Transcript SES4U ~ The Formation of Our Solar Systemstudentcopy

In a dense molecular cloud far, far away
Hot, ionized gas
M16: The Eagle
Nebula
Young stars
Dense Molecular gas
Molecules and dust
1. Allow efficient cooling of
gas
Ionizing radiation 2. “Self-shield” from ionizing
radiation
3. Act as a “Refrigerator”
Bright star
Ionized
Gas
Ionized
Gas
“Evaporating”
molecular gas
Dust grain absorbs visible/UV, re-radiates infrared,
which escapes
Dusty Molecular Gas
Excited molecule releases energy
Locally denser clouds survive, cool
Distinct Protostellar Nebula
1000 AU
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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.
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initial nebula presumably looked like the molecular cloud cores in Galaxy M 16,.
Properties of the pre-Solar Nebula
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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:
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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).
• 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)
•Note ring-like structures within
gas clouds (nebulae), surrounding
a central proto-star (here in Orion
Nebula)
•Stars older than a few Ma don’t 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)
• Beta Pectoris
– ~50 light years from sun
– Disk is 500 AU across
– ~100 million years old
• 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
• 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
• 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.
Cluster of planetesimals
• Now sufficiently large that turbulent gas
motions don’t blow them in, out, ‘round
and about
• Gravity takes over
– Planetesimals free to sink into middle plane of
disc
– Planetesimals gravitationally attract each
other
• Clusters of
planetesimals become
self-gravitating
• Collisions are
usually soft, since
everything is corotating together
• Soft “soil” on surface
allowed more
“sticking”
Evolutionary
Differentiation
– Denser materials
(iron, nickel) in center
– Lighter materials
(silicates) toward
surface
• Radioactivity heats,
melts protoplanet,
allowing
differentiation
“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
• 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
• 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.
• Interaction of Jupiter
and Saturn kicked out
planetesimals to form
Oort Cloud
• Interaction of Uranus
and Neptune kicked
out planetesimals to
form Kuiper belt
• 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.
Differentiation
Radial
Position
Temperature
(K)
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)
Dominant Solid
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
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.
• 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 (>1 Jupiter mass)
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First extrasolar planet confirmed
announced in late 1995 by astronomers studying
51 Pegasi, a spectral type G2-3 V mainsequence 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
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
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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.
• 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
• 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
• 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.