Introduction to the Solar System

1. 0 Introduction to the Solar System Chapter 6

2. 0 Guidepost You are a planetwalker. What does that mean? You live on an astronomical body, Earth, and it is time to find out how Earth is connected to the story of the stars. As you explore our solar system, you will find answers to four essential questions: • What theories account for the origin of the solar system? • What properties must a successful theory explain? • How do planets form? • Is our solar system unique?

3. 0 Guidepost (continued) Exploring the origin of the solar system will give you new insight into what you are, but it will also give you a chance to answer two important questions about science: • Why are scientists hesitant to accept catastrophic theories? • Why are scientists skeptical of new ideas? Once you understand the origin of the solar system, you will have a framework for understanding the rest of the planets and the mysterious smaller bodies that orbit the sun.

4. 0 Outline I. Theories of Solar System’s Origin A. Early Hypotheses B. The Solar Nebula Hypothesis II. A Survey of the Solar System A. A General View B. Two Kinds of Planets C. Space Debris: Comets, Asteroids, Meteorites… D. The Age of the Solar System III. Planets Orbiting Other Stars A. Planet-Forming Disks around Other Suns B. Extrasolar Planets

5. Misconceptions to be cleared out…. • The Solar System is very crowded. • The Solar System contains only the Sun, Moon, and planets. • Meteors are falling stars. • Comets sweep across the sky like meteors. • Comet tails stream behind the comet as the comet moves. • Comets are burning. • Planets are close together or are large compared with the distances between them. • The planets are always arranged in a straight line away from the Sun. • Alignments of planets are dangerous and can affect the Earth. • Your personality and future are determined by the positions of the Sun, Moon, and planets. • Planetary orbits are circular. • Mercury is hot everywhere on its surface. • Giant planets have solid surfaces. • Saturn is the only planet with rings. • Jupiter and Saturn are made of gas. • The Sun is made of molten lava and is solid in parts. • The asteroid belt is crowded and dangerous.

6. I. Theories of Earth’s Origin

7. 0 Early Hypotheses (1) • Catastrophic hypotheses, e.g., passing star hypothesis: Star passing the sun closely tore material out of the sun, from which planets could form 2. Evolutionary hypotheses, e.g., Laplace’s nebular hypothesis: Rings of material separate from the spinning cloud, carrying away angular momentum of the cloud cloud could contract further (forming the sun)

8. 0 Early Hypotheses (2): No Longer Considered • Catastrophic hypotheses, e.g., passing star hypothesis: • Stars very far apart – Very infrequent collisions • Gas pulled from Sun too hot Catastrophic hypotheses predict: Only few stars should have planets! 2. Evolutionary hypotheses, e.g., Laplace’s nebular hypothesis: • Sun would be rotating too fast, and planets slow. Evolutionary hypotheses predict: Most stars should have planets!

9. 0 The Solar Nebula Hypothesis: Video • By 1940, astronomers were beginning to understand how stars form and how they generated their energy. • Solar Nebula Theory: planets form in the rotating disks of gas and dust around young stars. • If planet formation is a natural part of star formation, most stars should have planets. Basis of modern theory of planet formation. Planets form at the same time from the same cloud as the star. Planet formation sites observed today as dust disks of T Tauri stars. Sun and our Solar system formed ~ 5 billion years ago.

11. Scientific Argument • Why does the solar nebula theory imply planets are common? • Why would a catastrophic hypothesis for the formation of the solar system suggest that planets are not common?

12. Chapter Summary (1) • The passing star hypothesis suggests that a passing star pulled matter out of the Sun to form the planets. • Laplace’s nebular hypothesis required a contracting nebula to leave behind rings that formed each planet. • The solar nebula theory is a more extensive version of the nebula hypothesis, which is an evolutionary hypothesis.

13. II. A Survey of the Solar System

15. 0 Survey of the Solar System Relative Sizes of the Planets Assume, we reduce all bodies in the solar system so that the Earth has diameter 0.3 mm. Sun: ~ size of a small plum. Mercury, Venus, Earth, Mars: ~ size of a grain of salt. Jupiter: ~ size of an apple seed. Saturn: ~ slightly smaller than Jupiter’s “apple seed”. Pluto: ~ Speck of pepper.

