Dawn of Small Worlds: Dwarf Planets, Asteroids, Comets

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This book gives a detailed introduction to the thousands and thousands of smaller bodies in the solar system. Written for interested laymen, amateur astronomers and students it describes the nature and origin of asteroids, dwarf planets and comets, and gives detailed information about their role in the solar system. The author nicely reviews the history of small-world-exploration and describes past, current and future space craft missions studying small worlds, and presents their results. Readers will learn that small solar system worlds have a dramatically different nature and appearance than the planets. Even though research activity on small worlds has increased in the recent past many of their properties are still in the dark and need further research.

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Michael Moltenbrey Dawn of Small Worlds Dwarf Planets Asteroids Comets

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Astronomers’ Universe

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More information about this series at http://www.springer.com/series/6960

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Michael Moltenbrey Dawn of Small Worlds Dwarf Planets Asteroids Comets

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Michael Moltenbrey Munich Germany ISSN 1614-659X ISSN 2197-6651 electronic Astronomers’ Universe ISBN 978-3-319-23002-3 ISBN 978-3-319-23003-0 eBook DOI 10.1007/978-3-319-23003-0 Library of Congress Control Number: 2015954813 Springer Cham Heidelberg New York Dordrecht London © Springer International Publishing Switzerland 2016 This work is subject to copyright. All rights are reserved by the Publisher whether the whole or part of the material is concerned specifically the rights of translation reprinting reuse of illustrations recitation broadcasting reproduction on microfilms or in any other physical way and transmission or information storage and retrieval electronic adaptation computer software or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names registered names trademarks service marks etc. in this publication does not imply even in the absence of a specific statement that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty express or implied with respect to the material contained herein or for any errors or omissions that may have been made. Cover illustration: Artist’s concept of an asteroid belt around Vega. Credit: NASA/JPL-Caltech Printed on acid-free paper Springer International Publishing AG Switzerland is part of Springer Science+Business Media www.springer.com

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Contents 1 Introduction........................................... 1 2 Asteroids.............................................. 17 3 Comets............................................... 73 4 Trans-Neptunian Objects................................. 141 5 Dwarf Planets.......................................... 175 6 Exploration of Small Solar System Bodies.................... 215 Further Reading........................................... 269 Index................................................... 271 v

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Chapter 1 Introduction Almost since the beginning of time human beings have been fascinated by the starry sky above their heads. What were these little points of light up there Not havingnamesforthemyetpeoplewonderedaboutthemoonandtheSun.Evidence for this fascination traces well back into the earliest stages of civilization as e.g.CavepaintingsfromtheStoneAgesuggest.ThepaintingsofLascauxinFrance are avery famous examplefor this.Theywere createdbetween17000 and15000 BC and epic the Pleiades the zodiac and probably also the summer sky. The observation of the starry sky had been limited to the naked eye for many thousands of years. The people identified the Sun the Moon and some of the planets Mercury to Saturn. The outermost gas giants Uranus and Neptune remained concealed to them. From time to time an omen appeared in the sky—a cometTheviewontheworldoutthereonlydramaticallychangedwiththeadvent of optical telescopes. When Galileo Galilei 1564–1642 pointed his scope at Jupiter he was one of the first to realize that there was more. It was probably the first time that a human being was able to see what a planet actually is. Before this eventtheywere oftenmerely consideredtobewhattheir namesuggests:Wander- ing stars. Their visual appearance was not different to any ofthe other stars except fortheirmovements.YesTheyactuallymovedTheyappearedtomoverelativeto the background stars. What were they Galilei was surprised to see a disk with structures on it when looking at the planet.Thiswassomethinghehadnotexpected.Yethesawevenmore.Therewere four bright dots near Jupiter. They appeared tochange positions over the course of several observing sessions. He correctly interpreted them as moons of Jupiter. To honor their discoverer they are named the four Galilean Moons today. Itwasthefirsttimethatnewbodiesinoursolarsystemwerefound.Theplanets Uranus and Neptune followed. Nevertheless a general notion developed that our solar system merely consisted of the Sun the planets and their moons and a few comets see Fig. 1.1. Scientists in the eighteenth and nineteenth century however were wondering when looking at the distribution of the planets within the solar system. A planet © Springer International Publishing Switzerland 2016 M. Moltenbrey Dawn of Small Worlds Astronomers’ Universe DOI 10.1007/978-3-319-23003-0_1 1

