Chapter 6 Formation of Planetary Systems Our Solar System and Beyond

Transcription

1 Chapter 6 Formation of Planetary Systems Our Solar System and Beyond

2 The solar system exhibits clear patterns of composition and motion.

3 Sun Over 99.9% of solar system s mass Made mostly of H/He gas (plasma) Converts 4 million tons of mass into energy each second

4 Mercury Made of metal and rock; large iron core Desolate, cratered; long, tall, steep cliffs Very hot and very cold: 425 C (day), 170 C (night)

5 Venus Nearly identical in size to Earth; surface hidden by clouds Hellish conditions due to an extreme greenhouse effect Even hotter than Mercury: 470 C, day and night

6 Earth Earth and Moon to scale An oasis of life The only surface liquid water in the solar system A surprisingly large moon

7 Mars Looks almost Earth-like Giant volcanoes, a huge canyon, polar caps, and more Water flowed in the distant past

8 Jupiter Much farther from Sun than inner planets Mostly H/He; no solid surface 300 times more massive than Earth Many moons, rings

9 Io (shown here): Active volcanoes all over Europa: Possible subsurface ocean Ganymede: Largest moon in solar system Callisto: A large, cratered ice ball Jupiter s moons can be as interesting as planets themselves, especially Jupiter s four Galilean moons

10 Saturn Giant and gaseous like Jupiter Spectacular rings Many moons, including cloudy Titan Cassini spacecraft currently studying it

11 Rings are NOT solid; they are made of countless small chunks of ice and rock, each orbiting like a tiny moon. Artist s conception The Rings of Saturn

12 Uranus Smaller than Jupiter/Saturn; much larger than Earth Made of H/He gas and hydrogen compounds (H 2 O, NH 3, CH 4 ) Extreme axis tilt Moons and rings

13 Neptune Similar to Uranus (except for axis tilt) Many moons (including Triton)

14 Pluto Much smaller than other planets Icy, comet-like composition Pluto s moon Charon is similar in size to Pluto

15 Which planets have a rocky, relatively dense composition? 1. Jupiter, Saturn, Earth, and Mars 2. Uranus, Neptune, Earth, and Mars 3. Jupiter, Saturn, Uranus, and Neptune 4. Mercury, Venus, Earth, and Mars

16 What features of our solar system provide clues to how it formed?

17 Motion of Large Bodies All large bodies in the solar system orbit in the same direction and in nearly the same plane. Most also rotate in that direction.

18 Two Major Planet Types

19 Swarms of Smaller Bodies

20 Notable Exceptions

21 What theory best explains the features of our solar system?

22 According to the nebular theory, our solar system formed from a giant cloud of interstellar gas. (nebula = cloud)

23 Galactic Recycling Elements that formed planets were made in stars and then recycled through interstellar space.

24 Evidence from Other Gas Clouds We can see stars forming in other interstellar gas clouds, lending support to the nebular theory. The Orion Nebula with Proplyds

25 Most of the solar system s planets: 1. Are made of rocks and minerals 2. Are made of gas 3. Revolve (orbit) around the Sun in the same direction 4. Rotate in the same direction as they orbit the Sun 5. 3 and 4

26 Why do we think the inner (terrestrial) planets became more dense than the outer planets? 1. In the collapsing solar nebula, denser materials sank toward the center 2. The sun s gravity pulled denser materials toward the center 3. The inner nebula was so hot that only metals and rocks were able to condense 4. The rotating disk in which the planets formed spun lighter elements outward by centrifugal force

27 What do we think the composition of the solar nebula was? 1. About half hydrogen and helium, half heavier elements (iron, carbon, silicon, etc.) 2. About 98% hydrogen and helium, and 2% heavier elements 3. A less hydrogen and helium, more heavier elements

28 Orbital and Rotational Properties of the Planets

29 Conservation of Angular Momentum

30 Collapse of the Solar Nebula Rotation of a contracting cloud speeds up for the same reason a skater speeds up as she pulls in her arms.

