Transcript Nebular Hypothesis

Astronomy 251
Life on Other Worlds
Mondays 4-6, MP102
Prof. Ray Jayawardhana
MP 1408
416 946 7291
[email protected]
Office hours: Tuesdays 11-12 or by appt.
Course website and syllabus:
http://www.astro.utoronto.ca/~rayjay/ast251
Course Textbook
Goldsmith & Owen
The Search for Life in the Universe
READING 1st WEEK:
Ch. 1, 2 & 7 (p. 163-166)
READING 2nd WEEK:
Ch. 6 & 3
READING 3rd WEEK:
Ch. 4 & 5
READING THIS WEEK:
Ch. 11 & 17
Astronomy 251
Who is my TA?
Parandis Khavari
Duy Nguyen
Tim Rothwell
Marija Stankovic
khavari@astro
nguyen@astro
rothwell@astro
stankovic@astro
A-C
D-La
Le-R
S-Z
Upcoming Dates
Sample paper topics available on website
TA consultation sessions (for paper topics):
Tue Jan 25
3-4pm McLennan 15th floor conference room
Fri Jan 28
12-1pm McLennan 15th floor conference room
Paper topics due in class:
Mon Jan 31 ***NEXT CLASS***
Topics approved by Feb 7; Modifications finalized by Feb 14
Paper outlines due in class:
Mon Feb 21
Midterm test:
Mon Feb 7 4pm
Sidney Smith Hall 2102
Sidney Smith Hall 2118
A-La
Le-Z
Midterm review sessions by TAs:
Tue Feb 1
3-4pm McLennan 15th floor conference room
Fri Feb 4
12-1pm McLennan 15th floor conference room
Instructions for Paper Topics
Turn in topic on paper (typed) in class on (or before) Jan 31
DO NOT SUBMIT PAPER TOPIC BY EMAIL
Must include
1. Your name and student ID
2. Email address
3. Proposed paper topic
4. Thesis statement
(e.g., Past life on Mars)
(e.g., There is good evidence that
bacterial life existed on Mars in the past.)
5. Two or three sample references (e.g., book title & author, title & author
of an article in Scientific American, a web
site)
Response from TAs by Feb 7 (at midterm); any final modifications must be done
by Feb 14.
10% penalty for each day’s delay after Jan 31; zero after one week’s delay
One letter grade deduction for not handing in paper topic
Another letter grade deduction for handing in paper outline (Feb 21)
Major Solar Flare Last Thursday
Huygens on Titan
Mosaic of river channel and
ridge area
Evidence of water ice
and methane springs
Huygens on Titan
Titan’s varied terrain:
(lighter-colored uplifted areas, marked with what appear
to be drainage channels, and darker lower areas)
Saturn now visible (on clear nights!)
http://SkyandTelescope.com
25 Msun
1 Msun
Points to Remember
1. Stars are in a state of hydrostatic balance between
pressure and gravity
2. Without a source of energy, they would contract. The
Sun would contract significantly in about 15 Myr.
3. Nuclear fusion of H to He, then on to heavier
elements, creates new heat to sustain a star in
energetic balance.
4. Elements with more protons require higher
core temperatures to undergo fusion
5. This only works up to 56Fe – heavier elements require
energy to create and give off energy only in fission
Points to Remember
6. Stars are created in clusters. There are more stars
like the Sun (G and M stars) than massive (O and B)
stars.
7. More massive stars are much more luminous, so they
live much shorter lives.
8. Stars more massive than 8 solar masses explode as
supernovae.
9. Supernovae draw energy from gravity to create the
elements heavier than 56Fe.
10. Stars get slightly brighter on the main sequence and
then much brighter and bigger as red giant stars.
AST 251
Life on Other Worlds
Lecture 4
Formation of Stars and Planets
Basic properties of the solar system
Historical ideas on the origin of the solar System
Modern theories and observations
Planet fact sheet – 1
Orbits and rotation
•All planets orbit in the same plane and in the same direction
as the Sun’s rotation (prograde).
•6 of the planets also rotate in that plane and direction
(prograde). The larger, close-in moons all do, too.
•In our Solar System, the orbits of eight (!) planets are nearly
circular. So are the orbits of the large, close-in moons.
•The mass of the Solar System is concentrated in the Sun
(99.95%), but the angular momentum is mainly (98%) in the
orbits of Jupiter and Saturn (mainly Jupiter).
Planet fact sheet – 2
Solar System Inhabitants
Inner (Terrestrial) Planets: Mercury, Venus, Earth, Mars
Composition: Dense, refractory (highly depleted of volatiles); otherwise, similar to
Solar composition.
Density: about 5-10 times that of water
Asteroid Belt: small rocky bodies between Mars & Jupiter
Gas Giant Planets: Jupiter, Saturn
Composition: almost the same as the Sun’s; somewhat depleted of H, He
Density: about that of water (same as the Sun). Mass: 1/1000 of the Sun’s.