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appeared to be missing. By then thanks to Johannes Kepler 1571–1630 and Sir IsaacNewton1642–1726ithadbeenpossibletodeterminetheorbitsofobjectsin our solar system. Yet there was an unexplainable gap between the orbits of Mars andJupiter.Abythenfamouslawtheso-calledTitius-Bodelawpostulatedthata planet should exist there. In 1800 Baron Franz Xaver von Zach 1754–1834 and Johann Hieronymus Schroeter1747–1816foundedtheprobablyfirstinternationalresearchprojectthe “Himmelspolizey” engl. the “sky police” while working at the observatory in Gotha/Germany.Thegroupdividedthestarryskyinto24distinctregionsandeach Fig. 1.1 Montage of planetary images taken by spacecraft managed by the Jet Propulsion Laboratory in Pasadena CA. Included are from top to bottom images of Mercury Venus Earth and Moon Mars Jupiter Saturn Uranus and Neptune credit: NASA/JPL 2 1 Introduction

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member located in different European countries was asked to systematically search his region for the missing planet. The “missing planet” was found by chance by the Italian astronomer and theologian Giuseppe Piazzi 1746–1826 who had not been a member of the “Himmelspolizey” during the night of New year’s day 1801 January 1 1801 while working at the observatory of Palermo/Italy. Soon after more and more planets were discovered in that region. All of them appeared to be small and faint. Even in the largest telescopes of that time it was not possible to see more than a faintstar-likedot.WerethesereallyplanetsTheyweresodifferentfromtheother planets that had already been known by that time. All of them showed details of their respective surfaces. Yet the new ones remained completely indistinct. Theastronomerscametotheconclusionthattheseobjectshadtobeanewgroup of solar system bodies. The German-British astronomer William Herschel 1738– 1822 coined the term “asteroid” meaning star-like for them. Soon after the discovered asteroids clearly outnumbered any of the other known solar system bodies.Todaymorethan600000asteroidsinthemainasteroidbeltbetweenMars and Jupiter are known. More exist at other locations in the solar system. The exploration of the solar system continued. It was found that another larger bodysomehowperturbedUranus’orbit.Mathematicalpredictionsweremadeofthe orbitofthepotentialnewplanet.Indeedaplanetwasfoundthere.Thediscoveryof Neptunein1846howeverdidnotsolvealltheproblemsbuteventriggeredanew search for another planet. Astronomers speculated that Uranus’s orbit was being disturbed by another planet besides Neptune. In 1930 a young astronomer Clyde Tombaugh1906–1997workingattheLowellObservatoryinFlagstaffArizona/ USA discovered Pluto which was then subsequently classified as the ninth planet inoursolarsystem.AnintensivedebatebeganaboutPlutoanditsstatusinoursolar system. Observations subsequent to its discovery revealed that Pluto was by far smaller than any of the other planets. It was not possible to resolve any planetary disc of Pluto even with the largest telescopes available. Could this tiny object be really responsible for the perturbations of Uranus’ and Neptune’s orbits During the course of the following decades Pluto’s size and mass was contin- uouslyreducedfrominitiallyaboutthemassofEarthdowntoapproximately1/500 of Earth’s mass. Was this a planet The debate continued and intensified after the discovery of other small bodies in the region beyond Neptune in the 1990s. The first to be discovered was a object called 1992 QB1 discovered by the American astronomers David Jewitt and Jane Luu in 1992. In the following years more and more of these objects were found in the trans-Neptunian region. Some of these objects were so large and similar to Pluto as to question its planetary status. Consequently in 2006 the International Astronomical Union IAU introduced the new class of dwarf planets and demoted Pluto into it. New populations of new types of objects popped up everywhere in the solar system. Up to then astronomers had only distinguished a few classes of solar system bodies: the Sun the planets the moons asteroids and comets. Yet some ofthenewlydiscoveredobjectssuchasthecentaurscouldnotbeclearlyclassified into any of these categories as they showed for example properties of comets and 1 Introduction 3

