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Our Solar System and Its Origin

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1 Our Solar System and Its Origin

2 6.1 A Brief Tour of the Solar System
Our Goals for Learning • What does the solar system look like?

3 What does the solar system look like?

4 as a whole… mercury mars earth venus saturn jupiter uranus neptun pluto The planets are tiny compared to the distances between them (a million times smaller than shown here). The distances between inner planets are much smaller than the distances among outer once.

5 Recall scale of solar system
Before embarking on the tour of the planets, you might wish to review the overall scale from ch. 1. Inner planets are much smaller than the outer planets.

6 Let’s take a planetary tour:

7 Sun Over 99.9% of solar system’s mass
Made mostly (98%) of H/He gas (plasma) Converts 4 million tons of mass into energy each second These slides follow the planetary tour pages in ch. 6. The surface is speckled with sun spots (slightly cooler) than surroundings. Solar storms sometimes send streamers of hot gas high above the surface. In the core it is a nuclear fusion power plant. Despite the fast conversion of H to He, it will continue to shine for about 5 billion years more.

8 Mercury The smallest planet.
Made of metal and rock. Made mostly of Iron, the most metal rich of planets. Has no atmosphere: no scatter of sun light (you could see stars in daytime), or to help retain heat. It is very hot and very cold: 425°C (day), –170°C (night) It is desolate, cratered; with long, tall, steep cliffs Nights about 3 months long. Visited just by one spacecraft, which determined its composition.

9 Venus nearly identical in size to Earth;
thick atmosphere, its pressure equivalent to pressure 1 km deep in oceans surface hidden by thick clouds; observed only recently, before thought of as a paradise hellish conditions due to an extreme greenhouse effect: even hotter than Mercury: 470°C, both day and night (the heat is trapped) has no oxygen, no water, … it rotates very slowly, in the opposite direction w.r.t. other planets it has mountain, valleys and craters, but the geology of its surface is very different from Earth’s perhaps more than any other planet, makes us ask: how did it end up so different from Earth?

10 Earth Earth and Moon to scale
An oasis of life: the only planet with oxygen and ozone (thanks to plants!) and water. Water vapor and carbon monoxide maintain a moderate greenhouse effect – no harsh temperatures. The only surface liquid water in the solar system; about 3/4 of surface covered by water A surprisingly large moon, how was it acquired was long a mystery

11 Mars Smaller than Venus and Earth (half of Earth’s diameter, 10% of Earth’s mass). Giant extinct volcanoes, a huge canyon (runs 1/5th of the way around planet), polar caps (of frozen CO2 and water), dried up river beds… Looks almost Earth-like, but don’t go without a spacesuit! Thin atmosphere (low air pressure), not enough oxygen to breath, no ozone temperature below freezing, … Two tiny moons (much like typical asteroids) Water flowed in the distant past; could there have been life?

12 Jupiter Much farther from Sun than inner 4 planets, more than twice Mars distance (Asteroid belt in between). Gigantic for a planet: 300  Earth mass; >1,000  Earth volume. Also very different in composition: mostly H/He; no solid surface. at least 60 moons, rings… Long lived storm, the size of 2, 3 Earths.

13 Moons can be as interesting as the planets themselves, especially Jupiter’s 4 large “Galilean moons” (first seen by Galileo) Io (shown here): active volcanoes all over Europa: has an icy crust that might hide subsurface ocean. Life? Ganymede: largest moon in solar system — larger than Mercury Callisto: a large, cratered “ice ball” with unexplained surface features

14 Saturn Giant and gaseous like Jupiter
most spectacular rings of the 4 jovian planets many moons, at least 31, including cloud-covered Titan currently under study by the Cassini spacecraft

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

16 Cassini probe arrived July 2004
Saturn Cassini probe arrived July 2004 (Launched in 1997) Inset art shows the Huygens probe separated from the main spacecraft on its descent to Titan… This is Huygens probe, separated from Cassini, in order to explore Titan. Interesting Project 1 topic: what did we learn from its visit?

17 Uranus much smaller than Jupiter/Saturn, but still much larger than Earth made of H/He gas and hydrogen compounds (H2O, NH3, methane CH4 gives it pale blue color) extreme axis tilt — nearly tipped on its “side” — makes extreme seasons during its 84-year orbit. moons and rings also tipped in their orbits… Visited only once by Voyager 2. No missions to Uranus planned.

