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Module 6: Modelling the Formation of the Solar System Activity 1: The Solar Nebula Trapezium cluster in the Orion nebula Summary: In this Activity, we will investigate: (a) the present-day Solar System; (b) regions of star formation; (c) how stars form; and (d) the Solar Nebula hypothesis of planet formation. Introduction The Solar System is made up of the Sun, nine planets, dozens of satellites, planetary ring systems, and thousands of asteroids and comets. The planets can be broken up into two distinct groups: the inner rocky terrestrials and the outer gaseous Jovians, plus Pluto, which fits into neither category. In order to understand the composition of the Solar System, we need to understand how it formed. In this Module we will look at how stars and planets form, and in particular note some of the differences between these two planetary groups and try to relate these differences to their formation histories and how they have evolved. (a) The Present-day Solar System At the centre of the Solar System lies our ruler – the majestic Sun. The Sun contains about 99.8% of the total mass of the Solar System and its gravity governs the motion of almost all the other members. While it is extremely important to the planets within the Solar System, on the grander scheme of things the Sun is quite an ordinary star powered by nuclear fusion. The planets, of course, make up a very important part of the Solar System. All nine planets orbit around the Sun in the same direction (anti-clockwise viewed from above the Earth’s north pole), Pluto SUN Earth Neptune Mars Jupiter Saturn Uranus and all lie in approximately the same plane, with the exception of Pluto, which is inclined by 17° to the plane of the Solar System. The planets make up only 0.18% of the total Solar System mass – and Jupiter makes up 70% of that! So in terms of mass, the planets are a pretty small part of the Solar System. We can divide the nine planets into two main groups: • the inner rocky terrestrials: Mercury, Venus, Earth, Mars • and the outer gas giant, the Jovians: Jupiter, Saturn, Uranus, Neptune Pluto fits in neither of these classes, and, as we’ll see later in this Unit, is more like a Jovian satellite. Just as we are familiar with our own Moon, some of the other planets also have natural satellites. In fact, the Jovians all have Earth-Moon system a large family of moons. And Saturn is not the only member of the Solar System to host planetary rings: again, all the the Jovians Saturn’s satellites have ring systems. As well as planets and their accompanying rings and satellites, other members of the Solar System include tens of thousands of rocky asteroids and icy comets. Asteroid Ida (and its satellite Dactyl) Jupiter’s ring system Halley’s comet Our traditional view of the Solar System usually stops a little past the orbit of Neptune and Pluto. Past the planets, however, astronomers are beginning to detect members of a population of ice and rock conglomerates in a region called the Kuiper Belt, which is believed to supply most of the short-period comets like Halley’s comet. Extending even further out into space, astronomers suspect that a spherical cloud called the Oort cloud contains billions of “dirty snowballs” which are the source of long-period comets that take millions of years to travel once round their orbit. In order to understand all the objects and motions within the Solar System, we should try to understand how the Solar System was formed. Since planets form around stars, to understand how planets form we first need to briefly look at how stars forms. Let’s take a quick tour of some star forming regions. (b) Regions of Star Formation Stars form in dense clouds of gas and dust that are found in the arms of our own Milky Way galaxy and other spiral galaxies. NGC253 M100 Spiral galaxies contain about 100 billion stars, as well as enormous clouds of gas and dust. It is within these clouds - called giant molecular clouds - that stars are born. Whirlpool galaxy Giant Molecular Clouds Giant molecular clouds are dusty gas clouds that are held together by gravity, with masses of 100,000 to 1,000,000 times that of the Sun, diameters of 50 to 300 light years* and temperatures of about 10 K*, which is very cold! Ophiuchus & Orion clouds in infrared Because these clouds are so cold, they are good hosts for molecules and can contain more than 60 different kinds of molecules – and hence their name. * where 1 light years = 9.46 x 1015m. To learn more about astronomical distances, click here. * where 273 K = 0°C, so 10K = -263°C. To learn more about the Kelvin temperature scale, click here. Stellar Nurseries The Hubble Space Telescope now allows us to probe these star forming regions in great detail. Three of the richest star forming regions in our Galaxy are the