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asteroids together.Furthermoresomebodiesthathadbeen consideredasasteroids turned out to be extinct comets i.e. comets that do not exhibit any activity anymore. The number of newly discovered objects increased and with them the chaos in classification. In2006togetherwiththedefinitionofaplanettheIAUthereforedecidedtoput them together in a new group of objects the so-called Small Solar System Bodies SSSB. This group comprises all solar system bodies except the planets dwarf planetsandplanetarysatellitesormoons.ThesecurrentlyincludemostoftheSolar System asteroids most Trans-Neptunian Objects TNOs comets and other small bodies even including interstellar dust. Thisbookwillprovideanoverviewofthesesmallbodiesanddwarfplanetsthat are present in our solar system. We will cover asteroids comets and trans- Neptunian objects. In addition we will also deal with dwarf planets and their most prominent representative Pluto. 1.1 Orbits and Resonances: A Brief Introduction Before starting our journey to the small bodies in our solar system we need to introduce some terms and concepts that will be necessary for the further under- standing.Thosereaderswhoarealreadyfamiliarwiththese conceptscanskipthis section and directly go to the following dealing with the formation of our solar system. 1.1.1 What Are Orbits All the objects in our solar system move along orbits around the Sun. Figure 1.2 shows the basic structure of our solar system including the orbits of the planets Mercury Venus Earth Mars Jupiter Saturn Neptune. Further shown are the asteroid belt between the orbits of Mars and Jupiter the Kuiper belt beyond Neptune and the dwarf planet Pluto. The orbits of the planets are not exact circles but so-called ellipses. You can thinkofellipsesassomekindofcirclethathasbeenstretchedtotheoneortheother direction.Theextenttowhichtheyarestretchediscalledtheeccentricitye.Ifwedo notstretchthecircleatallwehavenoeccentricitye¼0themorewestretchthe more elongated the circle becomes. Eccentricity e is increasing in the latter case. Ellipses are an important concept we have to be familiar with. Most of the planets have near circular orbits. However this is not true for many of the small bodies. They often have highly elliptical orbits i.e. extremely elongated circles. Figure1.3depictsthebasicstructureofanellipse.Eachellipsehastofocuses.In thecaseofoursolarsystemtheSunisatoneofthefocuses.Thedistancebetween thefocusesandthecentergivesanindicationonhowmuchtheellipseisstretched. 4 1 Introduction

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Ifwehaveacirclei.e.aneccentricityof0thetwofocuseswillfalltogetheratthe center.Inparticulartwoparameterswillbeimportantforthefollowingchapters.On theonehandthisistheso-calledsemi-majoraxisawhichissizeoftheorbitandthe eccentricity e which defines how much the ellipse deviates from a perfect circle. Fig 1.2 Basic structure of our solar system including the planets the asteroid belt and the Kuiper belt 1.1 Orbits and Resonances: A Brief Introduction 5

