Harlow Shapley. Through Rugged Ways to the Stars

Transcription

1 A hypothesis or theory is clear, decisive, and positive, but it is believed by no one but the person who created it. Experimental findings, on the other hand, are messy, inexact things, which are believed by everyone except the person who did that work. Harlow Shapley Through Rugged Ways to the Stars

2 Earthwatch jun2009 Solar system dynamics Frank Timmes Outline The big picture Four challenges Solar Nebula theory & observations cococubed.asu.edu

3 Learning about the origin of the solar system is easiest if we begin by looking at our solar system with a big picture view, rather than focusing on individual worlds.

4 Imagine we are making the first scientific survey of our solar system. What patterns and exceptions would we see?

5 All planets orbit the Sun in the same direction: counterclockwise when viewed from above the Earth s North Pole.!+#) *+#,-!"#$%#& '"(%) 3"/,%(" 45%,6 2#+(%) 1+,%#(.%/0,"# All planetary orbits lie in nearly the same plane. *+#,- '"(%) 1%(!"#$%#&!+#) Almost all the planets travel on nearly circular orbits, with a spacing that increases with distance according to a fairly regular trend. 45%,6 3"/,%(" 1+,%#(.%/0,"# 2#+(%)

6 Most planets rotate in the same direction they orbit: counterclockwise when viewed from above the Earth s North Pole.!+#) *+#,-!"#$%#& '"(%) 3"/,%(" 45%,6 2#+(%) 1+,%#(.%/0,"# The Sun rotates in the same direction in which the planets orbit. *+#,- '"(%) 1%(!"#$%#&!+#) Almost all moons orbit their planet in the same direction as the planet s rotation and near the planet s equatorial plane. 3"/,%(" 1+,%#(.%/0,"# 2#+(%) 45%,6

7 Most planets have fairly small axis tilts, usually less than 25º. Challenge #1: Why are motions in the solar system generally so orderly?

8 Can we categorize the planets into groups? How many categories do we need?

9 Terrestrial Smaller size and mass Higher density (rocks, metals) Solid surface Closer to Sun (and closer together) Warmer Few if any moons and no rings Jovian Larger size and mass Lower density (light gases) No solid surface Closer to Sun (and closer together) Cooler Many moons and all have rings Challenge #2: Why do the inner & outer planets divide so neatly into two classes?

10 No description would be complete without the most numerous objects in the solar system: asteroids and comets. Asteroids are small rocky bodies that orbit the Sun between the orbits of Mars and Jupiter, primarily in the asteroid belt.

11 Their orbits generally lie close to the plane of the planetary orbits, although they are usually tilted a bit more. Some have quite large eccentricities. Some 10,000 asteroids are known; these are likely only the largest ones. The largest asteroids have a radius of about 200 km - much less than half of the Moon s radius.

12 Comets are small, icy bodies (about 10 9 of them) residing in either the Kuiper Belt and the Oort Cloud.

13 The Kuiper belt begins near Neptune (~30 AU) and extends out to about 100 AU. Kuiper belt comets have orbits that lie fairly close to the plane of the planetary orbits, and they orbit in the same direction as the planets.

14 The Oort cloud is a huge, spherical region centered on the Sun that extends about halfway to the nearest stars. Comets in the Oort cloud comets have orbits with random inclinations, orbital directions, and eccentricities.

15 Challenge #3: Why are there a large number of asteroids & comets in two different locations?

16 Some object patterns don t fit the general patterns: Mercury and Pluto have much larger eccentricities and inclinations. The rotational axes of Uranus and Pluto are substantially tilted. #$#%&'()'*+,- " " " " 34.6&-/0+1(2 "!!!! " "! 7()-&! +1&)+89* 7()-&" +1&)+89* "! " "! " "! #$$.&-/0+1(2!!!! :(*;*+(1 3456&'()'*+,- 5<

17 Venus rotates backwards - clockwise, rather than counterclockwise, as viewed from above Earth s North Pole. Earth has an exceptionally large moon. Pluto s moon is almost as big as Pluto. While most jovian moons orbit with the same orientation as the planet s rotation, a few orbit in the opposite direction.

18 Challenge #4: Why are there exceptions to the general patterns?

19 In the last 20 years, a lot of evidence has accumulated in support of a model called solar nebula theory. This model holds that our solar system formed from a giant swirling interstellar cloud of gas and dust. Nebula is the Latin word for cloud. Orion Nebula : an active star-forming region.

20 Star systems are born within interstellar clouds where the gas is somewhat denser than 1 atom/sugarcube. Typical star-forming clouds contain enough material to form millions of stars.

