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

http://www.greatdreams.com/cosmic/orion852.jpg

EagleNebula(“Pillars ofCreation”)

From HubbleSpace Telescope

http://forums.airbase.ru/cache/sites/a/n/antwrp.gsfc.nasa.gov/apod/image/0310/468x468/horsehead_cfht.jpg

HorseheadNebula

(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

http://www.aerospaceweb.org/question/astronomy/solar-system/formation.jpg

OortCloud

&Kuiper

Belt

http://www.harmsy.freeuk.com/oimages/oort_cloud.jpg

• 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 embryosChambers, EPSL (2004), Fig. 2

Inner Solar System Evolution

Morbidelli et al., Meteoritics & Planetary Sci. (2000), Fig. 1

Eccentricity

e = b/aa = 1/2 major axisb = 1/2 distance between foci

Sun-Earth distancesAphelion: 1 + ePerihelion: 1 -

e

a

b

Today:e = 0.017

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 ME

14.5 ME

1 ME

17.2 ME

95 ME

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 (CaTiO3, Ca2Al2SiO7, MgAl2O4)

2. Metallic Fe-Ni alloy3. MgSiO3 (enstatite)

4. Alkali aluminosilicates5. FeS (troilite)6. FeO-silicates7. Hydrated silicates (kinetically

inhibited)*Ref.: Lewis and Prinn, Planets and their Atmospheres (1984), p. 60

Condensation sequence (cont.)

8. H2O9. NH3

10.CH4

11.H2

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?

Equilibriumcondensation

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