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TheaveragedistancefromEarthtoSunisroughlyequivalenttothesemi-major axisofEarth’sorbit.Afurtherimportantcharacteristicofanorbitisitsinclinationi towards the ecliptic plane. So what is the ecliptic plane The orbit of Earth around theSundefinesaplanewhichistermedtheecliptic.Mostoftheplanetshavetheir orbits moreor lessalsoin thisplane.Thevalueby which they deviate is called the orbit’s inclination. 1.1.2 Orbital Resonances Another important concept for the following chapters are the so-called orbital resonances. An orbital resonance occurs when two orbiting bodies exert a regular periodic gravitational influence on each other. This is the case when their orbital periodsarerelatedbyaratiooftwointegers.Orbitalresonancesgreatlyenhancethe mutual gravitational influence of the bodies i.e. their ability to alter or constrain each other’s orbits. In most cases the results are unstable orbits due to orbital perturbation caused by the gravitational interaction of the two bodies. Under some circumstances a resonant system can be stable and self-correcting so that the bodies permanently remain in resonance. A prominent example of such stabiliz- ing resonances is the 2:3 resonance between Pluto and Neptune. In this resonance PlutowillmaketwoorbitsforeverythreeorbitsNeptunemakes.Wewillcoverthis resonance later on in more detail. Fig. 1.3 Basic structure of an ellipse with a semi-major axis a. The Sun is in one of the focuses. The letters P and A stand for perihelion and aphelion 6 1 Introduction

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Many destabilizing resonances can be found in the main asteroid belt between Mars and Jupiter. The asteroids there are temporarily locked in resonance with Jupiter. The gas giant with its enormous mass causes strong gravitational interac- tion with the asteroids. Their orbits are perturbed to a smaller or larger extent. Figure 1.4 depicts the possible effects of an orbital resonance. In situation 1 object B is slowed down asit is “pulled back” by the gravitational interaction with A.Insituation2BisacceleratedasitispushedforwardbytheinteractionwithA. 1.2 Formation of Our Solar System Before we continue our journey to the small solar system bodies we first need to havealookathowoursolarsystemformed.Thisiscrucialfortheunderstandingof howthesmallbodiesformedandwhytheyarethewaywecanobservethemtoday. Furthermore only with this background it is possible to understand their develop- ment composition and orbits. We will go through the basic principles of the formation of our solar system and give an overview on the processes that took place. Further details will be provided in the following chapters e.g. on the formation of the asteroid belt or the Kuiper belt. Today the most widely accepted model describing the formation of our solar system is the so-called nebular hypothesis. It was originally proposed by Pierre- Simon Laplace 1740–1827 Emanuel Swedenborg 1688–1772 and Immanuel Kant 1724–1804 in the eighteenth century. Since then the basic model has undergone several more or less severe refinements but is in its basic assumptions still considered to be valid. SohowdidoursolarsystemformAllbeganabout4.6billionyearsagowitha giant molecular cloud spanning across about 65 lightyears. Nowadays we know Fig. 1.4 An example on the effects of orbital resonances of two objects 1.2 Formation of Our Solar System 7

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that many such clouds exist in the Universe and we are able to observe the star formation there as well as the formation of planetary systems see Fig. 1.5. Prominent examples of such star forming regions are the Orion nebula the Trifid nebula and the Eagle nebula. These giant molecular clouds mainly consist of hydrogen and helium. Over a period of several millions of years these clouds tend to collapse and fragment. In the case of the cloud from which our solar system formed each of the fragments had a size of about 3.25 lightyears. Of course you may ask what causes the fragmentation The cloud is floating in the galaxy why should it change Various processes are conceivable. Just imagine a nearby star is exploding in form of a supernova. The shock waves originating from the supernova will prop- agate through the cloud and may cause local instabilities likes waves in the ocean. Some parts may become denserthanothersand thereby theseed for fragmentation issowed.OneofthesecollapsingfragmentsformedwhatbecametheSolarSystem. The masses of such protostellar nebulae can range from merely fractions of the mass of our Sun to several times the mass of our central star. The process of collapsing will last about 100000 years for a nebula with having the approximate mass of our Sun. Fig. 1.5 An overview of protoplanetary discs that were discovered in the Orion Nebula credit: NASA/ESA and L. Ricci ESO 8 1 Introduction