Eagle Nebula, which is 7000 LY* away, the Orion nebula, 1500 LY* away, and the TaurusAuriga cloud at a distance of just 450 LY*. Each of these nebulae* contain hundreds of protostars. * Nebulae are just clouds of gas and dust. * 1 LY = 1 light year = 9.46 x 1015m Stellar Cocoons Stars are thought to be forming inside these dusty cocoons. Anglo-Australian Observatory optical image of the Eagle nebula, or M16, 7000 LY away. HST images of the Eagle nebula pillars, one light year in length. The Orion Nebula One of the richest stellar nurseries in our part of the Milky Way, extensively studied by the Hubble Space Telescope, is the Orion Nebula. This image is a mosaic of 45 images taken of15 separate fields in the centre of the Orion nebula, located in the middle of the “sword” region of the constellation of Orion - seen by southern hemisphere observers as the “handle of a saucepan”. Proplyds A detailed study of the Orion Nebula has revealed over 150 glowing proplyds - or protoplanetary disks around young stars. Proplyds are thought to be embryonic Solar Systems the sites of planet formation. The lumps in this part of the Orion nebula are called proplyds: they are cocoons of leftover gas and dust surrounding baby stars. Each of these images is about 30 times the size of the Solar System and the proplyd are 2–8 times the size of the Solar System. The red glow in the centre of each disk is a young, newly formed star, roughly one million years old. Click here to see a brief movie showing how these images fit together. Solar Systems in the Making This proplyd, Orion’s largest and about 17 times Solar System’s diameter, is the best example of a protostellar disk seen silhouetted against the back-lit nebula. The second image - taken through a different filter - clearly shows the hidden protostar. Solar Systems not in the Making... In this false colour mosaic we see the centre of the Trapezium cluster of the Orion nebula, which contains four massive energetic stars as well as a number of evaporating proplyds (seen as small white “blobs”). This close-up image shows the destruction of a disk surrounding a young star in the act of forming. If the disk had been left alone it would be a strong candidate for producing planets. This is a “false colour image”, with the colours chosen to bring out the details the disk is actually quite dark. Let’s now return to our giant molecular clouds and learn how stars form. (c) How Stars Form Giant molecular clouds are held up by gas pressure, rotation and magnetic fields. If the clouds become massive enough, they can collapse due to gravity. First let’s just consider pressure and gravity: Low-mass cloud High-mass cloud gravity gravity pressure pressure Pressure “wins”, cloud expands Gravity “wins”, cloud contracts There are actually three factors fighting against gravity: • thermal motions and gas pressure mean that the gas is in motion; • magnetic fields act upon the charged particles in the cloud, making them follow field lines rather than obey gravity; and • rotation gives an outward centrifugal force that keeps the cloud from collapsing. The particles are in motion (causing heat and pressure) Gravity works to collapse the cloud, but ... magnetism gravity Magnetic forces from moving charges act against collapse A rapidly rotating cloud tends to spread out Eventually gravity wins... Giant molecular clouds are extremely cold – only a few degrees warmer than the near-zero of space – and rotate very slowly. So at some stage gravity actually wins and the giant molecular cloud begins to collapse under its own gravity. The giant molecular cloud fragments, forming much smaller dense cloud cores within it. It is within these cloud cores that stars actually form. Triggered star formation Most star formation is probably triggered by an event that starts the gravitational collapse. Shock waves travelling through a nebula would cause it to bunch up in places, sometimes enough for gravity to be able to do its work and start the collapse process. Such shock waves can be caused by the outflows of nearby young stars or the supernova death of nearby old NGC 604, a huge, star-forming stars. nebula in the galaxy M33 The collapse of cloud cores Once the collapse process has begun, the cloud cores continue to attract surrounding gas and dust from the nebula, making them more massive, and continuing the gravitational collapse. Large cloud core: • high potential energy Both momentum and energy have • low kinetic energy to be conserved during the collapse • low spin speed process. • To conserve energy, the smaller the cloud core gets, the hotter it gets (potential energy is converted into kinetic energy, which heats the gas). • To conserve angular momentum, the smaller the cloud core gets, the faster it (or parts of it) will spin. Click here to find out more about angular momentum