Ice Giant Planets: Uranus, Neptune
Composition: Significantly depleted of H, He (just 5-20% by mass)
Density: a couple times water
Kuiper Belt: A flattened disk of asteroids, of which Pluto is considered the largest
Comets: a trillion of ‘em, in the (spherical) Oort cloud, extending 1/20 parsec away
Why are the planets in such a flat disk?
This question spawned two competing theories…
Where did the planets come from?
The Encounter Hypothesis (Georges Buffon, 1745)
The Sun was pulled apart by a nearby star, and the stream of gas
condensed to form the planets
Implications for Drake’s equation:
Planetary systems are very rare (fp<<1), because stellar collisions
essentially never happen (outside of globular clusters and the centre
of the Galaxy).
The Nebular Hypothesis (Immanuel Kant 1755; Laplace, 1796)
The Sun and planets formed together out of a swirling cloud of gas
(the “Solar Nebula”)
Implications for Drake’s equation:
Planetary systems can be quite common (fp~1).
Sun
Other star
Sketch of the Encounter Hypothesis
Tidal stream
Condensation into planets
Where did the planets come from?
The Encounter Hypothesis (Georges Buffon, 1745)
The Sun was pulled apart by a nearby star, and the stream of gas
condensed to form the planets
Implications for Drake’s equation:
Planetary systems are very rare (fp<<1), because stellar collisions
essentially never happen (outside of globular clusters and the centre
of the Galaxy).
The Nebular Hypothesis (Immanuel Kant 1755; Pierre-Simon Laplace,1796)
The Sun and planets formed together out of a swirling cloud of gas
(the “Solar Nebula”)
Implications for Drake’s equation:
Planetary systems can be quite common (fp~1).
Sketch of the Nebular Hypothesis
Gas cloud
Formation of planets
Condensation to
disk
The Nebular Hypothesis – Why flattened disks?
As gas clouds evolve, their fate is determined by
the conservation of angular momentum.
1. The cloud begins in a state of slow (even imperceptible)
rotation.
2. It contracts, and it radiates away heat (recall Kelvin’s theory
for the contraction of the Sun).
3. However, it tends to keep its angular momentum, because
this changes only if there is a torque on the cloud from
outside.
4. For the same angular momentum, the cloud
must spin faster as it contracts
5. Flattening is the result of angular
momentum conservation
Two major views of the Nebular Hypothesis
Geologists have drawn evidence primarily from the mineral and isotope
ratios in the Earth and in meteorites. They posit a hot primordial nebula
from which the minerals condensed as it cooled.
Astronomers have only had a good picture of star formation since the
advent of infrared astronomy, in the past few decades. They posit the
accretion of cold gas onto the star and its disk; so that the gas remains
cold unless it is close to the forming Sun. In their model, minerals and
ices are inherited from the interstellar material and get chemically
processed depending on location in the Nebula.
In the astronomical model, the protoplanetary disk (Solar Nebula) is the
remnant of the protostellar disk through which the Sun accreted much of
its material.
Important concept: the
Minimum Solar Nebula
The Minimum Solar Nebula is what you get if you take the
material that’s currently in the planets, and then add enough
volatile materials (mainly H & He) to “reconstitute” a gas disk
with the same composition as the Sun.
This is considered to be a lower limit to how much material
could have been in the Solar Nebula (but this assumes that the
planets did not move much after they formed).
The Minimum Solar Nebula has about 1% the mass of the
Sun. At Earth’s location, it has about 1 kilogram per square
centimeter – so it is roughly as “thick” as the Earth’s current
atmosphere.
Exercise 1
In what ways are massive stars (in
contrast to low mass stars) important for
life in the universe?
Exercise 2
Please explain why most stars are seen in the
strip of the Hertzprung-Russell Diagram known
as the “Main Sequence”.
Establishing the life history of a meteorite
using radioactive decays
time
What can be learned about the life history of a meteorite
– Remnants of the Solar System’s formative epoch –
1a. When it solidified from a liquid
1b. When it cooled to a temperature comparable to the Earth’s mantle
… from light, noble-gas daughters that leak away while it’s hot
The cooling rate indicates how big the parent body was.
2. The magnetic field there was in when it cooled
… is recorded in magnetic minerals
3. How long it spent floating around in space
… from radioactive isotopes created by cosmic-ray impact
4. When it fell to Earth
… from the decline of cosmic-ray isotopes after it’s protected by
the Earth’s atmosphere
Types of Meteorites
Differentiated Meteorites are broken out of the interiors of larger bodies
(planetesimals) whose insides melted because they were large enough
to retain heat (from radioactivity). Examples: Stony and iron-nickel
meteorites
Primitive Meteorites have not been inside a molten body. Many
primitive meteorites are carbonaceous chondrites – consisting of
chondrules within an organic substrate.