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Thenebulafurthercollapsed.Ithadacertainamountofangularmomentumand because of the conservation of exactly this momentum the nebula spun faster as it collapsed.Asthematerialwithinthenebulacondensedtheatomswithinitbeganto collide with increasing frequency converting their kinetic energy into heat. At the center of the nebula the concentration of molecules and atoms was the highest within the whole nebula. The frequency of collisions was also higher which eventually caused the center to become hotter than the surrounding disc. Over a period of about 100000 years the competing forces of gravity gas pressure magnetic fields and rotation caused the protostellar nebula to flatten into a protoplanetary disc. This disk extended up to about 200 AU. Its center formed a hot youngprotostar. Aprotostar is pre-stage toanormalstar. Howeverthe fusion of hydrogen has not yet begun in a protostar. Such protoplanetary discs have already been observed in distant regions e.g. in the Orion nebula see Fig. 1.6. The rotating protoplanetary disc further provided material to the protostar. Hence the contraction further continued. The protostar gained more mass and the temperatureandpressureinitcontinuouslyincreased.Afterabout50millionyears aturningpointwasreached.Temperatureandpressattheprotostarhadbecomeso high as to trigger nuclear fusion. A star is born our Sun was born. The nuclear fusion process created an internal heat source. The heat increased the pressure within the young star that countered gravitational contraction until hydrostaticequilibriumwasachievedi.e.untiltheoutwardsdirectedpressureand the inwards directed gravitation had the same strength. Our Sun became stable. Fig. 1.6 One of protoplanetary discs discovered in the Orion Nebula credit: NASA/ESA and L. Ricci ESO 1.2 Formation of Our Solar System 9

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1.2.1 Formation of the Planets The protoplanetary disc around our Sun yet remained and comprised the remnants of the solar nebula in the form of gas mainly hydrogen and helium and dust various other heavier molecules such as silicates. It is important to see that the disc contained both types of material. Otherwise a formation of a solar system as ours is not possible. While the protoplanetary disc continued to rotate around the Sun and delivered further material to it in an accretion process the disc also started to differentiate. Thedustbegantosettlethroughthegastothecentralplaneofthediscandformeda thin disc of dust there. Then the disc further differentiated into distinct regions owing the different temperatures present in various parts of the disc. The inner region close to the Sun ranging up to about 4 AU was too warm for volatile molecules like water and methanetocondenseandremainsolid. Theysimplyevaporated.Howeverheavier molecules with a higher melting point such as metals iron nickel aluminum and rocky silicates remained there and condensed. A bit farther out also carbonaceous compounds which have a slightly lower melting point compared to metals remained present in solid form. There was however a problem. All these particles that remained in the inner region are not very common inthe Universe. Their abundances are much lower than for volatiles such as methane and foremost hydrogen and helium. Hence the terrestrialplanets which should be born from them were limited in their growth. When we leave the inner region behind us we cross the so-called frost line which lies in the region of the current asteroid belt between the orbits of Mars and Jupiter. Beyond this line temperatures are low enough to allow the presence of water ice in solid form. The farther we leave the Sun behind us the lower the ambient temperatures become and further even more volatile molecules such as methane can remain frozen. From Dust Grains to Terrestrial Planets The terrestrial planets essentially formed in the inner region of the protoplanetary disc. The dust grains in orbit around the Sun in this inner region agglomerated trough direct contact to millimeter-sized objects. The particles were so small that the gravitational effects among them were negligible. Through further direct contactthesetinyseedsclumpedtogetheruptoseveralhundredmetersindiameter. This process continued until these objects had accreted enough material to form objects of about 10 km in size the so-called planetesimals. Thiswasthepointoftimewhengravityprevailed.Theplanetesimalsinfluenced each other gravitationally and collisions occurred. These collisions of course caused disruption but also could lead to further accretion. In this phase in total the accretion dominated the disruption by collisions. The planetesimals grew until only a few of them had survived and had formed planetary embryos of about 0.05 Earth masses. Subsequent mergers and collisions led to the formation of the terrestrial planets as we know them today. Some of these collisions must have 10 1 Introduction