18 Looks nearly like a twin to Uranus (but much smaller axis tilt)
Neptune Looks nearly like a twin to Uranus (but much smaller axis tilt) It has many moons, including unusual Triton: orbits “backward”; larger than Pluto. The explanation of this effect has given us a new insight into the history of outer solar system. Note: this is the same image as in the text but rotated 90° to show the axis tilt relative to the ecliptic plane as the horizontal on the page. (The orientation in the book chosen for its more dramatic effect.)

19 Pluto: hint of why it is not a planet.
A “misfit” among the planets: far from Sun like large jovian planets, but much smaller than any terrestrial planet. Comet-like composition (ices, rock) and orbit (eccentric, inclined to ecliptic plane, long years). Sun looks like a little light among other stars from farther part of his orbit. Its moon Charon is half Pluto’s size in diameter Best current photo above; New Horizons mission launch 2006, arrival 2015…

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21 This important summary table may be worth some time in class to make sure students understand how to read it…

22 What have we learned? • What does the solar system look like?
Our solar system consists of the Sun, nine planets and their moons, and vast numbers of asteroids and comets. Each world has its own unique character, but there are many clear patterns among the worlds.

23 6.2 Clues to the Formation of Our Solar System
Our Goals for Learning • What features of our solar system provide clues to how it formed? • What theory best explains the features of our solar system?

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

25 First feature: The Sun, planets, and large moons orbit and rotate in an organized way:
Almost all orbits are circular, except for Mercury, (we will not talk about Pluto anymore), and lie nearly in the same plane Most planets rotate in the same direction in which they orbit The same holds for the moons. And for the Sun.

26 Second: Four terrestrial planets are small, rocky, and close to the Sun. Four jovian planets are large, gas-rich, and far from the Sun.

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28 Third major feature: Rocky asteroids: mostly in asteroid belt between Mars & Jupiter. Much smaller than our Moon. Comets: made largely of ices (water, ammonia and methane). The vast majority of them never visits inner solar system. There are two major groupings of comets: Kuiper belt, beyond Neptunes orbit (orbit in the same plane and same direction as planets) Oourt cloud: much farther from the sun. Shows no simple pattern.

29 Fourth feature: A successful theory of solar system formation must allow for exceptions to general rules Venus rotates backwards! Many small moons have unusual orbits.

30 Summary: Four Major Features of our Solar System

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

32 According to the nebular theory our solar system formed from a giant cloud of interstellar gas.
(nebula: latin for “cloud”) This is Orion Nebula, a star forming cloud. Thousands of stars are yet to be born in this cloud. Suggested in 18th century by Imanuel Kant, and independently by Pierre Simon Laplace. Today, it is accepted theory, in more developed form than the original proposal.

33 What have we learned? • What features of our solar system provide clues to how it formed? Four major features provide clues: (1) The Sun, planets, and large moons generally rotate and orbit in a very organized way. (2) With the exception of Pluto, the planets divide clearly into two groups: terrestrial and jovian. (3) The solar system contains huge numbers of asteroids and comets. (4) There are some notable exceptions to these general patterns. • What theory best explains the features of our solar system? The nebular theory, which holds that the solar system formed from the gravitational collapse of a great cloud of gas.

34 6.3 The Birth of the Solar System
Our Goals for Learning • Where did the solar system come from? • What caused the orderly patterns of motion in our solar system?

35 Where did the solar system come from?

36 The cloud of gas that gave birth to our solar system resulted from the recycling of gas through many generations of stars within our galaxy.

37 Even though this process has been going on since the first stars formed, only about 2% of H and He has been converted to heavier elements before the Solar system was formed. Sun still has that composition… Terrestrial planets are then formed of this 2% of heavier elements, and the Jovian planets from the rest. Evidence for nebular theory from other forming stars: Stars that appear in a process of formation are always found within interstellar clouds.

38 What caused the orderly patterns of motion in our solar system?
In three steps…

39 Four Unexplained Features of our Solar System
--> 1) Why do large bodies in our solar system have orderly motions? 2) Why are there two types of planets? 3) Where did the comets and asteroids come from? 4) How can we explain the exceptions the the ‘rules’ above?

40 1) As gravity forced the cloud to become smaller, it began to heat up
Question: explain why. What types of energy are involved?

41 1) As gravity forced the cloud to become smaller, it began to heat up
Question: explain why. What types of energy are involved? Conservation of energy

42 2) As gravity causes cloud to shrink, its spin increases
Question: explain in more detail why the spin increases.

