Transcript FORMATION OF THE SOLAR SYSTEM

FORMATION OF THE SOLAR
SYSTEM
We’ll concentrate on our planetary
system now. At the end of the
course we will discuss how we
find planets around other stars.
WHEN, WHAT, HOW?
• WHEN: AGES VIA RADIOACTIVE DECAY
• Many heavy elements have UNSTABLE NUCLEAR
ISOTOPES.
• Remember: isotopes of the same element have
different numbers of neutrons
• Such nuclei can FISSION into lighter ones.
• Common is loss of 4He nuclei -- alpha particles.
Other PARENT NUCLEI lose neutrons, protons,
electrons or positrons
• Typically the resulting DAUGHTER NUCLEI are still
unstable, so there is a DECAY CHAIN to a final,
STABLE NUCLEAR ISOTOPE.
Radioactive Potassium turns into Argon
• Relative
amounts of
different
isotopes can be
measured in
rocks and used
to date them
• Half-life of 40K is
1.25 billion years
Half-Life and Nucleochronometers
• N(t) = N0 2-t/t_half
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So, 1/2 gone after 1 half-life; 3/4 gone after 2 halflives; 7/8 gone after 3 half-lives, etc.
Important NUCLEOCOSMOCHRONOMETERS:
235U231Th207Pb (7.3x108yr),
187Re187Os
238U234Th206Pb (4.5x109yr)
40K40Ar
These lead to oldest:
-earth rocks of 3.9 Gyr
--moon rocks of 4.4 Gyr
--meteorites of 4.55 Gyr.
THE AGE OF THE SOLAR SYSTEM IS 4.55 Gyr
This value agrees with calculations for the evolution
of a star with the mass and composition of the Sun.
Practice Question Answers
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True: a CCD is more linear and preferred over film.
False: Jupiter is 11.2 Earth radii but 318 Earth
masses, not about 100.
True: oldest rocks on Earth ~4 Gyr, oldest on moon
from Apollo ~4.4 Gyr
False: While most large telescopes are reflectors,
they spend most of their time taking spectra, not
pictures.
False: The earth’s magnetic field is generated in its
liquid outer core. (Mantle is rocky and plastic.)
False: Twice the wavelength means 1/2 the energy:
E = hf = hc/
True: liquids, solids & dense gases give continuum
thermal spectrum
More practice answers
8.
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True: this situation is stimulated emission
B: 293 K is earth or room temp. and thermal emission peaks @
m = 0.29cm-K / 293 K = 2.910-1 cm / 2. 9102 = 1.010-3 cm
= 1.010-5 m = 10 m which is in the IR, but some is emitted at
every wavelength
D: 500 atoms after 1 half-life of 30 yrs, 250 after 2, 125 after
three half-lives, or 90 years
E: angular momentum conservation means flattening, gravity
means condensation, collisions meant extra flattening as
vertical energy is lost.
A: getting above atmosphere means less turbulence and less
absorption by water vapor
D: LA/LB=(RA/RB)2(TA/TB)4= 22(1/2)4=4/16=1/4
WHAT MUST BE EXPLAINED?
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99.9% of SS MASS in the SUN
99% of SS ANGULAR MOMENTUM in the Planets
PLANETS are relatively ISOLATED
ECCENTRICITIES of planetary ORBITS are SMALL
INCLINATIONS also SMALL
Planetary ORBITS are ALL PROGRADE
MOST planetary SPINS are PROGRADE
MOST BIG MOONS have PROGRADE, nearly
EQUATORIAL ORBITS
TERRESTRIAL vs. JOVIAN DICHOTOMY
ASTEROIDS are like pieces of planets, but primitive
The KUIPER BELT has asteroid sized, but icy bodies
COMETS are primitive, icy & DISTANT (Oort cloud)
HOW CAN THIS BE UNDERSTOOD?
CONDENSATION WITHIN A NEBULA!
• Start with a COLLAPSING, ROTATING CLOUD of GAS
and DUST: a piece of a MOLECULAR CLOUD
• Gravity pulls in faster along rotation axis; angular
momentum fights collapse in the equatorial plane.
• PROTOSTAR forms at the center of a flattened nebula;
center gets most mass, little of the angular momentum.
• Dust grains stick, settle towards disk plane.
