1 credit: NASA L3: The formation of the Solar System UCL Certificate of astronomy Dr. Ingo Waldmann
2 A stable home The presence of life forms elsewhere in the Universe requires a stable environment where appropriate temperature and pressure conditions allow for the development and/or sustaining of biochemical reactions. Stars or the interstellar medium are not suitable for this. Planet/satellites/comets/asteroids around a star are the best choice to look for life Let us consider our own solar system as a start
3 Key attributes of the Solar System Key attributes of the Solar System Five most important attributes: 1. Terrestrial planets: Made of rocky substances, relativelly small. 2. Gas giants (Jupiter,Saturn): Made of H and He, large. 3. Ice giants (Uranus,Neptune): Similar composition as the gas giants but higher metallicity (300 times solar). 4. All planets orbit in the same direction and roughly on the same plane. 5. Terrestrial planets orbit close to the Sun, gas.ice giants orbit further away. The Solar System 3
4 Characteristics of Solar system: terrestrial planets Small ( km diameter) High density ( kg/m 3 ) Mainly composed of heavy elements (Silicate mantles and Iron cores) Shallow or non-existent atmospheres Close to the Sun (< 2au) Density = Mass Volume = M 4 3 R3
5 Characteristics of Solar system: jovian planets Large (50, ,000 km kg/m diameter 3 Low density (700-1,700 ) Mainly composed of light elements (H and He) Deep atmospheres (dominant feature of gas giants) Further away from to the Sun (> 5au)
6 Planetary abundances
7 General orbital characteristics In the solar system, all planets have close to circular orbits The orbits of the planets are all coplanar, i.e. lying on the same orbital plane All planets orbit in the same direction which is also the orientation of the solar spin.
8 Formation history: Protostellar nebular collapse The protosolar nebula is estimated to have 300,000 times the mass of the sun and a diameter of ~4 million au. Composition: ~99% H and He, ~1% metals and dust. A dense filament of an estimated diameter of 2,000-20,000 au is thought to have become gravitationally unstable and collapsed into itself. This is the onset of the solar system. This instability can be cause by shockwaves of nearby supernovae or tidal interactions between galaxies, nearby stars or other large gas clouds. We observe similar scenarios in the Orion Nebula, the Horse Head Nebula and others.
9 Protoplanetary disk in the Orion nebula Protostellar fillaments in the Horsehead Nebula Protostellar nebula
10 Formation history: Protostellar nebular collapse Contraction of the Solar Nebula Under its own gravitational pull, the Solar Nebula contracted Greatest concentration of matter occurred at the center protosun as the name suggests, the protosun was the progenitor to the Sun Falling material gained speed and the kinetic energy was converted into thermal energy nebula heated up (Kelvin-Helmholtz contraction) If the collapsing Solar Nebula did not have any rotation, then all the material would fall directly towards the proto-sun. However the Solar Nebula started out with a small degree of rotation The rotation velocity of the nebula increases as it reduces in size Conservation of angular momentum The Solar System 12
11 Conservation of angular momentum Conservation of angular momentum The principle of conservation of angular momentum valid at all scales (atomic... galactic) When the net external torque acting on a system is zero, the total angular momentum (L) of the system is constant (i.e. conserved) If a packet of gas orbits a central proto-star it has angular momentum L = mvr L = constant mass = constant hence can rearrange equation to give so velocity, v, is inversely proportional to the radial distance from the protosun, r. As r decreases, v increases Ice-skating analogy ᅠ v = L mr The Solar System 13
12 Formation history: Protoplanetary disk Due to the conservation of angular momentum, the rotating disk is stretched out into a thin disk This highly heated, fast spinning disk is called the Protoplanetary disk. It is usually only several 10s-100s of au thick and the gas inside is highly turbulent. It is initially only composed of gas and dust. Freedman & Kauffmann The Solar Syste
13 Formation history: Formation of planetesimals, planetesimals, planets asteroids and comets planets, asteroids and comets So we ve seen evidence of stars forming from the collapsing, rotating gas and dust clouds But how are planetesimals, planets, asteroids and comets formed within this spinning cloud? Definition of planetesimal: large solid bodies (larger than dust, smaller than planets) for the purpose of this course it refers to objects with diameters of ~1 km. An important concept to understand when considering planet formation is that of the Condensation temperature The Solar System 16
14 Formation history: Two planet formation mechanisms There are currently two theories of how planets form out of the protoplanetary disk Gravitational instability theory: Similar to the collapse of the protostellar nebula, we can imagine that small filaments in the protoplanetary disk form and collapse under their own gravity This is an instant formation model Core accretion theory: Small particles accrete onto each other and form small lumps that gravitationally attract more lumps until this conglomerate reaches the size of planetesimals and beyond. This is a slow and gradual formation process
15 Formation history: Two planet formation mechanisms There are currently two theories of how planets form out of the protoplanetary disk Gravitational instability theory: Similar to the collapse of the protostellar nebula, we can imagine that small filaments in the protoplanetary disk form and collapse under their own gravity This is an instant formation model Core accretion theory: Small particles accrete onto each other and form small lumps that gravitationally attract more lumps until this conglomerate reaches the size of planetesimals and beyond. This is a slow and gradual formation process
16 Gravitational instability model This formation process happens very quickly, within ~3 million years of the formation of the protoplanetary disk When the gaseous filament collapses, it forms a core of dense gas but no solids. This is called a top-down formation approach where the atmosphere forms first before the solid core. The gravitational potential of the gaseous core is now high enough to attract small dust grains which fall into the centre of the gaseous core to form a solid core. Jeans instability: - Gravity pulls gas cloud together (inwards force) - Pressure makes it expand (outwards force) - Above the Jeans mass, gravity wins and the cloud collapses into itself
