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

Slide 2 The Origin of the Solar System Chapter 19

Slide 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. Mercury, Venus, Earth, Mars: ~ size of a grain of salt. Sun: ~ size of a small plum. Jupiter: ~ size of an apple seed. Saturn: ~ slightly smaller than Jupiter’s “apple seed”. Pluto: ~ Speck of pepper.

Slide 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). The inner Solar System to scale, Fall, 1996 The entire Solar System to scale, Fall, 1996 Almost empty space Note rotation in one direction Note small eccentricity

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

Slide 6 e = R a - R p R a + R p Remember parameters: perihelion, aphelion, semimajor axis Elliptical orbits Eccentricity:

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

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

Slide 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

Slide 11 Sharp difference in masses and density

Slide 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.

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

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

Slide 15 Space debris: asteroids and comets

Slide 16 Comets Mostly objects in highly elliptical orbits, occasionally coming close to the sun. Icy nucleus, which evaporates and gets blown into space by solar wind pressure.

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Slide 20 Meteoroids Small (  m – mm sized) dust grains throughout the solar system If they collide with Earth, they evaporate in the atmosphere.  Visible as streaks of light: meteors.

Slide 21 Origin of the Solar System 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

Slide 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!

Slide 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

Slide 24 Prehistory: Big Bang and structure formation

Slide 25 The abundance of the elements in the universe

Slide 26 History: chronology of events

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

Slide 28 How old is the Earth? How does it work: 1.count relative numbers of different isotopes in nature 2.measure present ratio of unstable to stable isotope atoms in a rock 3.then know how many unstable atoms have decayed into stable atoms 4.for example, if 1/2 of unstable atoms have decayed, the rock's age is one half-life of the unstable element 5.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) Radioactive dating: oldest rocks 4.3 billion yr old

Slide 29 Parent Isotope Stable Daughter Product Currently Accepted Half-Life Values Uranium-238Lead billion years Uranium-235Lead million years Thorium-232Lead billion years Rubidium-87Strontium billion years Potassium-40Argon billion years Samarium- 147 Neodymium billion years 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.

Slide 30 Radioactive Decay (SLIDESHOW MODE ONLY)

Slide 31 In the Nebular Hypothesis, a cloud of gas and dust collapsed by gravity begins to spin faster because of angular momentum conservation 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.

Slide 32 Conservation of angular momentum

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Slide 34 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 The spinning nebula flattens 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.

Slide 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 (??)

Slide 36 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 ( 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. Composition of the disk: Hydrogen 75% by mass, Helium 23%, and heavier elements 2% total

Slide 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

Slide 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

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

Slide 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

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Slide 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.

Slide 44 Inner versus outer planets 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

Slide 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 less than ~ 15 Earth masses: Planets can not grow by gravitational collapse Mass of more than ~ 15 Earth masses: Planets can grow by gravitationally attracting material from the protostellar cloud Earthlike planets Jovian planets (gas giants)

Slide 46 Clearing the nebula 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.

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

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

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

Slide 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!

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Slide 52 Star-Birth Clouds in M16 (Eagle Nebula). J. Hester and P. Scowon (Arizona St. Univ.), November 2, Taken with NASA Hubble Space Telescope, WFPC2

Slide 53 Dust Disks Around Forming Stars Dust disks around some T Tauri stars can be imaged directly (HST).

Slide 54 Thin, warped disk: effect of planets?

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Slide 57 Indirect Detection of Extrasolar Planets 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 Over 100 extrasolar planets detected so far.

Slide 58 Our Solar System