Chapter 27 Planets of the solar System

1. Chapter 27Planets of the solar System

2. 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.

3. The Origin of the Solar SystemFour 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.

4. The Origin of the Solar SystemFour 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?

5. 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?

6. 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

9. 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.

11. We see evidence of accretion disk around other stars. • For example, b Pictoris.

12. 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.

18. Uh oh…

23. 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.

24. Section 2 Models of the Solar System

25. 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 sun-centered or heliocentric model.

26. 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)

27. 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

28. 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

29. 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

30. 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). “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 Johannes Kepler (1571-1630)

31. 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.

32. 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

33. 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.

34. 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.

35. 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

36. 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

37. Touring Our Solar System A Trip Through the Solar System

38. 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.

39. The Inner or Terrestrial Planets

40. 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

41. Visible and radar illumination 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.

42. 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

43. 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

44. Venus - named after the Roman goddess of beauty and love Greenhouse effect • Venus’ atmosphere is 95% CO2.

45. 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.

46. 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

47. 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

48. 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.

49. 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.

50. 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