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been very dramatic. One such collision is believed to have formed our Moon and another to have blown away Mercury’s outer mantle leaving behind merely its naked core see Fig. 1.7. Yet the terrestrial planets were still immersed in the protoplanetary disc. The gas within the disc of which the planets were surrounded did not move as rapidly around the Sun as the planets did. This resulted in a drag caused by the transfer of angular momentum. The planets slowed down and migrated to orbits closer to the Sun. When the protoplanetary disc finally dissipated we will come to that in a moment the migration stopped and the terrestrial planets had arrived at their current orbits. The Evolution of Giants Beyondthefrostlinetheformationofplanetswastheresultofdifferentprocesses. In that region water ice and other form of ices were present. Yet water ice dominated them all being the most abundant icy material in the protoplanetary disc and today’s solar system. In general the ices that formed the gas giants were more abundant than the metals and silicates that formed the terrestrial planets allowingthegiantplanetstogrowmassiveenoughtocapturehydrogenandhelium the lightest and most abundant elements. The planetesimals accumulated about up to four times the mass of Earth within a period of about 3 million years. A runway process began during which the young gas giants accreted further material rapidly and thereby increased their sizes drastically. Jupiter is believed to have formed first. It accreted much of the ices in the protoplanetary disc. Hence less material was left for the formation of the other gasplanets.ThenextonethatformedwasSaturnandforthejustmentionedreason is smaller and less massive than Jupiter. Uranus and Neptune are believed to have formed after Jupiter and Saturn. The latteroneshadalreadyusedupmostoftheicymaterials.InadditiontheyoungSun a so-called T Tauri star was more active and had a much stronger solar wind than Fig. 1.7 Artist’s conception showing a celestialbodyaboutthesize of our moon slamming at great speed into a body the size of Mercury credit: NASA/JPL-Caltech 1.2 Formation of Our Solar System 11

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we can experience today. This strong wind had already blown away much of the disc material when the two ice giants Neptune and Uranus formed. As a result the planets accumulated little hydrogen and helium. Their cores grew to masses equivalent to Earth. However there exists a timing problem regarding the formation of the gas giants.Attheirpresentpositionstheprotoplanetarydiscwaslessdenserpopulated and it would have taken about a hundred million years for their cores to have formed. By that time however the protoplanetary disc would have already been dissipated a long time ago leaving no material behind for the accretion process. Currentmodels suggestthatafter between 3and 10million years the young Sun’s solar wind had cleared away all the gas and dust in the protoplanetary disc by blowing it into interstellar space. The growth of the planets ended. How can this problem be solved Basically two possibilities are conceivable. First a yet unknown process was involved in forming the ice giants that somehow ledtoafasteraccretionofmaterialorprovidedthenecessarymaterialattheregion where they formed. Secondly the two planets did not form at their current locations. The latter possibility is considered to be the most plausible one. The known processes are at least partially well understood and can explain most of the formation features. No further unknown process would be necessary except for the problematic case of the ice giants. And to cite the Greek philosopher Ptolemy 90–168: “We consider it a good principle to explain the phenomena by the simplest hypothesis possible.” Hence if Uranus and Neptune did not form at their current locations they must have formed somewhere else and then migrated to their current orbits. Computer simulations can help us understand the various scenarios. In a currently well- acceptedscenarioitisassumedthatallofthegiantplanetsJupiterSaturnUranus andNeptunehaveformedclosertotheSunandthattheywerealsoclosertogether. They have reached their current orbits after a phase called planetary migration. As turns out the migration is also necessary to explain the remaining features of the outer solar system. The Asteroid Belt and the Outer Solar System Beforewe describethe process ofplanetary migrationwe first need toknow more about the so far neglected elements of our young solar system. These played a key role in the further evolution of the system. The early solar system after the dissipation of the protoplanetary disc did not only comprise the Sun and the planets. A huge amount of smaller bodies ranging fromdustparticlestosmallrubblesandtoplanetesimalswasstillleft.Ontheouter edge of the inner solar system between 2 and 4 AU a large gap exists in which no planethadformed.Itisfilledwiththousandsandthousandsofsmallerrockybodies which we call asteroids today. In the early phase this zone was much denser populated and there was enough material in form of smaller bodies and planetes- imals in there to form about two or three Earth sized planets. 12 1 Introduction

