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IV Planet formation http://sgoodwin.staff.shef.ac.uk/phy229.html 4.0 Introduction Here we examine how planets form and how planetary systems can change. Much of this was covered in PHY106. The key here is to see how that model is adapted in light of exoplants: in particular Migration. It is important to be clear on two modern theories of movement and restructuring of the Solar System: the Grand Tack model (during formation), and the Nice Model (500Myr after formation). 4.1 Protoplanetary discs Planets form in gas/dust discs around young stars (see the disc of HL Tau below). The discs are roughly 10% of the mass of the star, and roughly 1% of their mass is in solid dust grains (micron-sized initially). They are heated by the star and the temperature falls-off strongly as a function of distance as the disc self-shields (>1000K at 0.1AU, to 10sK at >50AU). Gas discs last <10 Myr, so that is the timescale for gas/ice giant formation. 4.1 Core accretion theory Planets form from dust grains: Coagulation: Coagulation Dust particles interact, sticking together to form larger and larger particles. Runaway growth: growth The larger a particle becomes, the faster it grows as it has a larger surface area. Once these condensations reach ~10s km in size they become 'planetesimals' and gravity becomes important. Oligarchic growth: growth The largest planetesimals grow faster, and the larger they become the more dominant their gravitational attraction becomes, allowing a small number to grow to planetary masses. Giant planet formation: formation In the outer solar system a planet can grow large enough to attract a significant H-He envelope (planet mass >a few Earth masses). 4.1 Core accretion theory Core accretion theory predicts that: In the inner parts of the disc planets will be fairly small, rocky bodies (most volatiles having been gaseous leaving only rocky dust): Terrestrial planets (e.g. Mercury, Venus, Earth & Mars). <4au In the middle of the disc large planets can form that will collect large H-He envelopes: Gas Giants (e.g. Jupiter & Saturn). 5-10au In the outer disc large icy planets will be able to form, but the low density will mean they do not collect very massive H-He envelopes: Ice Giants (e.g Uranus & Neptune). Remember the Nice model! 10-20au There will be significant amounts of debris remaining which will be rocky in the inner system (e.g. Asteroids), and icy in the outer system (e.g. Pluto, Kuiper Belt and the Oort Cloud). Everywhere, esp. >20au 4.2 Formation of the Moon As we shall see later the Moon might be an important factor in the development of advanced life on Earth. The Moon is thought to have formed after the collision of a Mars-sized body with the early Earth. This body presumably formed during the oligarchic growth phase in a Lagrange point in the same orbit as Earth. It was then perturbed from its Lagrange point and eventually collided with the Earth. This scenario suggests that large moons around terrestrial planets might not be rare as this should happen fairly (?) often in the core accretion scenario. Estimates are that maybe 10% of Earth-like planets will have a large Moon. 4.3 Hot Jupiters Core accretion says that gas giant planets can only form beyond the ice line at ~3-5 au. However, most of the planets discovered so far are gas giants well within the ice line. How can we explain hot Jupiters? Ice line 4.3 Migration: Type I High-mass planets (more than a few Earth masses) interact weakly with the discs causing a spiral density wave in the disc. At this point the discs are much more massive than the planets they contain. The planet interacts more strongly with the outer wave causing it to loose angular momentum/energy (transferring that angular momentum/energy to the wave) and so moves inwards. 4.3 Migration: Type I The dominant gravitational force of the outer (trailing) wave decelerates the planet causing it to move inwards as it looses kinetic energy. (Note it also gains some kinetic energy as it moves inwards by releasing gravitational potential energy, but the deceleration from the wave dominates). 4.3 Migration: Gap Clearing As the planet increases in mass, the strength of the interaction with the disc grows stronger. The transfer of angular momentum (outwards through interaction with the trailing wave, and inwards through interaction with the leading wave) causes the disc around the planet to clear. 4.3 Migration: Type II Once a planet has reached the mass of a small gas giant planet, the area around that planet will have been completely cleared. Some gas continues to accrete, caught by the planet's gravitational field. As the disc slowly moves inwards accreting onto the star, the planet will also slowly move inwards (the disc looses a.m. by viscous transport, and new material accreted by the planet will have lower a.m., causing the planet to move inwards also). 4.3 Migration: Type II Spiral structure and gaps (presumably due to planets) have been observed in a number of discs around young stars: 4.3 Migration: Problems Migration appears to solve the hot Jupiter problem: massive planets can move in from beyond the ice line to close to stars. However, migration suffers from two major problems: Firstly, the timescale for rapid (type I) migration is very short (O(105) yrs) – why do planets not fall into their parent star? Why are so many Jupiters at 1-3au? Secondly, why did migration not occur in the Solar System? It is argued that Jupiter and Saturn reached a resonance as they migrated so stopping migration. Is another gas giant required? How often does this happen? Some systems have hot Jupiters and distant Jupiters (55 Cnc, HD125612,Ups And, and others) – why? 4.4 Problems in the Solar System The Solar System sort-of fits the predictions of the core accretion model but there are some problems. 1. Why do the Terrestrial planets have volatiles (especially water)? They formed well within the ice line and so should be very depleted in volatiles. 2. Why is Mars so small? Mars should be much bigger than the Earth given where it is now. 3. Why are Uranus and Neptune where they are? They are too far out to have formed where they are (they have H-He envelopes so must have formed while the gas disc was in place). And why is Neptune more massive than Uranus? 4.5 Grand Tack model The Grand Tack model suggests that Jupiter and Saturn both migrated whilst forming. Jupiter migrated in to ~2AU. Its migration was stopped and reversed by the inward migration of Saturn to a 3:2 resonance. The inward migration of Jupiter brought volatile-rich material from ~5AU which was incorporated in the formation of the terrestrial planets. It also cleared much of the material from Mars' orbit meaning that the planet that formed there was low mass. 4.5 Grand Tack model 4.6 Nice model The Nice model suggests that the outer Solar System was restructured about 0.5Gyr after the formation of the Solar System (ie. much later than the Grand Tack model). Initially the outer Solar System was Jupiter-Saturn-Neptune-Uranus (and maybe another ice giant). The Kuiper Belt was more massive and extended in to the orbit of Jupiter. Over ~500Myr interactions with comets caused the planets to move orbit slightly. This is quite gentle until Jupiter and Saturn hit a 2:1 resonance which pumps energy into the orbits of the ice giants throwing them to a larger distance (Neptune further as it was closer to them). It also pushes J-S out of the resonance and clears the early Kuiper Belt. This also explains the Late Heavy Bombardment. 4.6 Nice model The Solar System before, during, and after restructuring in the Nice model. 4.5 Summary The core accretion model states that planets form from the build-up of dust into larger and larger bodies. But planet formation is not simple. Interactions with the disc can cause the migration of massive planets, and multiple massive planets can interact in complex ways. Planetary systems can also restructure themselves long after formation. An important point for astrobiology is how often do significant amounts of volatiles get introduced to the inner solar system? Grand Tack does this early, but Nice can do it later. A common question is: how 'special' is our Solar System?