Chapter 6: Planetological foundations for origins of life.

Chapter 6: Planetological foundations for origins of life

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

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?

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

Gas Accretion & Gap-formation

HH 30 (from HST)

Flared, gaseous, dusty disk

http://www.astro.psu.edu/users/niel/astro1/slideshows/class43/slides-43.html

Protoplanet

Star formation sets the stage for planet formation

Planet formation theories

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:

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: 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

Protoplanet

Tidal Torque

Viscous Torque

Disk

Disk

Gap opens in a disk when

Tidal Torque ~

Viscous Torque

When do giant planets quit growing?

Planetary masses: determined by gap opening

- Gap-opening mass ~ Final mass of a planet - Two competing forces (Tidal vs Viscous) - Smaller gap-opening masses in an inviscid disk

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

Disk Radius a [AU]

Disk pressure scale height h [AU]5

40

a

h

M

M

Star

Planet

Lin & Papaloizou (1993)

30

20

10

0

Dis

k R

adiu

s [A

U]

0 2×106 4×106 6×106 8×106 107

Time [years](w/o Dead Zone)

30

20

10

0D

isk

Rad

ius

[AU

]

0 2×106 4×106 6×106 8×106 107

Time [years](w/ Dead Zone)

=10-3 =10-3

=10-5

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)

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

Wolf & D’Angelo (2005)

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

astro-ph / 0410064

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

ALMA band 7 300 GHz = 1 mm

resolution = 1.4” to 0.015”

100 AU = 0.3” at d=300pc

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

~ Highest resolution at 850 GHz = 350 m

Condensation sequence: 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

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

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

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;

Giotto images of Halley’s comet

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

kg1612 1010

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?

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 Model1. 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

3. 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

a) 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