17 Gravitational instability model Disadvantages of theory: Does not explain the formation of terrestrial planets The mechanism can only act in early formation stages. Once the proto-sun is fully formed, the solar wind will dissipate much of the gaseous content of the inner protostellar disk regions -> protoplanetary disk differentiation. Does not explain the existence of ice-giants such as Neptune and Uranus
18 Protoplanetary disk differentiation The condensation temperature The protoplanety disk was not uniform. This is easy to understand if one considers the increasing distance from the hot protosun Near the protosun, protoplanetary disk was hot Hydrostatic equilibrium only refractory elements with high melting points could remain solid such as iron, nickel, silicon water, methane, ammonia, etc remain in gaseous form Further away disk was cooler lighter compounds were also able to remain solid such as water, methane, ammonia Solid or gaseous governed by the condensation temperature if the temperature of the element > condensation temperature, the element will be in gaseous form if the temperature of the element < condensation temperature, the element will be in solid form (dust/ice particles) K for water, methane, ice, etc >1300K for rock-forming elements such as silicates, iron, etc The Solar System 17
19 Freedman & Kauffmann SNOW LINE T=170K is the waterice sublimation point in a vacuum (Snow line) Result: out to ~ 4AU from Sun, disk devoid of solid icy compounds The Solar System 18
20 Formation history: Two planet formation mechanisms There are currently two theories of how planets form out of the protoplanetary disk Gravitational instability theory: Similar to the collapse of the protostellar nebula, we can imagine that small filaments in the protoplanetary disk form and collapse under their own gravity This is an instant formation model Core accretion theory: Small particles accrete onto each other and form small lumps that gravitationally attract more lumps until this conglomerate reaches the size of planetesimals and beyond. This is a slow and gradual formation process
21 Core accretion theory The formation of planetesimals It is a bottom-up formation approach Dust grains collide and stick together by electrostatic forces planetesimals up to 1km form Due to condensation temperatures In inner regions mainly refractory materials In outer regions combination of refractory and lighter compounds After a few million years, around 10 9 planetesimals of size ~1km within the Solar System an enormous amount of material, but necessary to eventually form planets The Solar System 19
22 Core accretion theory: formation of terrestrial planets Formation of the terrestrial planets As planetesimals grew larger, their gravitational pull increased km-sized planetesimals collide to produce lunar-sized objects collisional process is termed accretion Final episode of terrestrial planet formation unimaginably large collisions taking place ultimately resulted in the terrestrial planets we see today To test whether this model is feasible, computer simulations are used to see what happens when you take a large number of planetesimals in orbit around a star, moving under gravity alone. the result The Solar System 20
23 Core accretion theory: formation of terrestrial planets Computer simulation of the Solar System terrestrial planet formation Large numbers of simulations show that accretion typically continues for ~100 million years, and that, typically, 4-5 terrestrial planets are formed within 1.5 AU from the Sun. Can consistently account for terrestrial planets within ~100 million years The Solar System 21
24 Core accretion theory: formation of giant planets Formation of the giant planets: Core-accretion model More solid material available outside of snow line ices, dust Massive objects able to form quite quickly 5-15M Earth Massive gravity able to capture gas from local disc forming gaps in the disc (core-accretion model) End result: massive planet, consisting mostly of H and He e.g. Jupiter and Saturn Critical timing needed massive objects need to form within the timescale of the gas disc, otherwise there would be no gas for the objects to capture and gas giants wouldn t exist. The Solar System 22
25 Core accretion theory: Terrestrial vs Jovian planet formation Terrestrial Terrestrial vs Jovian vs planet Jovian formation planet formation Freedman & Kauffmann Freedman & Kauffmann The Solar System The Solar System 24
26 The curious case of: Neptune and Uranus Neptune and Uranus The jury is still out as to how Uranus and Neptune were formed Unlikely to have formed in their current locations: 19AU and 30AU from the Sun Solar nebula too sparse at these distances Believed to have formed between 4AU and 10AU and then gravitationally flung outwards to their current orbits The Solar System 25
27 The curious case of: Pluto Pluto is a special case Smaller than any of the terrestrial planets Intermediate average density of about 1,900 kg/m 3 Density suggests it is composed of a mixture of ice and rock Orbital eccentricity higher than for the rest of the planets (e=0.25), and tilted with respect to the plane of the other orbits (10 o ). Not the same origin as the other planets: Dwarf planet (along with Eris or Ceres) Pluto The Solar System 28
28 Other solar system bodies Other solar system bodies Some planetesimals never grew large enough to form planets Naturally split into two families based on distance from the Sun asteroids and comets Asteroids: probably prevented from forming planets by the gravitational influence of Jupiter continual heating by Sun s radiation causes evaporation of volatile ices only rocky substances remain Comets: formed at large distances from the Sun; spread too thinly to contribute to planet building Kuiper Belt (30AU+) and Oort cloud comets (~50,000AU) The Solar System 29
29 Kuiper belt Pluto s orbit Typical Kuiper belt object (KBO) orbit Oort cloud credit: NASA
30 Which one is right then? Core accretion or gravitational collapse? Most likely the process of planetary formation is a combination of both: core accretion and instant gaseous collapse. Until recently we only had one case study (our own solar system) to study. This is clearly not enough. What we observe in the solar and exoplanetary systems today is the product of planetary formation and a later stage of planetary migration which we will discuss in the next lecture.
31 Aims and objectives 1. Know what a protostellar nebula and the protoplanetary disk are. 2. Understand the formation of protoplanetary disks and the role of gravity and heat in transforming the protostellar nebular into a planetary system. 3. Understand the principle of conservation of angular momentum. 4. Be able to explain both planetary formation theories: 1) Gravitational collapse and 2) Core accretion. 5. Appreciate the different formation histories of terrestrial and jovian planets.