Small cloud core: • lower potential energy • higher kinetic energy • higher spin speed The collapse of cloud cores So as the cloud cores collapse, they become more centrally condensed and the central region heats up. It also spins faster as it gets smaller - particularly in the central regions. This rapid rotation creates large centrifugal forces,* just as you feel in a car when you go too fast around a corner - large sideways forces push you away from the corner. These centrifugal forces are greatest at the equator and the core starts to spread out and form a disk: * Follow this link to find about more about centrifugal forces. Inside a cocoon of dust If we could see inside one of these dense cloud cores, we would see a rotating disk - or proplyd - with a protostar in the centre. Young protostar is forming in centre Start with a vast, rotating, contracting cloud of gas, dust & molecules Nebula contracts to form a protostellar disk Disk gets cooler as you go further out Our Solar System This is why all the members of our Solar System – the planets, asteroids and comets – are all orbiting in the same direction. It’s the direction that the original cloud of gas and dust that formed our Solar System was spinning. A star is born In the centre of the collapsing cloud core, a protostar is forming. As gravity pulls more and more gas to the centre, the temperature, density and pressure of the core get so large that hydrogen atoms actually fuse together... …and once fusion reactions start, you have a baby star! We’ll learn more about star formation in the Unit Exploring Stars and 61H+ 4He++ + 2e+ + 2 + 2 + 21H+ the Milky Way. 6 hydrogen atoms fuse to become one helium nucleus, two positrons, two neutrinos, two gamma rays and two spare hydrogen atoms to keep the fusion going (d) The Solar Nebula Hypothesis Astronomers have long theorised that the Sun and planets were formed from the Solar Nebula - a vast swirling disk-shaped cloud of gas and dust. Hubble Space Telescope images of proplyds provide concrete examples to study in order to confirm, refine and, where necessary, revise our theories of planet formation. The fact that all the planets in the Solar System orbit around the Sun in the same direction and in nearly the same plane strongly constrains any theory of the Solar System’s formation. The Solar Nebula hypothesis was originally put forward by German philosopher Immanual Kant and French scientist Pierre-Simon de Laplace in the late-1700s. They suggested that the entire Solar System was formed from a huge rotating cloud of gas and dust called the Solar Nebula. As the rotating cloud collapsed, the proto-Sun formed in the centre, surrounded by a disk of material out of which the planets formed about 4.5 billion years ago. The formation of the solar nebula disk from the collapsing cloud core took about 100,000 years. During the collapse, the density and temperature of the nebula increased enormously. The temperatures were high enough to vaporise the primordial dust of the cloud core, thereby erasing any prior history of the nebula dust. The resulting protostellar disk was made up of predominantly hydrogen gas, with traces of other elements. Over time the disk radiated its energy and began to cool. Condensation In the conditions believed to exist in the solar nebula, whether a substance existed as a solid or as a gas depended on the local temperature. Temperature decreases from the centre As the solar nebula radiated its energy and gradually cooled, different elements and molecules started to condense out of the nebula, forming solid dust grains. Different substances have different condensation temperatures. The interior of the nebula is hotter than the outer regions, so materials stable at relatively high temperatures tended to dominate in the hot inner Solar System, Temperature decreases from the centre whereas more volatile (easily evaporated) compounds tended to dominate in the cooler outer Solar System. So metals and rock minerals could exist as solids near the Sun, whereas volatile compounds (like methane, ammonia and water ices) were stable further out. Approximate temperatures in the early Solar System 500 200 Pluto Neptune Uranus Saturn Mercury Temperature in Kelvin (K), where 0°C = 273K Jupiter 100 Venus Earth Mars Temperature (K) 1000 10 0.1 1.0 10.0 Distance from Sun (AU) Distance in AU, where 1AU = Sun-Earth distance = 1.5x1011m Examples of compounds and distances beyond which it was cool enough in the early Solar System for them to condense out: Temperature (K) 1000 500 200 metal oxides, silicates Iron oxide 100 water ammonia & methane 10 0.1 1.0 10.0 Distance from Sun (AU) Once solids began to condense out of the solar nebula, this effectively reset their “internal clocks”. Isotope dating of meteorites tells us that the metals began to condense as soon as the disk formed, about 4.55 – 4.56 billion years ago. The rocks (mostly silicates) condensed out a little later, about 4.4 – 4.5 billion years ago, once the disk began to cool. Planetesimal formation So now the solar nebula is full of tiny dust grains, with metal oxides, iron & nickel close to the Sun, silicate compounds out a little further, and ices (water, ammonia and methane) dominating the outer Solar System. These tiny newly formed grains began to stick together electrostatically through low speed collision. 