43 2) As gravity causes cloud to shrink, its spin increases
Question: explain in more detail why the spin increases. Conservation of angular momentum Collapse_of_solar_nebula.swf

44 3) Collisions flatten the cloud into a disk.
And 3) Collisions flatten the cloud into a disk. If particles are moving randomly but there is preferred direction of motion (cloud spinning in one direction) in collisions they will be scattered in such a way to be pushed to do the same spinning. Any upward or downward will also be suppressed (only rotation, in a disk prevails) More over, these collisions tend to make more circular orbits out of eccentric ones.

45 Collisions between gas particles in cloud gradually reduce random motions

46 Collisions between gas particles also reduce up and down motions

47 Spinning cloud flattens as it shrinks

48 Summary The planets all orbit the Sun in the same plane because they formed in the flat disk. The direction in which the disk was spinning became the direction of the Sun’s rotation and the orbits of planets. This was also preferred direction for spinning of planets. Also, collisions caused nearly circular orbits.

49 Evidence supporting this model:
The formation of a disk in which planets can form is a natural part of star formation process. We see plenty of evidence for spinning disks of gas and dust around other stars, especially newly formed stars Material orbiting a star

50 What have we learned? • Where did the solar system come from?
The cloud of gas that gave birth to our solar system was the product of recycling of gas through many generations of stars within our galaxy. This gas consisted of 98% hydrogen and helium and 2% everything else combined.

51 What have we learned? • What caused the orderly patterns of motion in our solar system? A collapsing gas cloud naturally tends to heat up, spin faster, and flatten out as it shrinks in size. Thus, our solar system began as a spinning disk of gas. The orderly motions we observe today all came from the orderly motion of this spinning disk of gas.

52 6.4 The Formation of Planets
Our Goals for Learning • Why are there two types of planets? • Where did asteroids and comets come from? • How do we explain the existence of our Moon and other “exceptions to the rules”? • When did the planets form?

53 Four Unexplained Features of our Solar System
√ Why do large bodies in our solar system have orderly motions? --> 2) Why are there two types of planets? 3) Where did the comets and asteroids come from? 4) How can we explain the exceptions the the ‘rules’ above?

54 Why are there two types of planet, when all planets formed from the same nebula?
Planet formation required the presence of seeds – solids bits of matter around which gravity will eventually built planets – much like formation of snowflakes. Based on the condensation (formation of solid particles) of gas.

55 Different material condense at different temperatures – the reason for two types of planets.
Hydrogen compounds could condense only beyond the frost line.

56 Inner parts of disk are hotter than outer parts.
Question: Explain why. Rock can be solid at much higher temperatures than ice.

57 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.

58 Question: Consider a region of solar nebula in which the temperature was about 1,300 K. What fraction of the material in this region was gaseous? How about the region with 100 K. Which of the regions is closer to the Sun?

59 Primary force at the beginning was electrostatic force.
Once we had primary seeds, they started to “glue” together after gentle collisions they experienced. Question: why collisions were “gentle”? How were particles in nebula moving when planets started to form? Primary force at the beginning was electrostatic force. Later, when they grew bigger gravity became important.

60 Tiny solid particles stick to form planetesimals.
Gravity draws planetesimals together to form planets. Collisions are at first violent and happen often. This process of assembly is called accretion Same as previous

61 Gravity of rock and ice in jovian planets draws in H and He gases which formed around icy cores.

62 Moons of jovian planets form in miniature disks, much like the planets formed around the Sun.

63 Clearing the Nebula: After planets were formed (lucky coincidence with Sun’s activity), solar wind blew away the leftover gases.

64 Question: what would happen if the solar wind happened before the planets were formed? What would happen if it didn’t happen at all?

65 Question: what would happen if the solar wind happened before the planets were formed? What would happen if it didn’t happen at all? If the solar wind was never strong enough to blow away particles, H and He would condense in the inner solar system after it cooled down enough, and accumulated on the terrestrial planets changing them significantly.

66 Why are there two types of planets?
Outer planets get bigger because abundant hydrogen compounds condense to form ICES. Outer planets accrete and keep H & He gas because they’re bigger.

67 Four Unexplained Features of our Solar System
√ Why do large bodies in our solar system have orderly motions? √ Why are there two types of planets? --> 3) Where did the comets and asteroids come from? 4) How can we explain the exceptions the the ‘rules’ above?

68 Comets and asteroids are leftover planetesimals.
• Asteroids are rocky because they formed inside the frostline. Most of them ended up in the asteroid belt region between Mars and Jupiter. • Comets are icy because they formed outside the frostline. We find comets mostly outside the Neptuns orbit, in Kuiper belt, or in Oort cloud. Simulations have shown that they fit well in nebular theory.

