OUR SOLAR SYSTEM

1. Chapter 29 OUR SOLAR SYSTEM

2. 29.1 Overview of our Solar System • The Earth is one of nine (9) planets that orbit the Sun • All planets, as well as their moons, orbit in the same direction • All planets except Pluto lie near the same plane • Each of our planets have various sizes, surface conditions, and internal structures

3. Early Observations • Early ideas about the solar system were developed solely on the basis of Earth-based observations of the sky • Early astronomers assumed the Sun, planets, and stars orbited a stationary Earth (Earth centered model) • Problem with Earth centered model is that some aspects of planetary motion were difficult to explain. For instance, occasionally a planet would move in the opposite direction across the sky, retrograde motion. • Retrograde motion caused astronomers to continue searching for a better model of our solar system

4. The Sun-centered model • Nicolaus Copernicus suggested the Sun to be the center of the solar system in 1953. • The sun-centered model is referred to as the heliocentric model, suggesting that the Earth and other planets orbited a stationary Sun. • Provided a simple explanation for retrograde motion, noting that the inner planets move faster so when they bypass the slower-moving outer planets it appears that the outer planets are moving backward

5. Kepler’s First Law • Johannes Kepler demonstrated that each planet orbits the Sun in a shape called an ellipse, rather than a circle • An ellipse is an oval shape that is centered on two, rather than one, points • The two points are called the foci • The major axis is the line that goes between the two foci

6. Astronomical Unit Each planet’s elliptical orbit is a different shape and size, with the Sun always being at one focus For each Sun – planet pair, half of the length of the major axis is called the semi-major axis The average distance between the Sun and the planet is measured in astronomical units (1.496 x 108) Earth is 1 AU from the Sun

7. Eccentricity • A planet in an elliptical orbit is not at a constant distance from the Sun • When a planet is closest to the Sun in its orbit it is said to be at perihelion • When a planet is furthest from the Sun in its orbit it is said to be at aphelion • The shape of a planet’s elliptical orbit is defined by eccentricity, which is the ratio of the distance between the foci to the length of the major axis • The length of time it takes a planet or other body to travel a complete elliptical orbit around the Sun is referred to as the orbital period

8. Kepler’s Second and Third Laws Kepler’s second law states that an imaginary line between the Sun and a planet sweeps out equal amounts of area in equal amounts of time. Kepler derived a mathematical relationship between the size of a planet’s ellipse and its orbital period. He found that the square of the orbital period (P) equals the cube of the semmi-major axis of the orbital ellipse (a). This mathematical relationship (P2 = a3) is Kepler’s third law. (P refers to a unit of time measured in Earth years and a is a unit of length measured in astronomical units.)

9. Gravity and Orbits • Isaac Newton developed an understanding of gravity by observing the Moon’s motion, the orbits of the planets, and the acceleration of falling objects on Earth. • Newton’s statement of the relationship among the masses of two bodies and the force and distance between them is known as the Law of Universal Gravitation. • This law is stated as follows: • 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 distance between them

10. 29.2 The Terrestrial Planets • The inner four planets • Close to the size of Earth • Each have rocky, solid surfaces • Mercury, Venus, Earth, and Mars

12. Mercury • Closest planet to the Sun • No moons • Approximately 1/3 the size of Earth • In 2 of Mercury’s years, 3 of Mercury’s days have passed • Essentially no atmosphere but what does exist is composed of primarily oxygen and sodium • Largest day / night temperature difference (427 C daytime / -173 C at night) • Surface is covered with craters and plains with a planet-wide system of cliffs • Extensive nickel-iron core

13. Venus • Second planet from the Sun • No moons, highest albedo of all planets • Thick clouds in atmosphere, composed of sulfuric acid • Extremely hot on surface • One day is 243 Earth days in length (slow rotation) • Atmosphere is primarily carbon dioxide and nitrogen • Greenhouse Effect causes Venus to be the hottest planet (although it is not closest to the Sun) • Surface has been smoothed by volcanic lava flows, only a few impact craters • Surface is relatively young with little evidence of tectonic activity • Internal structure is similar to Earth. • Considered to be Earth’s “twin”