16. 0 Planetary Orbits All planets in almost circular (elliptical) orbits around the sun, in approx. the same plane (ecliptic). Orbits generally inclined by no more than 3.4o Mercury Venus Sense of revolution: counter-clockwise Mars Exceptions: Mercury (7o) Pluto (17.2o) Jupiter Earth Sense of rotation: counter-clockwise (with exception of Venus, Uranus, and Pluto) Uranus Saturn Pluto Neptune (Distances and times reproduced to scale)

18. Scientific Argument • In what ways does the solar system resemble a disk?

19. 0 Two Kinds of Planets Planets of our solar system can be divided into two very different kinds: Terrestrial (earthlike) planets: Mercury, Venus, Earth, Mars Jovian (Jupiter-like) planets: Jupiter, Saturn, Uranus, Neptune

20. 0 Terrestrial Planets (Video Trailer) Four inner planets of the solar system Relatively small in size and mass (Earth is the largest and most massive) Rocky surface Surface of Venus can not be seen directly from Earth because of its dense cloud cover.

21. 0 Craters on Planets’ Surfaces Craters (like on our Moon’s surface) are common throughout the Solar System. Not seen on Jovian planets because they don’t have a solid surface.

22. 0 The Jovian Planets (Video Trailer) Much lower average density Mostly gas; no solid surface All have rings (not only Saturn!)

24. Scientific Argument • What are the distinguishing differences between the Terrestrial and Jovian planets?

25. Latest Pluto’s FlybyNew Horizons July 2015

26. 0 Space Debris (Video Trailer) In addition to planets, small bodies orbit the sun: Asteroids, comets, meteoroids Asteroid Eros, imaged by the NEAR spacecraft

27. Common Misconception • Asteroids are the remains of a planet that broke apart.

28. 0 Comets (Video Trailer) Icy nucleus, which evaporates and gets blown into space by solar wind pressure. Mostly objects in highly elliptical orbits, occasionally coming close to the sun.

29. 0 Meteoroids (Video Trailer) Small (mm – mm sized) dust grains throughout the solar system If they collide with Earth, they evaporate in the atmosphere. • Visible as streaks of light: meteors. • (Sound of a meteor shower)

30. (Video Trailer)

31. 0 The Age of the Solar System Sun and planets should have about the same age. Ages of rocks can be measured through radioactive dating: Measure abundance of a radioactively decaying element to find the time since formation of the rock Dating of rocks on Earth, on the Moon, and meteorites all give ages of ~ 4.6 billion years.

32. 0 Chapter Summary (2)

33. Review Questions 1. Which characteristic listed below describes the Jovian planets? • Small masses. • Low density. • Solid surfaces. • Slow rotational period. 2. All terrestrial planets have ________ • Thick atmospheres. • Strong magnetic fields. • Craters. • Liquid water oceans. 3. What are the bodies between Mars and Jupiter (sometimes called the ‘failed planet’)? • Kuiper belt objects • Ort cloud objects. • The asteroids. • The meteorites.

34. Why Isn't Pluto a Planet Any More? India response to Pluto’s Denomination!!!!

35. III. Planets Orbiting Other Stars Video Trailer 3: Hubble and Exoplanets Hubble Cast 14

36. 0 Evidence for Ongoing Planet Formation Many young stars in the Orion Nebula are surrounded by dust disks: Probably sites of ongoing planet formation right now!

37. 0 Dust Disks Around Forming Stars Dust disks around some T Tauri stars can be imaged directly (HST).

38. 0 Extrasolar Planets Modern theory of planet formation is evolutionary Many stars should have planets! planets orbiting around other stars = “Extrasolar planets” Extrasolar planets can not be imaged directly. Detection using same methods as in binary star systems: Look for “wobbling” motion of the star around the common center of mass.

39. 0 Indirect Detection of Extrasolar Planets (Video Trailer: How to Detect) / Video 2 Observing periodic Doppler shifts of stars with no visible companion: Evidence for the wobbling motion of the star around the common center of mass of a planetary system 3500 Over 250 extrasolar planets detected so far.

40. 0 Direct Detection of Extrasolar Planets Only in exceptional cases can extrasolar planets be observed directly. Preferentially in the infrared: Planets may still be warm and emit infrared light; stars tend to be less bright in the infrared than in the optical

43. Summary (3) Is our Solar System Unique? • Hot disks of gas and dust have been detected in early stages of star formation and are believed to be the kind of disk in which planets could form. • Planets orbiting other stars, extrasolar planets, have been detected • by the way they tug their stars about, creating small Doppler shifts in the stars’ spectra. • as they cross in front of their star and dim the star’s light. • Some extrasolar planets are big enough to be Jovian worlds but are quite close to their star and are called hot Jupiters. • A few of these planets have been detected when they crossed behind their star and their infrared radiation was cut off. • Nearly all extrasolar planets found so far are massive, Jovian worlds. Lower-mass terrestrial planets are harder to detect but are presumably common.