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The conditions sounded promising for the existence of a planet. Why did no planet form there The simple answer is: Jupiter Jupiter is the by far largest and heaviest body in the solar system besides the Sun. He is approximately 2.5 times more massive than all the remaining planets combined. This tremendous mass of about 1899 10 27 kg implies strong gravitational effects on the other planets and in particular on the small bodies of the solar system. The proximity of the primor- dialasteroidbelttoJupitermeantthatafterthegasgianthadformedabout3million years after the Sun the region’s history changed dramatically. Jupiter’s gravity destabilized the orbits of some of the bodies in this area and increased their velocities. The gas giant kicked some of the bodies out of the solar system or injected them into the inner or outer solar system. The increased veloc- ities further resulted in heavier collisions in which the bodies were shattered. The shattering clearly dominated any present accretion processes. Asimilararea evolvedbeyond the gas giants. Howeverwhile in the primordial asteroid belt bodies of rocky or metallic nature dominated the primordial outer solar system was the realm of small icy bodies. A large disc of these small icy bodiesisbelievedtohaveexistedtheretheprimordialKuiperbelt.Atthisdistance fromtheSunthedensityoftheprotoplanetarydiscwaslowandhencetheaccretion wastooslowtoallowplanetstoformbeforethedissipationofthediscandthusthe initial disc lacked enough mass density to consolidate into a planet. Today’she Kuiper belt lies between 30 and 55 AU from the Sun. The primordial Kuiper belt also called the proto-Kuiper belt was much denser and closer to the Sun with an outeredgeatapproximately30AU.Itsinneredgewouldhavebeenjustbeyondthe orbits of Uranus and Neptune which were in turn far closer to the Sun when they formed most likely in the range of 15–20 AU and in opposite locations with Uranus farther from the Sun than Neptune. Planetary Migration or Nothing Stays the Same We now have all the basic ingredients necessary to understand what was going on duringthephaseofplanetarymigration.Theearlysolarsystembeforetheplanetary migration consisted of the eight planets whereas Uranus and Neptune were in oppositelocations. Theouterplanets were muchcloselyspacedandmorecompact than in present days. The planets then migrated until they reached their current positions. How can we know that What did happen during the migration There are many open questions. Computer simulations help to understand better the processes that were involved. Various models have been proposed and discarded again. A currently promising model is the so-called Nice model named for the location of the Observatoire de la Co ˆte d’Azur in Nice where they were initially developedbyRodneyGomesHalLevinsonAlessandroMorbidelliandKleomenis Tsiganis. This model assumes that the four giant planets Jupiter Saturn Uranus and Neptune were originally found on near-circular orbits between about 5.5 and approximately 17 AU. Today their orbits lie between 5.2 and roughly 30 AU. In the early system Neptune and Uranus had swapped positions making Uranus the outermostofthegasgiants.Alargedensediskofsmallrockandiceplanetesimals 1.2 Formation of Our Solar System 13