14. Earth • Third planet from Sun • Unique properties in comparison to other planets • Water exists in all three states (solid, liquid, and gas) • Life is abundant on Earth • Moderately dense atmosphere comprised primarily of nitrogen and oxygen • Axis is tilted, which causes seasonal changes • Wobble in rotation on axis, called precession • Only has one moon

15. Fourth planet from Sun, outermost terrestrial planet • Often referred to as Red Planet, which is caused by a high iron content in the soil • Smaller, less dense than Earth. Has 2 irregularly-shaped moons – Phobos and Deimos • Atmosphere is thin and turbulent, constant wind and dust storms • Southern hemisphere is heavily cratered and northern hemisphere is dominated by plains • Four gigantic shield volcanoes, largest is Olympus Mons. (Also largest mountain in solar system) • Martian surfaces suggest that liquid water once existed on the surface as there are dried river and lake beds present • Polar ice caps on both poles • Hypothesize that the core of Mars is iron and nickel, possibly mixed with sulfur. Mars has no magnetic field, so it is believed that the core is solid • There is no evidence of tectonic activity on the surface or crust of Mars Mars

16. 29.3 The Gas Giant Planets • Remaining planets (minus Pluto) in solar system • Larger than Earth in size • Gaseous, lack solid surface • Jupiter, Saturn, Uranus, and Neptune

18. Jupiter • Largest planet, 11 times larger than Earth, fifth from Sun • Banded appearance, due to a result of flow patterns in the atmosphere • Four major satellites • Low density, atmosphere is made up mostly of hydrogen and helium – which exist in a liquid form • Magnetic field is present • Rotation is rapid, spinning on axis in a little less than 10 hours, making shortest day in solar system • Belts (low, warm, dark-colored clouds that sink) and zones (high, cool, light-colored clouds that rise) are present throughout Jupiter • Great Red Spot – atmospheric storm that has been rotating around Jupiter for more than 300 years • Four major moons composed of ice and rock mixtures. • Discovery of one large ring, 6400km wide, that first proved Saturn is not the only gas giant with rings

19. Sixth planet from Sun, second largest in solar system • Density is lower than that of water • Rotates rapidly for its size, has several zones and belts (as Jupiter) • Atmosphere is dominated by hydrogen and helium, also contains ammonia ice • Internal structure is likely fluid, with solid core and strong magnetic field • Most intricate ring system in solar system. Rings are composed of rocks and ice that ranges from microscopic to the size of an average house!! • Many satellites, including its largest moon, Titan, and a number of smaller moons • Titan is larger than Earth’s moon and its atmosphere is made of nitrogen and methane Saturn

20. Uranus Seventh planet from the Sun Discovered accidentally in 1781 Titaniaand Oberon are two of larger moons; however, there are several that orbit Uranus’ equatorial plane Exact moon count is constantly changing Dark, black rings 4 times as large and 15 times as massive as Earth Atmospheric composition causes a blue, velvety appearance Methane gas reflects blue light back into space although most of the atmosphere is composed of hydrogen and helium Very few clouds present on Uranus and they are difficult to detect due to their similar brightness and color to the surrounding atmosphere No distinct zones or belts like those observed on Jupiter and Saturn Internal structure is similar to Jupiter – it is completely fluid with exception to a small, solid core Strong magnetic field Rotational axis is tipped so far that the North Pole nearly lies in its orbital plane It is believed that Uranus was knocked on its side by a massive collision with a passing object

21. Neptune • Discovered in 1846 • Smaller and more dense than Uranus, but four times as large as Earth • Bluish color caused by methane in the atmosphere, atmospheric compositions, temperatures, magnetic fields, interiors, and particle belts • Distinctive clouds and atmospheric belts and zones similar to Jupiter and Saturn • Had a persistent storm (The Great Dark Spot) that disappeared in 1994 • Many moons, largest being Triton • Triton has a retrograde orbit (goes backwards) and a thin atmosphere with nitrogen geysers. • Six rings composed of microscopic-sized dust particles

22. Pluto • Discovered in 1930 • Solid surface, but not classified as terrestrial due to low density and small size • Made of half ice and half rock • Pluto is smaller than the Earth’s moon, at only about 1,400 miles wide (That is about half the width of the United States) • It takes Pluto 248 years to make one revolution around the sun. Pluto takes 6 1/2 days to rotate • Atmosphere is composed of methane and nitrogen • Orbit is eccentric – at aphelion it is 50 AU from the Sun and at perihelion is it almost 30 AU from the Sun, which means at times it is closer to the Sun than Neptune • Rotational axis is tipped, making its North Pole point “south” of its orbital plane • Satellite – Charon – orbits in Pluto’s equatorial plane