•Chondrules are 1-mm spheres that were briefly melted to about 2000 K
and cooled (in a region of high magnetic field). They are depleted of
volatiles.
•The organic substrate is material that
never got heated to the temperatures
that made the chondrules.
Ages of Meteorites
Differentiated meteorites
4.4 – 4.56 Gyr
Primitive meteorites
4.56±0.02 Gyr
The Condensation Sequence in the Geological Nebular Theory
Substance
1760 K
Al203
Corundum
1510 K
Temp.
Substance
550-330 K
Hydrated
Minerals
MgAl204
Spinel
173 K
H2O
Water ice
1470 K
Ni-Fe Metal
120 K
NH3H2O
Ammonia ice
1440 K
(Mg,Fe)2SiO4
Olivine
70 K
CH46H2O
Methane Ice
1000 K
(Na,K) AlSi3O8
Alkali
Feldspars
70 K
N26H2O
Nitrogen ice
The Condensation Sequence indicates which substances
can exist (or might form) below each temperature.
Volatile
Refractory
Temp.
The astronomical view of the Nebular Hypothesis
1. Stars are born in relatively dense “molecular clouds”
The astronomical view of the Nebular Hypothesis
2. The cloud clumps collapse (contract) to form stars and disks
Orion Disks in Silhouette
Edge-on disks in Taurus
The astronomical view of the Nebular Hypothesis
3. Within the dark, dense regions of molecular clouds, the shieldingout of ultraviolet radiation (by dust) allows relatively complex
molecules – including organics like sugars and amino acids – to be
created. These fall into the Solar nebula and, since it remains
relatively cold, are not destroyed (if they stay a few AU out).
How do planets form out of the disk?
1. Direct gravitational collapse
This occurs if the disk gets dense enough that its own gravity pulls
it together faster than its orbit can shear it apart --> portions of the
disk collapse under their own gravity to make giant planets directly
Pro:
Con:
Giant planets can form quickly (thousands of years)
Most observed disks don’t seem to be dense enough
How do planets form out of the disk?
2. Planetesimal accretion Model
In this theory, dust grains combine to form pebble-size objects,
which combine to form rocks, then boulders, then up to planets like
the Earth.
Pro: Both terrestrial and gas giant planets form the same way
Con: Takes a while to make Jupiter (~5-10 million years): will there be
enough gas in the disk for that long?
© William K. Hartmann
Runaway growth of kilometer-sized planetesimals
Gravitational Focusing:
Once a planetesimal grows to mountain sizes (1 km), it begins to pull other
objects toward it as they pass by – increasing its “feeding zone” beyond its
own size.
Smaller than 1 km
Larger than 1 km
This leads to a runaway growth in which the largest body eats all its neighbors.
This continues until its feeding zone is cleared of other objects (when the object
is about the size of the Moon)… at which point growth slows down a lot, and
relies on objects getting “kicked” into the feeding zone.
The next big step occurs if it exceeds 15 Earth masses and gas is still around
Forming Giant Planets
When the planet mass exceeds about 15 Earth masses, it can begin to
accrete gas as well as rocks. (Why? Then, its escape velocity exceeds
the speed of gas particles.)
This triggers another stage of
runaway growth until the planet
has opened a gap in the disk.
Open questions:
1. Does the gas disk last long enough for the planets to get this big?
2. Once the planet gets big enough to affect the gas disk, it is rapidly
shoved inward by the disk. How can giant planets ever stay put rather
than being eaten by their stars?
Planetesimals to Planets: Consequences
Location Matters: Since planets are built up from solids in the disk, it
matters a lot what solids exist at which location. Inside Mercury’s orbit, even
refractory grains are evaporated. From Jupiter’s orbit outward, ices exist in
the nebula.
Planetesimals to Planets: Consequences
The last stages of planet growth involve some very serious collisions…
•The Moon is made of the same material as Earth’s mantle, and has no core.
A popular explanation is that the Moon formed from material ripped out of
the Earth by a collision with a planet somewhat smaller than Mars.
•Mercury is a lot like the iron-nickel core of a planet that has no silicate
mantle. Does this mean its mantle was stripped away in a collision?
•The collisions that built the planets are still with us, but much rarer now than
in the early Solar System.
[The current impact rate is not sufficient to create the cratering seen on the
Moon in the age of the Solar System.]
There was a phase of heavy bombardment in our first 80 Myr.
Observational clues to planet formation
0.5 Myr old star
2 Myr old star
Gemini North Telescope, Hawaii
Disk around 10-Myr-old star
Cerro Tololo Inter-American
Observatory,
Chile
James Clerk Maxwell Telescope,
Hawaii
Disk around 500-Myr-old star
Solar System
Epsilon Eridani
Atacama Large Millimeter Array
(now being built in Chile)
64 12-meter antennae with 10-km baseline