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Download The Planets of the Solar System
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Chapter 27 Planets of the solar System The Nebular Hypothesis • In 1796: Laplace’s hypothesis states that the sun and the planets condensed about the same time out of a rotating cloud of gas and dust. The Origin of the Solar System Four Challenges 1. Patterns of Motion Planets orbit in the same direction... ...in nearly the same plane... ...in nearly circular orbits. Most planets rotate in the same direction. Most moons orbit in the same direction. 2. Categorizing Planets Planets are either rocky or gas-rich. The Origin of the Solar System Four Challenges 3. Asteroids and Comets Most asteroids are found between Mars and Jupiter. Most comets have highly elliptical orbits. 4. Exceptions to the Rules What about Pluto’s elliptical orbit and composition? What about the odd rotation of Venus and Uranus? Formation of the Solar System • The solar system is thought to have formed from a cloud of gas and dust in a process know as accretion. • Our Sun is thought to be a second generation star. • What does that mean? Formation • • http://observe.phy.sfasu.edu/courses/ast105/lectures105/chapter06/formation_protoplanet_disk .htm http://observe.phy.sfasu.edu/courses/ast105/lectures105/chapter06/accretion_and_planets.htm • During the first few million years, matter in the accretion disk of our protosun coalesced (joined into a single mass)… • ...in the larger objects called planetesimals, with diameters of about 100 km. • We see evidence of accretion disk around other stars. • For example, b Pictoris. • Collisions of planetesimals dominated the early solar system… • ...and these objects combined to form our planets. • We see evidence of early collisions in our solar system in the form of impact craters on the planets and their moons. Uh oh… • In addition to the 8 major planets, there are at least 100 moons in our solar system. • While some of these moons are spherical, most look roughly like potatoes. • There is still minor debris left over from the formation of the solar system: – asteroids and comets. Section 2 Models of the Solar System Early Models of the Solar System • 2000 years ago Aristotle suggested an Earth-centered or geocentric solar system. • Around 130 AD Ptolemy proposed changes to the model to account for problems with Aristotle’s model. • In 1543 Copernicus proposed a suncentered or heliocentric model. Giants of Science Tycho Brahe & Johannes Kepler These two scientists showed that the Universe was not some ideal perfection as Ptolemy proposed and worked towards acceptance of Copernicus’ heliocentric model Tycho Brahe – made the most accurate observations of stars and planets up to that time. – was a flamboyant Danish nobleman who wore a silver nose when part of his nose was cut off in a duel! Tycho Brahe (1546-1601) Tycho Brahe and Uraniborg • He lived in a mansion/observatory on an island off the coast of Denmark. • The mansion had very sophisticated equipment (but no telescopes!) to help him and his assistants to measure the positions of stars and planets. • He named the mansion Uraniborg (Sky Castle). Some of the equipment used at Uraniborg Tycho Brahe’s Discoveries • As a young man he proved that comets had to be farther from Earth than the Moon. • He also proved that a star which appeared to brighten dramatically over a few weeks was also beyond the Moon. • Both observations showed that the heavens could change like the Earth. • He also came up with his own compromise model of the Universe. Brahe’s compromise: All the planets went around the Sun while the Sun moved around a fixed Earth Tycho Brahe & Johannes Kepler • A few years before he died, Brahe hired Johannes Kepler to help in analyzing the data he had collected. • Brahe started him out on his hardest problem: determine the orbit of Mars. • Mars has the largest observed retrograde motion and no circular orbit could be found to match Brahe’s observations. Brahe and assistants making observations Kepler’s Models After years of work, the most accurate circle he could find for Mars’ orbit still left an error of 8 arcminutes (about 1/4 the angular size of the full Moon). Johannes Kepler (1571-1630) “If I had believed that we could ignore these eight minutes [of arc], I would have patched up my hypothesis accordingly. But since it was not permissible to ignore, those eight minutes pointed the road to a complete reformation in astronomy” - Kepler Kepler’s Breakthrough • Kepler’s key discovery – planets do not orbit in circles but rather in ellipses. – the Sun was not at the center of the ellipse but rather at one focus. • With this breakthrough he obtained excellent agreement between his model and observations. Properties of Ellipses • Each point marked by a tack is called a focus. • The farther apart one focus is from another the more eccentric the ellipse. • The line cutting the ellipse in half that passes through each focus is called a major axis. Half the major axis is called a semimajor axis. • The semimiajor axis is the average distance of the planet from the Sun Kepler’s 3 Laws of Planetary Motion • These laws describe the observed planetary motions but do not describe why these motions occur as they do. Kepler’s First Law of Planetary Motion The orbit of each planet around the Sun is an ellipse with the Sun at one focus. – There is nothing at the other focus. – The average distance of the planet from the Sun is the semimajor axis. – Throws out Ptolemy’s perfect circular orbits. Kepler’s Second Law of Planetary Motion • As a planet moves around its orbit, it sweeps out equal areas in equal times. – A planet travels faster when it is nearer the Sun and slower farther away – Throws out Ptolemy’s uniform motion Kepler’s Third Law of Planetary Motion • The amount of time it takes a planet to orbit the Sun is related to the size of its orbit by P2(years) = a3(AU) – 1 AU (astronomical unit) is the semimajor axis of the Earth’s orbit. Earth’s average distance from the Sun. – It doesn’t matter how elliptical the orbit as long as the average distance is the same Touring Our Solar System A Trip Through the Solar System The Inner or Terrestrial Planets • Mercury, Venus, Earth and Mars share certain characteristics: – All are rocky bodies. – All