10 micron interplanetary dust grain Slowly they grew in size until they formed large rocky and icy bodies called planetesimals, which were the size of boulders and small asteroids. The sticking mechanism that turned micron sized grains into metre sized rocks is not well understood… Small clumps of planetesimals would have formed gradually. Planetesimals began to condense out: - roughly 4.5 billion years ago As the planetesimals formed, regions of slightly higher density would have accumulated more of the surrounding material by gravitational attraction. Protoplanet formation Once the planetesimals are metre sized, runaway growth occurred. The planetesimals with sizeable masses and therefore appreciable gravity quickly became larger, accumulating all solids in their orbital path, becoming protoplanets of several hundred kilometres. The resulting size of a protoplanet depended on its position in the solar nebula, since location determined the local density and composition. Protoplanet size In the inner Solar System, protoplanets were the size of asteroids and small moons, made up of metals and rocky materials. In the outer Solar System, protoplanets grew much larger, between one and 15 Earth masses. The large size jump of protoplanets at the Mars-Jupiter boundary was due to the availability of materials. Since the solar nebula contained a much higher proportion of volatiles than metals and silicates, this meant that there was much more material available in the outer Solar System to go into forming planets, resulting in much larger protoplanets. The formation timescale of protoplanets was a few 100,000 to several million years. The Solar Wind After about a million years, the solar wind would have begun to blow, clearing the nebula of any remaining gas. This puts some strong time constraints on the formation of the giant planets, since their huge atmospheres are made up of gas from the solar nebula. Therefore the cores of the giants must have formed within a million years. Protoplanetary Disk Once the solar wind had blown away the remaining gas of the solar nebula, all that remained were the protoplanets and planetesimals. Such a disk of solid material is called a protoplanetary disk. The protoplanets continued to grow slowly via collisions, clearing up the remaining solids in the disk. After about 10 to100 million years, the Solar System was made, with 9 newly formed planets in stable orbits, as well some remaining debris - the asteroids and comets. Summary In this Activity, we have looked at star and planet formation in a contracting rotating cloud of gas and dust. Due to the rotation, the cloud collapsed in a disk out of which the planets formed. This theory of planet formation explains the overall shape and motions within the Solar System. As the disk cooled, rocky materials condensed out in the inner Solar System and volatiles in the outer Solar System, producing inner rocky protoplanets and giant outer icy protoplanets. In the next Activity we’ll look at how these newly formed protoplanets evolved into fully fledged planets. Image Credits Trapezium cluster in Orion nebula Credit: John Bally, Dave Devine & Ralph Sutherland, HST and NASA http://antwrp.gsfc.nasa.gov/apod/ap971118.html Montage of the nine planets - JPL http://photojournal.jpl.nasa.gov/catalog/PIA01341.jpg Image of the Earth and Moon from Galileo - NASA http://nssdc.gsfc.nasa.gov/image/planetary/earth/gal_earth_moon.jpg Montage of Saturn and some of its satellites - JPL http://photojournal.jpl.nasa.gov/catalog/PIA01482.jpg Jupiter and its rings in infrared - NASA http://antwrp.gsfc.nasa.gov/apod/ap970205.html Ida and Dactyl - NASA http://nssdc.gsfc.nasa.gov/image/planetary/asteroid/idasmoon.jpg Comet Halley © David Malin, AAO, used with permission http://www.aao.gov.au/local/www/dfm/image/uks019.gif Image Credits Spiral galaxy M100 © David Malin, AAO, used with kind permission http://www.aao.gov.au/images.html/captions/aat058.html Central region of spiral galaxy NGC253 Credit: Hubble Heritage Team (AURA/STScI/NASA) http://oposite.stsci.edu/pubinfo/pr/1998/42/ Central region of Whirlpool Galaxy (M51) Credit: Nino Panagia (STScI and ESA) and NASA http://oposite.stsci.edu/pubinfo/pr/96/17.html Orion nebula mosaic, O’Dell & Wong (Rise U.) and NASA http://antwrp.gsfc.nasa.gov/apod/ap951121.html Eagle nebula in M16 © David