21 An individual star system forms from a small part of a giant interstellar cloud. We call the piece of cloud that formed our solar system the solar nebula. At its start, the solar nebula was probably a few light-years in diameter. It then began to collapse under its own gravity. As it collapsed to a diameter of about 200 AU, about twice the diameter of Pluto s orbit, three processes gave form to our solar system: Heating Spinning, Colliding

22 As the cloud shrank, its gravitational energy was converted into the energy of motion of gas particles falling inwards. These particles crashed into one another, converting energy of motion into the random motions of thermal energy. Heating is energy conservation in action.

23 The solar nebula became hottest at the center, where much of the mass collected to form the protosun. The protosun eventually became hot enough that fusion ignited at its core - at which point the protosun became a full-fledged star.

24 Spinning represents conservation of angular momentum. mass distance speed = constant Like an ice skater pulling in her arms, the solar nebula rotated faster and faster as it shrank in radius. The spinning helped ensure that not all of the material of the solar nebula collapsed onto the protosun. The greater the angular momentum of a rotating cloud, the more spread out it will be.

25 Flattening is a natural consequence of collisions, which is why disks are common in the universe.!"#$%$&'()* A cloud may start with any size or shape, and different clumps may be moving in random directions with random speeds. +,-'./0$12--'('23($45$607/-%6$26,'8( As the cloud collapses these clumps collide and merge, giving the new clumps the average of the old speeds. 90%&423$12--'('23($45$(:%--06$2,;018

26 Thus, random motions in the cloud become more orderly as the cloud collapses, changing the cloud s original lumpy shape into a rotating, flattened disk. Collisions also reduce their eccentricities, making the orbits more circular.

27 Flattening of the disk explains why all the planets orbit in nearly the same plane. Spinning explains why planets orbit in the same direction, and also plays a role in making most of the planets rotate in the same direction. Colliding explains why most planets have nearly circular orbits. Heating, spinning, flattening, and colliding explain the tidy layout of our solar system, thus answering why the orderliness of Challenge #1.

28 Evidence of nebular collapse Four dusty protoplanetary disks around young stars in the Orion nebula. The red glow in the center of each disk is a newly formed star, roughly a million years old. Each image is a composite of emission lines from ionized oxygen (blue), hydrogen (green), and nitrogen (red).

29 Churning and mixing of the gas in the solar nebula ensured that its composition was about the same everywhere: 98% hydrogen and helium, and 2% heavier elements. How did the planets end up with such a wide variety of compositions when they came from uniform material?

30 The formation of a solid or liquid particles from a gas is called condensation. Pressures in the solar nebula were so low that liquid droplets rarely formed, but solid particles could condense like snowflakes condense from water vapor in our atmosphere. Such solid particles are called condensates. snowflake sublimating

31 The ingredients of the solar nebula condense at different temperatures: Category Ingredients Condensation Temperature Amount in Nebula Metals Rocks Iron, nickel, aluminum Silicon-based minerals 1600 K 0.2% K 0.4% Hydrogen compounds CH 4, NH 3, H 2 O 150 K 1.4% Light gases Hydrogen, helium Never condensed 98.0%

32 The temperature differences between the hot inner regions and the cool outer regions determined what kinds of condensates were available to form planets. Near Mercury s orbit, metal started to condense. Moving outwards to Venus and Earth, more rock condensed. Only beyond the frost line, which lay between the present orbits of Mars and Jupiter, were temperatures low enough for hydrogen compounds to condense. 3%*/-)0(#)+'102-)*%(#'(-'. 4"#$%&'()*%+,%5(#-)-10")60,%$78'#!"#$%&'()*%+,%(#-.)$%*/-. 0(#)+'102-)*%(#'(-' 9$%-1):7('

33 So, the outer solar system contained condensates of all kinds. Since ice was nearly three times more abundant, ice flakes dominated the mixture. 88EE 8EEE!""#$%&'()& IHEE B'C4'1%+)1'#?DA IGEE I<EE I8EE IEEE HEE GEE *'+%"#(,-.'& /'#%0.#0-#$1%-0& 2-"-3%+'& /'".&4%1& /'2 <EE 8EE E 5%16(0%3'()&#$1%-0& :7 ; 57 < =3'& *'13)1K L'0)& M%1+N *%1& O)4-+'1 P")+( :'4+)0' EF8 EF< EFG EFH IFE 8 < H IE 8E ;E <E JE 2("%1#>-&+%03'#?!@A

34 The composition of objects should gradually change with distance from the Sun. Objects closest to the Sun should be rich in metals, and the most distant should be rich in ice. Shiny flakes of metal are clearly visible in this meteorite; just what we would expect if condensation really did occur in the solar nebula.