• Both Size (adhesion) and Gravity enforce pre-planetary
Republicanism: the rich (big) get richer (bigger)
• PLANETESIMALS grow via ACCRETION of gas and
dust; have eccentric and somewhat inclined orbits
• Then they COLLIDE: typically bigger ones grow, smaller
are smashed; some of these fragments are accreted
later, others escape: disk flattens more
Summary:
Details to
Follow
Stars Form in Molecular Clouds
• Large scale recycling: gas and dust clouds collapse to
form stars (and planets)
• Old stars lose eject of their gas into the interstellar
medium from which new stars (as in Orion nebula) form
Condensation in a NEBULA
Naturally Explains:
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More mass to the center (Sun)
More angular momentum to the outside
Gaps between planets larger further from the Sun
Prograde orbits, low inclinations and low
eccentricities for planets
• Collisions USUALLY give prograde spins (but can
give exceptions, like Venus and Uranus)
• Mini-solar systems with disks around Jovian planets
producing moons near equatorial planes
Evidence for Flattened Nebulae
around Young Stars
• HST pictures of nearby stars with flattened rotating disks
probably en route to making planetary systems
• Protoplanetary Disk Formation
Materials in the Solar Nebula
• Hydrogen and Helium: 98% -- gases that
never condense to solids
• Hydrogen compounds: 1.4%-- form ices at
T < 150 K (H2O, CH4, NH3)
• Rock: 0.4%--condense to solids between
500K and 1300K depending on mineral
chemistry (SiO2, Al2O3, Mg2SiO4, etc.)
• Metal: 0.2%--Fe, Ni, Al, etc. condense to
solids at very high T: 1000--1600 K (most
refractory)
Temperatures
in the Solar
Nebula
• Only metals survive in
the hottest part
• Then rocks can live in
the areas where
terrestrial planets form
• Different ices can
condense at different
cold temperatures,
making up much of
outer solar system
Temperature Gradient Yields
Differentiation
• Hottest near new star; only refractory elements
condense there
Inner cores stay rocky with metals
Outer cores accrete more gas with prograde
spin: yields many outer planet moons
• T Tauri wind blows gas from inner SS
 INNER and OUTER SS differences
• Also roughly gives moon distributions & orbits
To some degree, explains nature and location
of asteroids and comets
Frost Line is Key to Difference
between Terrestrial & Jovian Planets
Temperature distribution
applet
• Outside: hydrogen
compounds can
condense along w/ rocks
and metals and planets
can grow big
• Inside: only rocks and
metals can condense so
planets stay small
• Biggest cores can attract
some of the H and He
gas via gravity
Terrestrial Planet Formation
• Dust grains sink to disk plane, can stick to each other (cm-m)
• Slow collisions cause growth via more surface & more gravity;
orbits are altered and cross more
• Randomly some grow much bigger, then: Disk Flattening
T Tauri Phase and Inner Planet Formation
Outer Planet Formation
• Beyond the ice line rocky cores would form too, but
• These cores would collect mass from larger volumes
so they could grow bigger
• Their gravity could be strong enough to also attract
ices and some (Uranus, Neptune) or a lot (Jupiter,
Saturn) gas to them
• The clouds of gas attracted to them had a lot of
angular momentum and could flatten to form disks
that fragmented into the bigger moons -like mini-solar systems
• Smaller moons are usually captured asteroids
Alternative Model: Possible
Outer Planet Formation
from Direct Disk Instabilities
Debris Ejection to Kuiper Belt
and Oort Cloud explains
asteroid & comet locations
Important Exceptions to Rules:
Peculiar Spins and Tilts
• Most planets have
prograde spins but Venus
doesn’t
• Most have spin axis
nearly perpendicular to
orbital plane but Uranus
doesn’t
• Major collisions were
much more common in
the first 108 yr of SS life
• They could cause angular
momentum oddities like
these
Very Important Exception: Our Big Moon
• Our Moon’s diameter = 0.27 Earth’s
• Our Moon’s mass = Earth’s/81
• Almost certainly formed via a massive rocky
planetesimal striking earth after metals sunk to center:
only hypothesis that explains lower density but big size
Summary
in a
Figure
Summary: Time-Line of SS Formation
Peer Instruction Question
• Planet Goola has a mass 8 times that of
the Earth and a radius 2 times that of
the Earth. Goola is most likely to be
• 1) a terrestrial planet
• 2) a jovian planet
• 3) an asteroid
• 4) One can’t really tell from this
information
Peer Instruction Question
• Planet Goola has a mass 8 times that of
the Earth and a radius 2 times that of
the Earth. Goola is most likely to be
• 1) a terrestrial planet
• 2) a jovian planet
• 3) an asteroid
• 4) One can’t really tell from this
information