Average: 88 Median: 88

1. Average: 88 Median: 88 Test 2: 86 Test 1: 73

2. The Origin of the Solar System Chapter 19

3. 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.

4. As discovered by Kepler, the planets orbit on ellipses with the Sun at one focus. In addition, the planets all revolve in the same direction on their orbits (counter-clockwise as viewed from the North). Note rotation in one direction Almost empty space Note small eccentricity

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

6. Elliptical orbits Remember parameters: perihelion, aphelion, semimajor axis Ra - Rp Eccentricity: e = Ra + Rp

8. Note small tilt (largest for Pluto: 17o) Side view of entire Solar System, Fall, 1996 Note small tilt (largest for Mercury: 7o) Side view of the inner Solar System, Fall, 1996

9. Two kinds of planets: Terrestrial and Jovian • Location • Size • Mass • Density

10. 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

11. Sharp difference in masses and density

12. Terrestrial Planets 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.

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

14. Space Debris In addition to planets, small bodies orbit the sun: Asteroids, comets, meteoroids Asteroid Eros, imaged by the NEAR spacecraft

15. Space debris: asteroids and comets

16. Comets 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.

20. Meteoroids 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.

21. Same direction of revolution (counterclockwise as viewed from the North) Same direction of rotation for most planets and their moons (except for Venus, Uranus, and Pluto) Small eccentricity Small tilt All that suggests common origin of the Sun and planets from the rotating disk Origin of the Solar System

22. Early Hypotheses • catastrophic hypotheses, e.g., passing star hypothesis: Star passing the sun closely tore material out of the sun, from which planets could form (no longer considered) Catastrophic hypotheses predict: Only few stars should have planets! • 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) Evolutionary hypotheses predict: Most stars should have planets!

23. Solar nebula hypothesis Kant 1755; Laplace 1796 The Sun and the planets were born simultaneously from gravitational contraction of a cloud of gas and dust Planets are the by-product of star formation and must be common in the Universe

24. Prehistory: Big Bang and structure formation

25. The abundance of the elements in the universe

26. History: chronology of events

27. The age: from abundance of radioactive elements Radioactive clock 238U -> 206Pb Half-life 4.5 billion yr 40K -> 40Ca, 40Ar: 1.3 billion yr Oldest rocks on Earth: 4.3 billion yr Oldest rocks on Moon: 4.48 billion yr Meteorite from Mars: 4.6 billion yr

28. How old is the Earth? Radioactive dating: oldest rocks 4.3 billion yr old • How does it work: • count relative numbers of different isotopes in nature • measure present ratio of unstable to stable isotope atoms in a rock • then know how many unstable atoms have decayed into stable atoms • for example, if 1/2 of unstable atoms have decayed, the rock's age is one half-life of the unstable element • use a set of isotopes, where one has a half-life that is approximately the age of what you want to measure (Uranium-238 has a half-life of 4.5 billion years, Carbon-14 has a half-life of 5730 years)

29. A technician of the U.S. Geological Survey uses a mass spectrometer to determine the proportions of neodymium isotopes contained in a sample of igneous rock.

30. Radioactive Decay (SLIDESHOW MODE ONLY)

31. A cloud of interstellar gas and/or dust (the "solar nebula") is disturbed and collapses under its own gravity. The disturbance could be, for example, the shock wave from a nearby supernova. In the Nebular Hypothesis, a cloud of gas and dust collapsed by gravity begins to spin faster because of angular momentum conservation

32. Conservation of angular momentum

34. The spinning nebula flattens Because of the competing forces associated with gravity, gas pressure, and rotation, the contracting nebula begins to flatten into a spinning pancake shape with a bulge at the center In several 100,000 yr, the center compresses enough to become a protostar and the rest of the gas orbits/flows around it. 99.8% of that gas flows inward and adds to the mass of the forming star, but the gas is rotating. The centrifugal force from that prevents some of the gas from reaching the forming star. Instead, it forms an "accretion disk" around the star. The disk radiates away its energy and cools off.

35. Depending on the details, the gas orbiting star/protostar may be unstable and start to compress under its own gravity. That produces a double star. Jupiter could be a star (??)

36. Composition of the disk: Hydrogen 75% by mass, Helium 23%, and heavier elements 2% total The gas cools off enough for the metal, rock and (far enough from the forming star) ice to condense out into tiny particles. (i.e. some of the gas turns back into dust). The metals condense almost as soon as the accretion disk forms (4.55-4.56 billion years ago according to isotope measurements of certain meteors); the rock condenses a bit later (between 4.4 and 4.55 billion years ago). The dust particles collide with each other and form into larger particles. This goes on until the particles get to the size of boulders or small asteroids.

37. Runaway growth of protoplanets. Three stages: Condensation and accretion of solid particles into planetesimals Size up to ~ 1 km Coalescing of planetesimals into protoplanets Clearing the nebula from most of the remaining gas and dust

38. Formation of planetesimals: Distance from the Sun and temperature play the key role • Inner planets formed from high-density metal oxides • Outer planets formed from low-density ices

40. The coalescing of planetesimals into protoplanets Gravitational instabilities led to clumping

41. The Growth of Protoplanets Simplest form of planet growth: Unchanged composition of accreted matter over time As rocks melted, heavier elements sink to the center differentiation This also produces a secondary atmosphere outgassing Improvement of this scenario: Gradual change of grain composition due to cooling of nebula and storing of heat from potential energy

43. The Jovian Problem Two problems for the theory of planet formation: 1) Observations of extrasolar planets indicate that Jovian planets are common. 2) Protoplanetary disks tend to be evaporated quickly (typically within ~ 100,000 years) by the radiation of nearby massive stars. Too short for Jovian planets to grow! Solution: Computer simulations show that Jovian planets can grow by direct gas accretion without forming rocky planetesimals.

44. Terrestrial planets had to condense from solid particles: metals and silicates. Most compounds of the nebula were gaseous . They grew longer (100 million yr) and became small and dense. Jovian planets formed in the outer nebula and could grow by accreting ices. Once they became ~ 15 times more massive than Earth, they had enough gravity to capture the gas (hydrogen and helium) directly from the nebula. They grew rapidly (< 10 million yr) and became very massive. Comets: the remains of icy planetesimals Asteroid belt: remains of rocky planetesimals. There could have been a planet but it never formed due to the effect of Jupiter Inner versus outer planets

45. The Story of Planet Building Planets formed from the same protostellar material as the sun, still found in the Sun’s atmosphere. Rocky planet material formed from clumping together of dust grains in the protostellar cloud. Mass of more than ~ 15 Earth masses: Mass of less than ~ 15 Earth masses: Planets can grow by gravitationally attracting material from the protostellar cloud Planets can not grow by gravitational collapse Earthlike planets Jovian planets (gas giants)

46. Radiation pressure from the protosolar radiation The solar wind: flow of ionized gas The sweeping up of debris by the planets. Heavy bombardment ~ 4 billion yr ago! Ejection of remains to the outskirts of the Solar system in close encounters with planets. Clearing the nebula

47. Clearing the Nebula Remains of the protostellar nebula were cleared away by: • Radiation pressure of the sun • Sweeping-up of space debris by planets • Solar wind • Ejection by close encounters with planets Surfaces of the Moon and Mercury show evidence for heavy bombardment by asteroids.

48. Observing formation of other stars Observing extrasolar planets Looking for traces of the past in the Solar system How do we know?

49. Formation of stars and protoplanetary disks in the Orion nebula (500 pc from Earth)

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