Solar System Formation

1
Solar System Formation
2
SOLAR SYSTEM FORMATION MODEL REQUIREMENTS
3
SOLAR SYSTEM FORMATION MODEL REQUIREMENTS
4
When the Solar System Formed
5
A long time ago

• In a dense molecular cloud far, far away

Hot, ionized gas
M16 The Eagle Nebula
Young stars
Dense Molecular gas
6

• Molecules and dust
• Allow efficient cooling of gas
• Self-shield from ionizing radiation
• Act as a Refrigerator

Bright star
Ionizing radiation
Ionized Gas
Ionized Gas
Evaporating molecular gas
Dust grain absorbs visible/UV, re-radiates
infrared, which escapes
Dusty Molecular Gas
Ionization Front
Excited molecule releases energy
7
Whats Going On
8
Locally denser clouds survive, cool
9
Self Gravity
10
M42 The Orion Nebula
Distinct Protostellar Nebula
1000 AU
11
The Nebular Hypothesis for Planet Formation
12
The Nebular Hypothesis

• proposed originally by Kant, Laplace, and others
  in the 1700's
• solar system formed from a nebula (cloud of
  interstellar gas) that evolved into a disk.
• initial nebula presumably looked like the
  molecular cloud cores in Galaxy M 16,.
• Properties of the pre-Solar Nebula
• Low density---102cm-3. Compare this to the
  density of air, 2x1019 cm-3.
• The minimum mass is a few times the mass of the
  sun.
• The material was well mixed (homogeneous).
• Solid material included interstellar grains
  (dust), nebular condensates, and diamonds.
• Chemical composition
• H and He 98
• C, N, O 1.33
• Ne 0.17
• Mg, Al, Si, S, Ca, Fe, Ni 0.365
• Initially Low Temperature---probably around
  50-100K (-200C).

13
Nebula

• Numerous regions of gas and dust dispersed in our
  galaxy
• Largest are known as giant molecular clouds and
  contain enough gas to make 100000 Sun masses
• Closest large molecular cloud is the Orion nebula
  (1500 light years away)
• Most of gas is H and He, but about one in every
  thousand atoms is heavier than He
• Virtually all atoms are combined in molecules (eg
  H2) (due to low T and relatively high density of
  atoms densest areas have about 107 atoms per
  cm3)
• Chemistry, density and temperature distribution
  of clouds is complex and varies between and
  within clouds
• Molecules detected inlcude H2, CO
• In coldest parts of clouds most of the molecules
  are bound in rocky-icy grains.
• Newly formed stars are known as T-Tauri stars
  (very bright)

14
The Collapse of a Nebula
15
The Accreation Disk
16

• Note ring-like structures within
• gas clouds (nebulae), surrounding
• a central proto-star (here in Orion Nebula)
• Stars older than a few Ma dont have proplyds,
  hence planets must form rapidly following
  proto-star formation. Gas giants may have formed
  quicker than terrestrial planets
• Planet formation in discs not
• actually observed, hence circumstellar disks is
  a better
• phrase
• Gaps in disks may indicate
• presence of planet
• Note planets may also form around old stars (eg
  binary systems were one has released dust and
  gas, that is captured by its companion, some
  planets also found around single red giants)

Proplyds
17
Proplyds

• Beta Pectoris
• 50 light years from sun
• Disk is 500 AU across
• 100 million years old

18
Planetary Formation
19
(No Transcript)
20
Grains to Planetesimals

• Condensation slow growth of grains, atom by
  atom by random collision
• Like snowflakes in a snow cloud

Dust grain, 1 micron
Very slowly
Atom, 0.0001 micron
21
Grains to Planetesimals

• Accretion Somewhat faster sticking of grains
• Condensation makes grains chemically sticky
• Friction generates static electricity

Dust grain, 10 microns 1 cm
Dust grain, 10 microns 1 cm
22
From Dust to Planetesimals

• The condensates take the form of (1 micron size)
  dust grains in the solar disk.
• These grains will settle to the disk midplane
  since they are heavier than the H and He gas.
  What happens next is uncertain.
• One possibility is that the thin disk of dust is
  gravitationally unstable, leading to the
  formation of roughly 1 kilometer size objects
  known as planetesimals.
• Another possibility is that the flow in the disk
  is turbulent, so that the dust cannot settle out
  and form an unstable thin disk. In this picture
  the dust grains collide with each other and stick
  to form slightly larger bodies, which in turn
  collide to form yet larger bodies. This picture
  suffers from the difficulty that bodies between
  the size of dust and planetesimals suffer the
  effects of drag, and so tend to spiral into the
  sun.

23
Planetesimals when 1km in Size
Cluster of planetesimals
24
Planetesimals

• Now sufficiently large that turbulent gas motions
  dont blow them in, out, round and about
• Gravity takes over
• Planetesimals free to sink into middle plane of
  disc
• Planetesimals gravitationally attract each other

25
Planetesimals to Protoplanets

• Clusters of planetesimals become
  self-gravitating
• Collisions are usually soft, since everything
  is co-rotating together
• Soft soil on surface allowed more sticking

26
Protoplanets Theory 1

• Evolutionary Differentiation
• Denser materials (iron, nickel) in center
• Lighter materials (silicates) toward surface
• Radioactivity heats, melts protoplanet, allowing
  differentiation

27
Protoplanets Theory 2

• In situ stratification
• The first planetesimals are mostly metals, while
  solar nebula is hot
• As nebula cools, lighter elements for
  planetesimals
• Melting, differentiation during formation

28
Planetary Formation

• Once the larger of these particles get big enough
  to have a nontrivial gravity, their growth
  accelerates.
• Their gravity pulls in more, smaller particles,
  and very quickly, the large objects have
  accumulated all of the solid matter close to
  their own orbit.
• The accretion of these "planetesimals" is
  believed to take a few hundred thousand to about
  twenty million years

29
Planets
30
Solar System Differentiation

• A wind of charged particles from star(s) now acts
  to erode disk from inside out (solar wind is a
  remnant of this).
• In solar system gas giants may have formed at
  this time, before most of gas of disk was blown
  away.

