Lecture 2—Planetary Formation Abiol 574

Let’s start with topics that we won’t talk about at any great length in this course First, one has to form the universe (the Big Bang) Then, one needs to form galaxies Then, one needs to form stars 

Orion Nebula Photo from HST The Orion nebula is a dense interstellar cloud of gas and dust in which stars are being formed

Eagle Nebula (“Pillars of Creation”) From Hubble Space Telescope

Horsehead Nebula (also from HST)

Cloud collapse/disk formation Then, one needs to form disks (circumstellar nebulae) This happens quite naturally if the interstellar material was spinning  astronomy/solar-system/formation.jpg

Oort Cloud & Kuiper Belt The Solar System also includes comets, both within the Kuiper Belt (within the disk) and the Oort Cloud (spherical shell)

Beta Pictoris (from HST) Beta Pic was the first such interstellar disk to be actually observed

Early stages of planet formation Dust settles to the midplane of the solar nebula The dust orbits slightly faster than the gas because it doesn’t feel the effects of pressure Gas drag causes some of the dust to spiral inwards Turbulence is generated, lifting some of the dust out of the midplane If the dust density is great enough, then gravitational instability sets in, forming km- size planetesimals Chambers, EPSL (2004), Fig. 1

Bipolar outflows From: The New Solar System, ed. 4, J.K Beatty et al., eds., p. 16 Material falls into the star along the midplane of the disk and is ejected towards the poles of the star Mass flows inward, angular momentum outward

Runaway growth stage Initially, the planetesimals were small Collisions make them grow if the relative velocities are small Dynamical friction keeps orbits circular and relative velocities low Gravitational focusing causes the largest bodies to grow the fastest  Runaway growth of planetary embryos Chambers, EPSL (2004), Fig. 2

Inner Solar System Evolution Morbidelli et al., Meteoritics & Planetary Sci. (2000), Fig. 1

Eccentricity e = b/a a = 1/2 major axis b = 1/2 distance between foci Sun-Earth distances Aphelion: 1 + e Perihelion: 1 - e a b Today: e = Range: 0 to 0.06 Cycles: 100,000 yrs

Final stage of accretion Chambers, EPSL (2004), Fig. 3 Results of four different simulations. Segments in the pie chart show the fraction of material coming from different parts of the Solar System.

Back to generalities. Let’s look at the results of planetary formation in more detail…

Titius-Bode Law Ref.: J. K. Beatty et al., The New Solar System (1999), Ch. 2. The logarithmic, or geometric, spacing is probably not an accident! The Solar System is “packed”, i.e., it holds as many planets as it can. If one tries to stick even a small planet inside it (except in the asteroid belt), it will be ejected.

Different planetary types There is a pattern to the planets in our Solar System –Small, rocky planets on the inside –Gas giant planets in the middle –Ice giant planets on the outside Why does this happen this way, and should we expect this same pattern to apply elsewhere? 318 M E 14.5 M E 1 M E 17.2 M E 95 M E

Solar nebula composition Ref.: J. K. Beatty et al., The New Solar System (1999), Ch. 14. The solar nebula is assumed to have the same elemental composition as the Sun We’ll talk later about how solar composition is obtained Different compounds condense out at different temperatures…

Condensation sequence (high temperatures to low) * 1.Refractory oxides (CaTiO 3, Ca 2 Al 2 SiO 7, MgAl 2 O 4 ) 2.Metallic Fe-Ni alloy 3.MgSiO 3 (enstatite) 4.Alkali aluminosilicates 5.FeS (troilite) 6.FeO-silicates 7.Hydrated silicates (kinetically inhibited) * Ref.: Lewis and Prinn, Planets and their Atmospheres (1984), p. 60

Condensation sequence (cont.) 8.H 2 O 9.NH 3 10.CH 4 11.H 2 12.He Collectively, these last 5 compounds (or elements) are referred to as “volatiles” because they are either liquids or gases at room temperature Volatiles are important, as they are the compounds on which life depends most strongly So, how did planets acquire them?

Equilibrium condensation model Ref.: J. S. Lewis and R. G. Prinn, Planets and Their Atmospheres (1984) 1 M  solar nebula (which is too high!) -- Nebula would be unstable if over ~0.1 M  -- Minimum mass solar nebula  0.03 M  The curve along which the planets lie is an adiabat running along the midplane of the nebula Earth Mars Venus

Problems with the equilibrium condensation model Assumed nebular mass (and thus pressure) was too high Formation of hydrated silicates is kinetically inhibited –Gas-solid reactions are slow Actual planetary accretion problem is time-dependent –The equilibrium condensation model applies only at a given instant in time Planetesimals can move from one part of the solar nebula to another –This will be the key to understanding the origin of Earth’s volatiles