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theproto-Kuiperbeltextendedfromtheorbitoftheoutermostgiantplanettosome 35AU.Todaywehaveroughlythreepopulationsofsmallbodiesintheoutersolar system: the Kuiper belt the Scattered Disk and the Oort cloud. Small bodies at the proto-Kuiper belt’s inner edge occasionally passed through gravitational encounters with the outermost giant planet which change the small bodies’ orbits. Most likely these small bodies were scattered inwards towards the innersolarsystem. By scattering thesmallbodiesinwardsanexchangeofangular momentum takes place which in turn pushes the planet slightly outwards. The inwards moving objects then come closer to the next gas giants influence zone itsHillssphere.Theprocessrepeatsandtheplanetesimalisfurthercastedinwards. So step by step not only the small bodies move inwards but also the gas giants change their orbits to farther distances from the Sun. Of course the push the gas giants received while interacting with the small bodiesisverysmallalmostnegligibleandhardlyinfluencestheplanet’sorbit.Yet thecumulativeeffectofmanysmallbodiesencountersshiftedtheorbitsofthegas giants significantly over time. ThisprocesscomestoanendwhenthesmallbodiesencounteredJupitertheby farmostmassiveplanetinoursolarsystemcombininginitmorethantwotimesthe mass of the sum of all other planets together. Jupiter’s enormous mass leads to a differentkindofinteraction.Inmostcasesthegasgiantforcedthesmallbodieson highly eccentric orbits or even kicked them out of the solar system. This is also consideredto bethe hourofbirthofthe Oortcloud. By scattering the small bodies outwards Jupiter very slowly moves inwards due to the preservation of angular momentum. About500–600millionyearsofslowbutgradualmigrationpassedwhenJupiter and Saturn reached their 1:2 mean motion resonance which increased their respec- tive orbital eccentricities. This strong resonance caused a destabilization of the entiresolarsysteminwhichessentiallyJupiterpushesSaturntoitscurrentposition also due to mutual interactions with the two ice giants Uranus and Neptune. Also these two endupwith by farmore eccentric orbits andfinally Uranus andNeptune swap positions making Neptune the outermost planet. In particular the migration of Neptune had severe consequences for the proto- Kuiper belt in which it now fully immersed. The ice giant thereby approached the small bodies therein. By its gravitational influence Neptune succeeded to capture someofthemintoresonanceswhileitpushedothersintomoreorlesschaoticorbits with higher eccentricities. This caused temporary chaos within the belt. Many of the planetesimals however were casted inwards closer to the Sun. Their fates were then finally decided by their encounter with Jupiter. This process thinnedouttheproto-Kuiperbeltdrastically.Somescientistsbelievethatmorethan 90 of its small icy bodies were lost thereby. The surviving bodies either captured into resonances or thrown into more chaotic orbits were in consequence pushed outwards. Some of them formed what we call the Kuiper belt nowadays others especially those with more inclined and eccentric and potentially unstable orbits established the scattered disk. 14 1 Introduction

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Yetnotonlytheplanetesimalswereaffectedbutalsothetwoicegiants.Friction within the belt made their orbits more circular again leading to the situation of the solar system as we know it today. This1:2meanmotionresonanceofJupiterandSaturnalsohadabiginfluenceon the primordial asteroid belt. A large number of planetesimals was captured in the outer asteroid belt at distances larger than 2.6 AU from the Sun. In that part a collisional erosion occurred in which much smaller fragments of the original planetesimals were created by collisions. These smaller bodies were so small that they could be influenced by the solar wind and blown away by it. This removed about 90 of the original material from the primordial belt. Furthermore while Jupiter migrated further inward the gas giant perturbed the orbitsofmanybodiesintheprimordialbelt.Someofthemwereeithercastedeither inwardsintotheinnersolarsystemoroutwardstowardstheKuiperbeltortowardsa region at the fringe of our solar system the Oort cloud. Some of them were even kickedoutofthesystem.Thisledtoafurtherlossofmaterial.Astronomersbelieve thattheprimordialbelthadatotalmassofaboutonetimethemassofEarth.Atthe end of its formation it had been reduced to about 0.1 of that original mass. Ourpreparatoryworkisdoneandwecanstartourjourneytothesmallbodiesof our solar system. 1.2 Formation of Our Solar System 15

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