69 Four Unexplained Features of our Solar System
√ Why do large bodies in our solar system have orderly motions? √ Why are there two types of planets? √ Where did the comets and asteroids come from? --> 4) How do we explain the existence of our Moon and other “exceptions to the rules”?

70 Probably due to collisions with leftover planetesimals
Probably due to collisions with leftover planetesimals. Heavy bombardment period occurred just after the planets were formed: Earth’s moon was probably created when a big planetesimal slammed into the newly forming Earth, tilting its axes and forming the Moon from outer layers of earth. In fact water was probably brought to rocky earth in one of those encounters. Question: were did planetesimals carrying water must have came from? Other large impacts may be responsible for other exceptions like rotation of Venus and Uranus

71 Review of nebular theory

72 Four Features of our Solar System - Explained
√ Why do large bodies in our solar system have orderly motions? √ Why are there two types of planets? √ Where did the comets and asteroids come from? √ How do we explain the existence of our Moon and other “exceptions to the rules”? Add ‘what have we learned’ slide here

73 When did the planets form?

74 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

75 The decay of radioactive elements into other elements is a key tool in finding the ages of rocks
Question: Let’s say there was 1 mg of potassium in the rock, to start with. After 1.25 billion years, how many mg of potassium and argon-40 would we have? And after next 1.25 billion years? Question: if we find a rock with equal amounts of potassium and argon, what could we conclude? What assumption we have to make about the amount of argon present initially in the rock if we want to use this method?

76 The decay of radioactive elements into other elements is a key tool in finding the ages of rocks
Question: Let’s say there was 1 mg of potassium in the rock, to start with. After 1.25 billion years, how many mg of potassium and argon-40 would we have? And after next 1.25 billion years? Question: if we find a rock with equal amounts of potassium and argon, what could we conclude? What assumption we have to make about the amount of argon present initially in the rock if we want to use this method? Turns out that the assumption that there was no Argon initially present works fine. Argon is a gas that never combines with other elements and didn’t condense in solar nebula.

77 Age dating of meteorites that are unchanged since they condensed and accreted tell us that the solar system is about 4.6 billion years old.

78 What have we learned? • Why are there two types of planets?
Planets formed around solid “seeds” that condensed from gas and then grew through accretion. In the inner solar system, temperatures were so high that only metal and rock could condense. In the outer solar system, cold temperatures allowed more abundant ices to condense along with metal and rock.

79 What have we learned? • How do we explain the existence of our Moon and other “exceptions to the rules”? Most of the exceptions probably arose from collisions or close encounters with leftover planetesimals, especially during the heavy bombardment that occurred early in the solar system’s history. Our Moon is probably the result of a giant impact between a Mars-size planetesimal and the young Earth. • Where did asteroids and comets come from? Asteroids are the rocky leftover planetesimals of the inner solar system, and comets are the icy leftover planetesimals of the outer solar system.

80 What have we learned? • When did the planets form?
The planets began to accrete in the solar nebula about 4.6 billion years ago, a fact we determine from radiometric dating of the oldest meteorites.

81 6.5 Other Planetary Systems
Our Goals for Learning • How do we detect planets around other stars? • What have other planetary systems taught us about our own?

82 How do we detect planets around other stars?

83 We detect planets around other stars by looking for a periodic motion of the stars they orbit.
We measure the motion through the Doppler shift of the star’s spectrum

84 The size of the wobble tells us the planet’s mass
The period of the wobble tells us the radius of its orbit (Kepler’s 3rd law)

85 We can also detect planets if they eclipse their star
Fraction of starlight blocked tells us planet’s size

86 What have other planetary systems taught us about our own?

87 Over 120 known extrasolar planets as of 2004
Most are more massive than Jupiter and closer to their star than Earth is to Sun Revisions to the nebular theory are necessary! Planets can apparently migrate inward from their birthplaces. Question: Does our theory of solar system formation qualify as a scientific theory, even though we have learned it needs modification? Does this mean the theory was wrong?

88 Is Earth Unusual? No Earth-like planets found yet.
Data aren’t good enough to tell if they are common or rare Available methods can only detect BIG planets.

89 What have we learned? • How do we detect planets around other stars?
So far, we are only able to detect extrasolar planets indirectly by observing the planet’s effects on the star it orbits. Most discoveries to date have been made with the Doppler technique, in which Doppler shifts reveal the gravitational tug of a planet (or more than one planet) on a star.

90 What have we learned? • What have other planetary systems taught us about our own? Planetary systems exhibit a surprising range of layouts, suggesting that jovian planets sometimes migrate inward from where they are born. This lesson has taught us that despite the successes of the nebular theory, it remains incomplete.


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