Malin, AAO, used with kind permission http://www.aao.gov.au/images.html/captions/aat047.html Gas pillars in M16, Eagle nebula Credit: Jeff Hester & Paul Scowen (Arizona State University), and NASA http://www.stsci.edu/pubinfo/pr/95/44.html Image Credits Closeup of “Proplyds” in Orion Nebula - Robert O’Dell (Rice U.) & NASA http://oposite.stsci.edu/pubinfo/gif/OrionProplyds.gif Four proplyds in Orion Mark McCaughrean (MPIA), Robert O’Dell (Rice U.) & NASA http://oposite.stsci.edu/pubinfo/gif/OriProp4.gif Edge on protoplanetary disks in Orion nebula Credit: Mark McCaughrean (MPIA), Robert O'Dell (Rice U.) & NASA http://oposite.stsci.edu/pubinfo/jpeg/OriEODsk.jpg Closeup of “Proplyds” in Orion Nebula Credit: John Bally, Dave Devine & Ralph Sutherland, HST and NASA http://www.cita.utoronto.ca/~johnston/orion.html#figures Trapezium cluster of Orion Nebula Credit: John Bally, Dave Devine & Ralph Sutherland, HST and NASA http://www.cita.utoronto.ca/~johnston/orion.html#figures NGC 604, courtesy of Hui Yang (University of Illinois) and NASA http://oposite.stsci.edu/pubinfo/gif/NGC604.gif Image Credits Interplanetary dust grain http://stardust.jpl.nasa.gov/science/sd-particle.html#idp-s Protoplanet disk (artists conception) - Credit: Pat Rawlings (JPL) http://eis.jpl.nasa.gov/origins/poster/protodisk.html Solar flare - Credit: SOHO - EIT Consortium, ESA, NASA http://antwrp.gsfc.nasa.gov/apod/ap970918.html Dusty disk of Beta Pictoris - Credit: C. Burrows & J. Krist (STScI) & NASA http://oposite.stsci.edu/pubinfo/jpeg/BetaPicB.jpg Now return to the Module home page, and read more about modelling the formation of the Solar System in the Textbook Readings. Hit the Esc key (escape) to return to the Module 6 Home Page Astronomical Distances Distances in the Solar System are very large! To compare the average distances between the Sun and each of the planets, it’s convenient to do it in terms of the average Earth - Sun separation. Astronomers define a convenient unit of length: The AU (astronomical unit) = average distance between Sun and Earth = 1.496 x 1011 m 1 AU Another astronomical unit of measure is the lightyear, which comes from the knowledge that light takes a finite length of time to travel through space. The lightyear (LY) is the distance that light will travel in a year, where: 1 LY = 9.461 x 1015 m = 63,240 AU 1 ly (distance) … and is seen here a year later An event happens here ... Click here to return to Activity The Kelvin temperature scale The Kelvin temperature scale is the same as the Celsius scale, except that the definition of zero is different. The Celsius scale specifies 0 degrees as the temperature at which water freezes (0°C). On the other hand, the Kelvin scale specifies 0 degrees as the temperature of an object in which the kinetic energy of the particles making up the object is at a minimum. This is called absolute zero (0°K). Therefore, 273.15 degrees Kelvin is the freezing point of water and 373.15 degrees Kelvin is the boiling point of water. The Celsius scale is 273.15 degrees “out of sync”: Kelvin 0 Celsius -273 100 200 300 400 500 600 -173 -73 27 127 227 327 Melting point of ice Boiling point of water Click here to return to Activity Angular momentum If something is spinning, it has angular momentum which is a conserved quantity. The angular momentum of an object depends on how the mass is arranged about the axis it is spinning around, and the speed at which it spins. For each part of an object, the angular momentum radius r m Speed v AM = mvr where m = mass v = speed r = distance from centre The classic ice skater concept If an iceskater is spinning with his arms and a leg out, he will spin slowly. But if he pulls these limbs closer to his body, he will spin faster. Angular momentum is conserved Distance large, speed small axis Distance small, speed large This is because the total sum, for each part of his body, of the angular momentum mass x speed x distance from axis must stay the same. If the distances of his hands, arms and legs from the axis get smaller, the speeds must get bigger to compensate! Angular momentum in space Distance large, speed small Distance small, speed large If a molecular cloud, for instance, contracts under gravity, it will spin faster. Click here to return to Activity Centrifugal Forces Centifugal force is not a “real” force in the same way that body forces such as gravity are - it is “fictitious forces” that is only felt in an accelerating reference frame. For an observer in a rotating (and therefore accelerating) reference frame, such as a child sitting on a merry-go-round or a passenger sitting in a cornering car, the observer feels an outward fictitious force. In fact, this sensation just comes from their body wanting to continue in a straight line. As seen from the merry-go-round As seen from above Click here to return to Activity