have solid surfaces. – Except for Mercury all have at least a thin atmosphere • They are called Terrestrial planets because of their resemblance to Earth. The Inner or Terrestrial Planets Mercury - named after the speedy messenger of the Roman gods • • • • • • Closest planet to the sun Revolution around the sun = 88 Earth days Rotation on its axis = 59 Earth days Crater-covered surface with steep cliffs Almost no atmosphere Temperature range – as high as 427 degrees C – as low as -170 degrees C Venus - named after the Roman goddess of beauty and love • Venus is the second planet from the sun and has an orbital period of 225 days. • Venus rotates very slowly, only once every 243 days. • Venus and Earth are of almost the same size, mass, and density, but differ greatly in other areas. Visible and radar illumination Venus - named after the Roman goddess of beauty and love • Second planet from the sun • About the same size as Earth • Thick, cloudy atmosphere – sulfuric acid – carbon dioxide • Highest temperature range of inner planets – as high as 480 degrees C Visible and radar illumination Venus - named after the Roman goddess of beauty and love • Surface pressure = 91 times more than Earth’s • Surface has… – deep canyons and tall mountains – craters – vast plains • Revolution around the sun = 224 Earth days • Rotation on its axis = 243 Earth days Visible and radar illumination Venus - named after the Roman goddess of beauty and love Greenhouse effect • Venus’ atmosphere is 95% CO2. Earth • Earth is the third planet from the sun. • The orbital period of Earth is 365 1/4 days. Earth completes one rotation on its axis every day. • Earth has one large moon. • Geologic records indicate that over the last 250 million years, Earth’s surface has undergone many changes. Earth • Third planet from the sun • Revolution around the sun = 365 days • Rotation on its axis = 24 hours • Because the axis of the Earth is tilted, this creates distinct “seasons” throughout the year Earth • Temperature range depends on the location, altitude and season • Gravitation pull of the moon creates tide changes (rise and fall of the ocean levels) • Surface – – – – Mountains Plains Deserts Heavy vegetation Mars - named after the Roman god of war • Mars is the fourth planet from the sun. • Mars is about 50% farther from the sun than Earth is. • Its orbital period is 687 days • it rotates on its axis every 24 hours and 37 minutes. • Mars’s seasons are like Earth’s seasons because the same axis. Mars - named after the Roman god of war • Fourth planet from the sun • Surface – rocky – large craters – soil is similar to Earth’s soil in many ways – Has the largest volcano in the solar system, Olympus Mons. Mars - named after the Roman god of war • Very thin CO2 atmosphere, polar caps of mostly frozen CO2. Since its atmosphere is thin and cold there is very little greenhouse effect. • High winds often create dust storms • Temperate falls well below 0 degrees C all the time Mars - named after the Roman god of war • About half the size of Earth. No geological activity likely now. No magnetic field. • Evidence of massive water erosion some time in the past. Scientists are searching for liquid water now. • Two satellites, Phobos and Deimos – (possibly captured asteroids) Mars - named after the Roman god of war The Outer or Jovian Planets • Jupiter, Saturn, Uranus and Neptune share certain characteristics: – All are large, gaseous bodies. – All have very thick atmospheres, with possibly liquid interiors and solid cores – All have rings • They are called Jovian planets because of their resemblance to Jupiter. The Outer or Jovian Planets Jupiter - named after the king of the Roman gods • Fifth planet from the sun • Made of mainly – hydrogen – helium • Temperature range – very cold at the cloud tops – as high as 30,000 degree C at the core Jupiter - named after the king of the Roman gods • Jupiter is the largest planet in the solar system and has a mass more than 300 times that of Earth. • The orbital period of Jupiter is almost 12 years. Jupiter rotates on its axis faster than any other planet—once every 9 h and 50 min. • Jupiter has at least 60 moons. • It also has several thin rings that are made up of millions of particles. Jupiter - named after the king of the Roman gods • Atmosphere – hydrogen – helium – ammonia – methane • Great Red Spot – hurricane-like storm – (as much as 20,000 years old) Jupiter - named after the king of the Roman gods • Very high atmospheric pressure • Giant magnetic field – created by the liquid metallic layer – called magnetosphere Saturn - named after the Roman god • Sixth planet from the sun • Surrounded by rings – made of icy particles – has at least seven major rings • Made of mainly – hydrogen – helium Saturn - named after the Roman god • The orbital period of Saturn is 29.5 years. • Saturn rotates on its axis every 10 h and 30 min. • Saturn is very cold and has an average cloud-top temperature of –176°C. • Saturn has at least 60 moons. Saturn - named after the Roman god • • • • Violent atmospheric storms Very cold Has a large magnetic field Lowest density of all the planets Uranus - named after the father of Saturn in Roman mythology • Seventh planet from the sun • Atmosphere – hydrogen – helium – methane • Temperature range – as low as -220 degree C at the cloud tops Uranus - named after the father of Saturn in Roman mythology • Extreme atmospheric pressure – atmosphere is 11,000 kilometers thick • Rotates on its axis at a 90 degree angle – appears laying on its side • Rings of methane ice surround it Neptune - named after the Roman god of the sea • Eighth planet from the sun • Atmosphere – hydrogen – helium – methane • Temperature – as low as -220 degrees C Neptune - named after the Roman god of the sea • Surface – ocean of water and liquid methane – rocky core • Five rings surround Neptune – made of dust particles formed from meteorites Pluto - named after the Roman god of the underworld • • • • • Ninth planet from the sun Smallest planet Least dense planet Seems to be made primarily of methane ice Thin atmosphere (only on the sunny side) – methane ice evaporated to form this Pluto - named after the Roman god of the underworld • Pluto may have been a satellite of Neptune that was displaced from its original orbit and split into two pieces. Charon Pluto