the Sun was the center of the solar system, which is referred to as a heliocentric model. This would explain that the planets closer to the Sun orbit quicker, thus when Earth moves past Mars’ orbit it appears as though Mars is moving backwards in the sky – retrograde motion. Other scientists started to build on these astronomy discoveries. Tyco Brahe made several accurate observations of planetary positions during his time. Johannes Kepler used Brahe’s data to explain that planets orbit the Sun in an ellipse, rather than in a circle. This is known as Kepler’s first law.

When a planet is farthest away from the Sun we call this aphelion. An ellipse is an oval shape that is centered on 2 points instead of a single fixed point. Minilab – pg. 777 Each planet’s average distance between it and the Sun is called astronomical units. The Earth has a AU of 1, which is the standard of all the other planets. When a planet is closest to the Sun in its orbit we call this perihelion. When a planet is farthest away from the Sun we call this aphelion. The shape of the planet’s elliptical orbit is defined as eccentricity. This value ranges from 0 to 1 O being a perfect circle & 1 being a parabola.

Kepler’s 2nd law described that an imaginary line between the Sun and a planet sweeps out equal amounts of area in equal amounts of time. Fig. 29-4 (pg. 778) Kepler’s 3rd law found that the sqaure of the orbital period equals the cube of the semimajor axis. P2 = a3. Galileo used the first telescope to view many celestial bodies in the solar system during this time. Jupiter’s 4 moons provide that not all objects orbit the Earth. Newton then proved the discoveries mathematically by stating that every pair of bodies in the universe attract each other with a force that is proportional to the product of their masses and inversely proportional to the square of the distances between them Universal Gravitation F = G (M1*M2/r2).

The Terrestrial Planets Terrestrial Planets are the inner 4 planets, which are close in size and have solid, rocky surfaces. Mercury – closest to the Sun. 1/3 the size of Earth No moons Slow rotation – 1407.6 hours Orbit – 87.969 days In 2 of Mercury’s years, 3 of Mercury’s days have passed. No Atmosphere – primarily oxygen & sodium Surface temp. of 427°C (day) & -173°C (night)

Mercury (Cont.) Venus Surface is covered with craters and plains Surface gravity is much greater than the Moon. Observation suggest that Mercury may have been much bigger in size (Earth-like). It is similar to Earth without a mantle or crust. Venus No Moons Brightest planet in Earth’s nighttime sky. Albedo of .75 1 Venus day is 243 Earth days  Clockwise spin The Sun would rise in the West and set in the East Venus is most similar to Earth with diameter, mass, and density.

Venus (Cont.) Mars Average surface temp. = 464°C The atmospheric pressure is 92 atm., compared to Earth’s 1 atm. at sea level. Atmosphere is mainly carbon dioxide and nitrogen. Lots of clouds with sulfuric acid instead of water. Hottest planet Smooth surface with volcanic flows . The interior of Venus is most likely similar to Earth because of similar densities and size. Mars 4th from the Sun Called the red planet High Iron content

Mars (Cont.) Smaller and less dense than Earth. 2 irregularly shaped moons Phobos and Deimos Atmosphere similar to Venus The density and pressure are much lower than Venus. Thin atmosphere with constant wind. Dust storms that can last for weeks. The southern hemisphere surface is covered in craters, while the northern hemisphere is plains. The largest volcano is Olympus Mons This is the largest mountain in the solar system. Several erosion features suggests that liquid water once existed on the surface of Mars. If Mars had a thicker atmosphere it would hold liquid water. Both poles have caps of “dry ice” – solid carbon dioxide.

The Gas Giant Planets The gas giant plants are much larger, more gaseous, and lack solid surfaces. Jupiter Largest planet & 5th from the Sun 11 times larger than Earth Has 4 major satellites & several smaller ones Io actually had volcanic activity. Jupiter has a low density (lightweight elements) The atmosphere is mainly H & He. Has a layer of liquid metallic hydrogen. Jupiter spins on it’s axis every 10 hours. Shortest rotation in the solar system.

Jupiter (Cont.) Jupiter’s Great Red Spot is an atmospheric storm that has been rotating around Jupiter for more than 300 years. All 4 major moons have composed of ice and rock mixtures. Europa is thought to still have a subsurface ocean of liquid water. Jupiter’s ring is 6400 km wide. All four gas giant planets. Saturn 6th planet from the Sun. 2nd largest planet Atmosphere is primarily H & He.

Saturn (Cont.) Uranus Broadest rings for among the planets. There are 7 major rings which are made of ringlets. Titan is the largest moon of Saturn. Uranus 7th planet from the Sun. Discovered accidentally in 1781. Uranus is 4 times as large and 15 times as massive as Earth. Most of Uranus’ atmosphere is He & H, but the methane reflects light which cause a bluish appearance. The rotation axis of Uranus is tipped over so far that the north pole almost lies in its orbital plane.

Uranus (Cont.) Each pole spends 42 Earth years in darkness and 42 years in sunlight due to this tilt and Uranus’s long trip around the Sun. The planet’s temp. is around -215° C. New moons are frequently being discovered just like Jupiter and Saturn, so the moon count constantly changes. Neptune Neptune was predicted to be present before it was ever observed due to the law of gravitation. Neptune is slightly smaller and denser than Uranus. Neptune is very similar to Uranus, except for an atmosphere with clouds and belts. Neptune had a persistent storm, the Great Dark Spot, but it disappeared in 1994. Triton, Neptune’s moon, orbits backward unlike virtually every other satellite in the solar system.

Formation of Our Solar System Scientists use observations and data from probes to take a look at the formation of the solar system in regard to the shape of our solar system, the differences among the planets, and the oldest planetary surfaces, asteroids, meteorites, and comets. Stars and planets form from clouds of gas and dust called interstellar clouds. Made primarily of He & H. Interstellar clouds may start to condense as a result of gravity and become concentrated enough to form a star and possibly planets.

As a collapsing cloud spins, the cloud flattens and the cloud concentration becomes dense at the center. The disk of dust and gas has now formed a solar nebula – a beginning star. The temps reach 2000 K. The disk of gases condenses towards the outside and elements form --- W, Al2O3, Fe, Si, etc. As the tiny grains of condensed material started to accumulate and merge together to form larger bodies. These larger bodies gather to form planeteimals. Hundreds of Km in diameter. Jupiter was the first large planet to develop.

There are impact craters on Earth. There re 1000s of bodies that orbit the Sun within the planetary orbits – these are called asteroids. They range from a few Km to 1000 Km in diameter. Most are located between Mars and Jupiter. When any interplanetary material enter’s Earth’s atmosphere it is called a meteoroid. The streak of light produced by this is called a meteor, as the meteoroid burns up. If it doesn’t burn up completely and collides with the ground its then called a meteorite. There are impact craters on Earth. Meteor Crater – Arizona Gosses Bluff in Central Australia

Comets are small, icy bodies that have highly eccentric orbits around the Sun. When comets are within 3 AU of the Sun, it begins to evaporate, becomes brighter, and forms a head and one or more tails. Hale-Bopp (fig. 29-30) The coma is the head of the comet. The nucleus is the solid core. It releases gas and dust when heat – thus forming a tail. The tails are pushed away from the coma by particles, ions, and radiation coming from the Sun. Tails always point away from the Sun because of this. Each time a comet pass the Sun it vaporizes some of the comet’s ice and it loses some of its matter. I may eventually break apart completely as the remaining ice evaporates.