31 Flattening Collisions between particles in the cloud caused it to flatten into a disk.

32 Formation of Circular Orbits Collisions between gas particles in a cloud gradually reduce random motions.

33 Why does the Disk Flatten? Collisions between gas particles also reduce up and down motions.

34 Formation of the Protoplanetary Disk The spinning cloud flattens as it shrinks.

35 Disks Around Other Stars Observations of disks around other stars support the nebular hypothesis.

36 Why are there two major types of planets?

37 Conservation of Energy As gravity causes the cloud to contract, it heats up. Collapse of the Solar Nebula

38 Inner parts of the disk are hotter than outer parts. Rock can be solid at much higher temperatures than ice. Temperature Distribution of the Disk and the Frost Line

39 Fig 9.5 Inside the frost line: Too hot for hydrogen compounds to form ices Outside the frost line: Cold enough for ices to form

40 Tiny solid particles stick to form planetesimals. Summary of the Condensates in the Protoplanetary Disk

41 Gravity draws planetesimals together to form planets. This process of assembly is called accretion. Summary of the Condensates in the Protoplanetary Disk

42 Accretion of Planetesimals Many smaller objects collected into just a few large ones.

43 The gravity of rock and ice in jovian planets draws in H and He gases. Nebular Capture and the Formation of the Jovian Planets

44 Moons of jovian planets form in miniature disks.

45 The Solar Wind Radiation and outflowing matter from the Sun the solar wind blew away the leftover gases.

46 Asteroids and Comets Leftovers from the accretion process Rocky asteroids inside frost line Icy comets outside frost line

47 Heavy Bombardment Leftover planetesimals bombarded other objects in the late stages of solar system formation.

48 When the first solid bits in the solar nebula became large enough to be called planetesimals, what began to increase their growth rate? 1. Gravity 2. They were mostly made of sticky stuff 3. Electrical forces ( static electricity ) 4. Pressure 5. Ice

49 Why could the Jovian planets grow to be much larger than the terrestrial 1. They were further from the Sun and gravity was weaker 2. They formed beyond the frost line where ices can condense so they included hydrogen compounds 3. They were far enough from the Sun to escape the heavy bombardment that battered the early solar system planets?

50 Origin of Earth s Water Water may have come to Earth by way of icy planetesimals from the outer solar system.

51 How do we explain the existence of our Moon and other exceptions to the rules?

52 Captured Moons The unusual moons of some planets may be captured planetesimals.

53 Giant Impact Giant impact stripped matter from Earth s s crust Stripped matter began to orbit Then accreted into Moon

54 Odd Rotation Giant impacts might also explain the different rotation axes of some planets.

55 What is the solar wind? 1. Strong radiation that comes from the Sun 2. Similar to winds on earth but faster and stronger 3. Similar to winds on earth but less dense and weaker 4. Atoms and parts of atoms ejected from the Sun at high speed

56 What do we think happened to the solar nebula after the planets formed? 1. The gas was all used up 2. The rest of the gas gradually drifted away 3. The solar wind helped blow the gas away 4. The gas is still there we just can t see it.

57 Where do asteroids come from? 1. A planet between Mars and Jupiter that broke up 2. They are escaped small moons 3. Leftover planetesimals from the inner solar system 4. Leftover planetesimals from the outer solar system

58 Where do comets come from? 1. A planet between Mars and Jupiter that broke up 2. They are escaped small moons 3. Leftover planetesimals from the inner solar system 4. Leftover planetesimals from the outer solar system

59 How would the solar system be different if the solar nebula had cooled with a temperature half its current value? 1. Jovian planets would have formed closer to the Sun. 2. There would be no asteroids. 3. There would be no comets. 4. Terrestrial planets would be larger.

60 Which of these facts is NOT explained by the nebular theory? 1. There are two main types of planets: terrestrial and jovian. 2. Planets orbit in the same direction and plane. 3. Asteroids and comets exist. 4. There are four terrestrial and four jovian planets.

61 When did the planets form? We cannot find the age of a planet, but we can find the ages of the rocks that make it up. We can determine the age of a rock through careful analysis of the proportions of various atoms and isotopes within it.

62 Radioactive Decay Some isotopes decay into other nuclei. A half-life is the time for half the nuclei in a substance to decay.