23. Kicked out… Today, Pluto is called a dwarf planet, which is round and orbits the sun just like the eight major planets But unlike a planet, a dwarf planet does not have enough gravity to attract all of the space dust and tiny objects in its path A dwarf planet also is much smaller than a planet, but it is not a moon Pluto is in a region called the Kuiper Belt The Kuiper Belt is a large band of thousands of small, icy objects that orbit the sun beyond Neptune

24. In 2003, U.S. astronomer Mike Brown discovered a new object beyond Pluto. He thought he had discovered a new planet because the object, which he named Eris, was larger than Pluto. • The discovery of Eris caused other astronomers to talk about what makes a planet a "planet,” and it was determined that Pluto and objects like it were not really planets due to their size and location in the solar system. It was determined that these objects should now be referred to as dwarf planets. • Pluto also is called a plutoid, which is a dwarf planet located further out in space than Neptune.

25. Current PLUTOIDS

26. 29.4 Formation of our Solar System • We use Earth based observations and data from probes to derive theories about how our solar system formed • Significant observations include the shape of our solar system, the differences among the planets, and the oldest planetary surfaces, asteroids, meteorites, and comets

27. A Collapsing Interstellar Cloud • Stars and planets form from clouds of gas and dust, called interstellar clouds • Interstellar clouds consist primarily of helium and hydrogen • They often appear as blotches of light and dark spots, due to the dust particles being partially illuminated by the reflection from stars • The density of an interstellar cloud is very low (due to gas content) but they can start to condense due to gravity. • The cloud then becomes concentrated enough to form a star and possibly planets. • Astronomers agree that our solar system began this way.

28. Orion’s Nebula is one of the most bright and beautiful interstellar clouds visible to the naked eye within the night sky

29. Sun and Planet Formation • The disk of dust and gas that formed the Sun and planets is called a solar nebula • The condensed concentration at the center eventually became the Sun • As the condensing slowed, tiny bits of condensed materials accumulated and merged together to form larger bodies called planetesimals. • Further growth of planetesimals continued, through mergers and collisions, sometimes destroying them and making smaller bodies (still relatively large) we now call planets

30. Order of Formation • In the outer solar system, the first large planet to develop was Jupiter. As Jupiter increased in size through the merger of icy planetesimals, gravity began to attract additional gas, dust, and planetesimals… making it grow even larger. • Saturn and other gas giants then formed in similar patterns, but could not grow as large because Jupiter had collected so much material in the vicinity. • As each gas giant gathered material from its surroundings, a disk formed within the equatorial plane – much like that of the early solar system – which then formed satellites • Inner planets then began forming in the same fashion; however, the gravitational pull of the Sun swept up much of the gas in the area and prevented them from acquiring much additional materials. This is believed to be the reasoning why they are so much smaller in size than the gas giants.

32. Asteroids • Thousands of bodies that orbit the Sun within the planetary orbit are called asteroids • Range in size from a few km to about 1000 km in diameter • Pitted, irregular surfaces • Most are located in the asteroid belt, between Mars and Jupiter • Asteroids are thought to be leftover planetesimal pieces from the time of the solar system’s formation • As they orbit, they occasionally collide and cause broken fragments to fall toward other planets (such as Earth) • When interplanetary material falls toward Earth and enters Earth’s atmosphere it is called a meteoroid • When the meteoroid falls toward Earth but burns up in the atmosphere it is called a meteor • If the meteoroid does not completely burn up and part of it collides with the ground it is called a meteorite

33. Comets • Small, icy bodies that have highly eccentric orbits around the Sun • Made of ice and rock, ranging from 1 to 10km in diameter • Head of comet is called the coma and the center, solid core is called the nucleus • Sometimes, a comet intersects with Earth and a meteor shower occurs as result of particles burning up upon entering the Earth’s atmosphere • Two clusters, or clouds, of comets: • Kuiper Belt – close to Pluto, 30-50 AU from the Sun • Oort Cloud – more than 100,000AU from the Sun