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Chapter 6: Planetological foundations for origins of life. 2 Planet formation – magic in the residue of stellar formation!. Emmanual Kant and Pierre-Simon Laplace: 18 th century giants. Kant-Laplace hypothesis: planets form in disks… verification 200 years later!

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2 planet formation magic in the residue of stellar formation
2 Planet formation – magic in the residue of stellar formation!

Emmanual Kant and Pierre-Simon Laplace: 18th century giants

Kant-Laplace hypothesis: planets form in disks…

verification 200 years later!

Two major kinds: terrestrial (rocky) planets: like Earth

giants (gaseous) planets: like Jupiter.

Formation: terrestrial planets form by collisions of smaller bodies like asteroids?

gas giants – gas accreting onto a massive rocky core; or by gravitational instability of disk?

Flared, gaseous, dusty disk formation!

HH 30 (from HST)

Star formation sets the stage for planet formation

Gas Accretion & Gap-formation


Planet formation theories
Planet formation theories formation!

  • Giant planet formation; two mechanisms under intense investigation:

    1. Core accretion model…. Coagulation of planetesimals that when exceeding 10 Earth masses, gravitationally captures gaseous envelope (eg. Bodenheimer & Pollack 1986)

    2. Gravitational instability model …. GI in Toomre unstable disk produces Jovian mass objects in one go (eg. Boss 1998).

  • For either 1 or 2 – final mass determined by “gap opening” in face of disk “viscosity”.

  • Terrestrial planet formation; model 1 - do gaps open too?

  • Core accretion formation!:

    3 phases: rapid growth of rocky core, slow accretion of planetesimals and gas, runaway gas accretion after critical mass achieved (near 10 ME)

    Problem: formation time still uncomfortably long: Jupiter at 5 AU forms in

    - 1Myr with 10 ME core

    - 5 Myr with 5 ME core

Hubickyj et al 2005, Icarus

  • GI: formation!rapid formation within few thousand yrs

    - disk must have Toomre Q < 1

    - disk must cool quickly (less than ½ orbital period – Gammie 2001)

    Problem:latter point not satisfied in detailed simulations (eg. Cai et al 2004)

Mayer et al 2002

When do giant planets quit growing? formation!

Gap opens in a disk when

Tidal Torque ~

Viscous Torque


Tidal Torque


Viscous Torque


Planetary masses: determined by gap opening formation!

Disk pressure scale height h [AU]

Disk Radius a [AU]

- Gap-opening mass ~ Final mass of a planet

- Two competing forces (Tidal vs Viscous)

- Smaller gap-opening masses in an inviscid disk

Lin & Papaloizou (1993)

Depends on disk physics!

- disk flaring (h/a) – governed by heating of disk (ie central star

- disk viscosity: very low in central region or dead zone

Migration of planets - by tidal interaction with disk: a planet moves in very rapidly (within a million years!) but can be saved by dead zone ( Matsumura, Pudritz, & Thommes 2006)





Dead Zone

Disk Radius [AU]

Disk Radius [AU]





0 2×106 4×106 6×106 8×106 107

0 2×106 4×106 6×106 8×106 107

Time [years]

Time [years]

(w/o Dead Zone)

(w/ Dead Zone)




Detecting Jovian planets in other disks...close-up view with ALMA

Mplanet / Mstar = 0.5 MJup / 1 Msun

Orbital radius: 5 AU

Disk mass as in the circumstellar disk as around the Butterfly Star in Taurus

50 pc

100 pc

Wolf & D’Angelo (2005)

astro-ph / 0410064

Birth of a Solar System: what ALMA can do….. with ALMA

ALMA band 7 300 GHz = 1 mm resolution = 1.4” to 0.015”

~ Highest resolution at 300 GHz = 1 mm (0.015”)

100 AU = 0.3” at d=300pc

~ Highest resolution at 850 GHz = 350 mm

Condensation sequence accounting for compositions of planets
Condensation sequence with ALMA: accounting for compositions of planets

  • Temperature of disk drops as radius increases.

  • All materials whose condensation temperatures are higher than disk temperature at that radius can condense out into solids

  • - so hot innner region of disk has metals – outer cool regions have ices

Biomolecule formation organic molecules made in protostellar disks
Biomolecule formation: with ALMA organic molecules made in protostellar disks

  • Organic chemistry in “molecular layer” – 3 layer vertical structure at r > 100AU

  • 2D, stellar ultra-violet irradiation of disks:

    -molecules dissociated in surface layer,

    - abundant in gas phase in intermediate layer,

    - frozen out onto grains in densest layer. (Zadelhoff et al 2003, A&A).

  • Delivery system of biomolecules to Earth?

    Water, and biomolecules: by asteroids? comets?

  • Simulations: Typically find a few Earth ocean’s worth delivered by asteroids from beyond 2.5 AU.

Comets dirty snowballs

Cometary nucleus – few km in diameter; passage near Sun heats up coma of dust and gas; coma can be 100,000 in size; hydrogen envelope extends millions of km;

Comets: Dirty snowballs

Halley’s comet as seen in May 1910: May 10 – 30 deg tail; May 12 - 40 deg tail. Period of comet: 76 years

Giotto images of halley s comet
Giotto images of Halley’s comet heats up coma of dust and gas; coma can be 100,000 in size; hydrogen envelope extends millions of km;

Evaporating dust and gas from Halley’s nucleus: 30 tons per second for comet inside 1AU – Halley’s comet would evaporate in 5000 orbits

In general: density 100 kg/ cubic metre; temperature, few 10s of Kelvins; mass ; composition, dust mixed with methane, ammonia & water ices

Cometary orbits evidence for two distinct reservoirs of comets
Cometary orbits – evidence for two distinct reservoirs of comets

Isotropic distribution of comets at 50,000 AU: result of gravitational scattering? Oort cloud

Disk-like distribution of comets beyond Neptune: remnant of original disk? Kuiper Belt

Origin of oceans delivery of water by comets or asteroids
Origin of oceans…. delivery of water by comets or asteroids?

  • Clue to origin of Earth’s water:

    HDO/H2O = 150 ppm = ½ of cometary value

  • Asteroids (carbonaceous chondrites) beyond ice line (2.5 AU) can have high water content

  • No more than 10% of Earth’s water from comets

  • Perturbations by Jupiter of asteroid system perturbs their orbits into ellipses that cross Earth’s orbit and collide,… bringing in water.

  • Do amino acids survive during this bombardment?

  • Evidence for bombardment: craters on Moon and elsewhere… and formation of the Moon itself in late heavy bombardment…

Formation of the moon impact model
Formation of the Moon – Impact Model asteroids?

1. Mars – sized object collides with proto-Earth which has already formed iron core: much of impactor and debris encounters Earth a 2nd time.

2. Collision tears off Earth’s mantle material – Moon ends up with composition similar to Earth’s mantle

  • Debris from collision in orbit around Earth collects together to form the Moon:

  • < 10% of initial ejected material ends up accreting to form the Moon.

Brief history of the moon
Brief history of the Moon asteroids?

  • Just after the end of the major meteoritic bombardment

  • b) Lunar vulcanism floods maria with lava ending 3 billion years ago

  • c) Original maria pitted with craters over last 3 billion yr