63 Suppose you find a rock originally made of potassium-40, half of which decays into argon-40 every 1.25 billion years. You open the rock and find 15 atoms of argon-40 for every atom of potassium-40. How long ago did the rock form? billion years ago billion years ago billion years ago 4. 5 billion years ago

64 Dating the Solar System Age dating of meteorites that are unchanged since they condensed and accreted tells us that the solar system is about 4.6 billion years old.

65 Planet Detection Direct: Pictures or spectra of the planets themselves Indirect: Measurements of stellar properties revealing the effects of orbiting planets

66 Gravitational Tugs The Sun and Jupiter orbit around their common center of mass. The Sun therefore wobbles around that center of mass with the same period as Jupiter. Stellar Motion due to Planetary Orbits

67 Gravitational Tugs Sun s motion around solar system s center of mass depends on tugs from all the planets. Astronomers who measured this motion around other stars could determine masses and orbits of all the planets.

68 Astrometric Technique We can detect planets by measuring the change in a star s position in the sky. However, these tiny motions are very difficult to measure (~0.001 arcsecond).

69 Doppler Technique Oscillation of a Star's Absorption Line

70 First Extrasolar Planet Detected Doppler shifts of star 51 Pegasi indirectly reveal planet with 4- day orbital period Short period means small orbital distance First extrasolar planet to be discovered (1995)

71 First Extrasolar Planet Detected The planet around 51 Pegasi has a mass similar to Jupiter s, despite its small orbital distance.

72 How can the Doppler shift be used to search for planets around other stars? 1. Astronomers look for a periodic, red-then-blue shift in the spectrum of the planet 2. Astronomers look for a periodic, red-then-blue shift in the spectrum of the star 3. Astronomers look to see if the star wobbles in the sky, shifting slightly back and forth

73 How do astronomers look for planets whose orbits might cause them to pass in front of a star outside our solar system? 1. They look for a small black dot passing in front of the star 2. The look to see if the star s position shifts or wobbles slightly in the sky 3. The measure the star s brightness, and look for periodic dimming ( transits )

74 Suppose you found a star with the same mass as the Sun moving back and forth with a period of 16 months. What could you conclude? 1. It has a planet orbiting at less than 1 AU. 2. It has a planet orbiting at greater than 1 AU. 3. It has a planet orbiting at exactly 1 AU. 4. It has a planet, but we do not have enough information to know its orbital distance.

75 Transits and Eclipses A transit is when a planet crosses in front of a star. The resulting eclipse reduces the star s apparent brightness and tells us the planet s radius. When there is no orbital tilt, an accurate measurement of planet mass can be obtained. Planetary Transits

76 Direct Detection Special techniques for concentrating or eliminating bright starlight are enabling the direct detection of planets.

77 Measurable Properties Orbital period, distance, and shape Planet mass, size, and density Composition

78 Orbits of Extrasolar Planets Most of the detected planets have greater mass than Jupiter. Planets with smaller masses are harder to detect with the Doppler technique.

79 Planets: Common or Rare? One in ten stars examined so far have turned out to have planets. The others may still have smaller (Earthsized) planets that cannot be detected using current techniques.

80 Surprising Characteristics Some extrasolar planets have highly elliptical orbits. Some massive planets orbit very close to their stars: Hot Jupiters.

81 Hot Jupiters

82 Revisiting the Nebular Theory Nebular theory predicts that massive Jupiter-like planets should not form inside the frost line (at << 5 AU). The discovery of hot Jupiters has forced a reexamination of nebular theory. Planetary migration or gravitational encounters may explain hot Jupiters.

83 Planetary Migration A young planet s motion can create waves in a planetforming disk. Models show that matter in these waves can tug on a planet, causing its orbit to migrate inward.

84 Gravitational Encounters Close gravitational encounters between two massive planets can eject one planet while flinging the other into a highly elliptical orbit. Multiple close encounters with smaller planetesimals can also cause inward migration.

85 What happens in a gravitational encounter that allows a planet s orbit to move inward? 1. It transfers energy and angular momentum to another object. 2. The gravity of the other object forces the planet to move inward. 3. It gains mass from the other object, causing its gravitational pull to become stronger.

86 Modifying the Nebular Theory Observations of extrasolar planets have shown that the nebular theory was incomplete. Effects like planet migration and gravitational encounters might be more important than previously thought.