31
Ejection of Planetesimals

• Interaction of Jupiter and Saturn kicked out
  planetesimals to form Oort Cloud
• Interaction of Uranus and Neptune kicked out
  planetesimals to form Kuiper belt

32
Solar System Differentiation

• Formation of Condensates and differentiation
• The solar nebula was originally gas,
• as the density of the gas increased solid
  material began to condense out.
• The process is the inverse of sublimation, in
  which a solid such as ice goes directly to the
  gas phase (water vapor in this example).
• A solid formed by condensation is called a
  condensate.

33
Differentiation
Radial Position Temperature (K) Dominant Solid
1. 1700 Refractory minerals, (CaO, Al2O3 TiO)
2. 1470 Metals (Fe, Ni, Co, and their alloys)
3. 1450 Magnesium rich silicates
4. 1000 Alkali feldspars (silicates abundant in alkali elements (Na, K, Rb)
5. 700 Iron sulfide FeS (triolite)
6. 400 Fe condenses
7. 350 Hydrated minerals rich in calcium
8. 300 Hydrated minerals rich in Iron and Magnesium
9. 273 Water ice
10. 150 Other ices (NH3, H2O, etc)
Differentiation
34
Formation of Gas Giants
35
Planetary Differentiation

• Cold Homogeneous Accretion model
• Terrestrial planets accreted as a homogeneous
  masses of disk material
• Later differentiated by internal heating
• Heat supplied by
• A) Accretionary Heating
• Meteorite bombardment
• B) Core Formation and the Heat of
  Differentiation.
• Gravitational collapse and release of heat energy
• C) Radiogenic Heating
• Decay of radioactive elements

36

• Hot Heterogeneous Accretion
• Most scientists today prefer a model where large
  chunks of material, some of which were metal
  silicate, others predifferentiated as one or the
  other, violently coalesce to form the Earth.
• Metal sinks to the core due to negative buoyancy
  (silicate is hot enough to be plastic and
  "squishy" if not actually molten).
• Conditions are HOT volatile elements were lost
  to a significant degree.
• Based on
• Asteroids
• "unassembled" planets,
• already differentiated into chemically different
  types.
• Therefore, Protoplanetary material was already
  differentiated.

Planetary Differentiation
37
Planetary Differentiation
38
Extrasolar Planets
39
Extrasolar Planets

• Many other planetary systems have been discovered
  within the past 18 years.
• Cannot be imaged directly, since too far
• Indirect evidence
• Analysis of light from parent star
• Wobbling of star due to mass of planets causes
  Doppler shift
• Most extrasolar planets are Large (gt1 Jupiter
  mass)

40
51 Pegasi

• First extrasolar planet confirmed
• announced in late 1995 by astronomers studying 51
  Pegasi, a spectral type G2-3 V main-sequence star
  42 light-years from Earth.
• High-resolution spectrograph found that the
  star's line-of-sight velocity changes by some 70
  meters per second every 4.2 days (a doppler
  shift).
• planet lies only 7 million kilometers from 51
  Pegasi
• much closer than Mercury is to the Sun
• planet has a mass at least half that of Jupiter.
  
• temperature of about 1,000 degrees Celsius
• Probably lacking an atmosphere,
• planet may be a nearly molten ball of iron and
  rock with seven times the Earth's diameter and
  seven times its surface gravity.
• One side may permanently face the star, much as
  the Moon's does the Earth

41
51 Pegasi
42
Upsilon Andromedea

• 3 planet system
• Planet 1
• 0.7 Jupiter masses
• 0.06 AU orbit
• Planet 2
• 2.1 Jupiter masses
• 0.83 AU orbit
• Planet 3
• 4.3 Jupiter masses
• 2.6 AU orbit

43

• At last a planet has been confirmed with an orbit
  comparable to Jupiter's
• a distance of 5.5 AU from the star, (Jupiter's is
  5.2 AU).
• the 13 year orbit is slightly elliptical rather
  than round,
• the world is 3.5 to 5 times the mass of Jupiter
• the closest astronomers have come to date in
  finding a system that resembles our own.
• two other confirmed worlds in this system are
  shown as small dots of light to the left and
  right of the parent star.
• The innermost gas giant was discovered in 1996
  and has a 14.6-day orbit.
• The middle world orbits 55 Cancri in 44.3 days.

55 Cancri
44

• located 137 light years away
• in the constellation Orion.
• confirmed 0.77 Jupiter mass planet whips around
  its star in 14.3 days at an average distance of
  0.13 AU.
• There is evidence in the data that a second
  companion may exist farther out, shown here as a
  large ringed planet with three satellites.
• The moon close up has icy sheets and ridges
  similar to those found on Europa and a thin
  atmosphere

HD38529
45

• A second planet has been discovered orbiting
  Gliese 876, making it one of the most bizarre
  systems found to date.
• The two planets are eternally locked in sync,
  with periods of 60 and 30 days.
• Because of this 2-to-1 ratio, the inner planet
  goes around twice for each orbit of the outer
  one.
• They gravitationally shepherd one other to
  maintain this synchrony.
• A lunar landscape is shown at the bottom

Gliese 876
46
47 Ursa Majoris

• Inner planet has a period a little over three
  years (1100 days),
• mass about three times that of Jupiter,
• orbital radius about twice the Earth's distance
  from the Sun.