35 The process of growing by colliding and sticking is called accretion. At first, the condensed flakes were far too small to attract one another by gravity, but they were able to stick together by electromagnetic forces and slowly grow. As the clumps grew in mass, gravity began to aid the process of sticking together, accelerating their growth into planetesimals.!"#$%&'()' *#&+,-./0&!"#$%&%'()#"'

36 Small planetesimals probably came in a variety of shapes, still reflected in many small asteroids today. Planetesimals that reached about 200 km across started to become spherical due to the force of gravity pulling everything towards the center.

37 The sizes and compositions of the planetesimals depended on the temperature of the surrounding solar nebula. In the inner solar system only rocky and metal flakes condensed. This is why the terrestrial planets ended up composed of rocks and metals.

38 Since rocks and metals only made up 0.6% of the material in the solar nebula, the planetesimals in the inner solar system could not grow very large, which explains why the terrestrial planets are relatively small.

39 Beyond the frost line planetesimals could also be built from ice flakes. Since ice flakes was more abundant, these planetesimals could grow much larger. These large icy planetesimals became the cores of the jovian planets.

40 The gravity from these larger cores was strong enough to capture the far more abundant hydrogen and helium gas from the surrounding nebula. Accretion created a miniature solar system around each jovian planet. The spinning disks of the jovian nebulae explains why most of the jovian moons orbit in nearly circular paths lying close to the equatorial plane of their parent planet and why they orbit in the same direction as the planet rotates.

41 Condensation, accretion and nebular capture meet Challenge #2 for our solar system formation theory: It can explain the general differences between the terrestrial and jovian planets.

42 After about 10 million years, there were hundreds of protoplanets in the inner solar system and a few large ones in the outer solar system. About this time our protosun ignited hydrogen in its core to become a real star. Young stars tend to blow strong winds - a flow of hot hydrogen and helium gas ejected in all directions.

43 That wind is much weaker today, but still with us. The strong wind from the young Sun blew away the excess gas, but many planetesimals remained scattered between the newly formed planets. These leftovers became asteroids and comets. Rock and metal inside the frost line, icy ones outside the frost line.

44 Today, most of the rocky leftover planetesimals are concentrated in the extra-wide gap between Mars and Jupiter that contains the asteroid belt. Jupiter s large gravitational influence made the asteroid belt a zone of frustrated planet formation.

45 The icy leftover planetesimals that cruised the space between Jupiter and Neptune couldn t grow more than a few km before being perturbed by the gravity of jovians. Those that avoided being swallowed by the jovian planets were flung off at high speeds in random directions into the far reaches of the solar system. These are the comets of the spherical Oort cloud.

46 Beyond the orbit of Neptune, the icy planetesimals were much less likely to be swallowed or cast off by gravitational encounters. Instead they remain in orbits going in the same direction as planetary orbits. They were able to continue accreting, and many have grown to 1000s of km. These are the comets of the Kuiper belt. Pluto is probably one of the largest member of this class. Quaoar (2002 in the Kuiper Belt) is the name of the creation force of the Tongva tribe who were the original inhabitants of the Los Angeles basin, where the Caltech campus is located. Chad Trujillo & Michael Brown.

47 The solar nebula theory has thus met Challenge #3: explaining why there are numerous asteroids and comets, why they have different compositions, and why comets reside in two large reservoirs. May 1996, Hyakutake

48 After the young Sun cleared out most of the leftover gas from the solar nebula, a period of consolidation took place - the era of bombardment. This era of collisions between planetesimals and protoplanets would have resembled a rain of rock and ice. <-#$-0$%2&*2")=*$-#!"#$%&'%()"#$)%*"+,-.%,*/"(#0 A$)5%4,.?#%2&*2")=*$-# : ; 1,*$%2$'&)$%/)$0$-#%32,44,&-0%&'%5$")06

49 These plots show the results of a simulation of the formation of the inner planets. a) The simulation begins with 100 planetesimals. b) After 30 million years, the planetesimals have coalesced into 22 protoplanets. c) After 150 million years, the inner planets are essentially complete.

50 Impacts were extremely common in the young solar system. They are a normal part of the accretion process during the late stages of planet formation. This process has slowed down considerable, but still continues today.

51 Random, giant impacts during the era of bombardment are the most promising explanation for meeting the why the exceptions? of Challenge #4. Mercury and Pluto have larger eccentricities and inclinations. The rotational axes of Uranus and Pluto are substantially tilted. Venus rotates backwards. Earth has an exceptionally large moon. Unfortunately, this idea is hard to prove. But no other idea so effectively explains the oddities we ve discussed.

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53 On Valentine's Day 1990, cruising four billion miles from the Sun, Voyager 1 looked back to make this first ever family portrait of our Solar System. The portrait is a 60 frame mosaic imaged from 32 degrees above the ecliptic plane. Unseen are Mercury, too close to the Sun to be detected; Mars, hidden by sunlight scattered in the camera's optical system; and Pluto, whose position was not covered.