Transcript Document

ASTA01 @ UTSC – Lecture 16
Chapter 12. Origins. Extrasolar Systems
Giant impact epoch
Solar system: Theory vs. observations
Dusty disks in other planetary systems:
Beta Pictoris and other system
Extrasolar planet discovery:
- Pulsar planets
- wobble method (radial velocity)
- transit (occultation, eclipse) method
-examples and statistics
1
Continuing Bombardment of the Planets
• Astronomers have good reason to believe
that comets and asteroids can hit not only
Earth but other planets as well.
Continuing Bombardment of the Planets
• this bombardment represents the slow
continuation of the accretion of the
planets.
• Earth’s moon, Mercury, Venus, Mars, and
most of the moons in the solar system are
covered with craters.
Continuing Bombardment of the Planets
• Most of the craters you see appear to
have been formed roughly
4 billion years ago as the last of the debris
in the solar nebula was swept up by the
planets.
• This is called the heavy bombardment.
4
Continuing Bombardment of the Planets
• 65 million years ago, at the end of the
Cretaceous period, over 75% of the
species on Earth, especially many large
animals like the dinosaurs and
pterodactyls, went extinct over a short
period of time (less than 1000 years)
• Other earlier extinctions also were defining the boundaries
of geological epochs
5
Cretacious-Tertiary extinction
• Scientists have found a thin layer of clay
all over the world that was laid down at
that time.
• It is rich in the element Iridium – common in
meteorites, but rare in Earth’s crust.
• This suggests that a large impact altered Earth’s
climate and caused the worldwide extinction.
6
Continuing Bombardment of the Planets
• Alvarez, L. W., et al. 1980. Extraterrestrial
cause for the Cretaceous-Tertiary extinction.
Science 208: 1095-1108.
- in this paper the theory and observations were
proposed for an asteroid impactor
Mathematical models indicate that a major
impact would eject huge amounts of
pulverized/red hot rock high above the
atmosphere.
7
Continuing Bombardment of the Planets
• As this material fell back, Earth’s
atmosphere would be turned into a
glowing oven of red-hot meteorites
streaming through the air.
• This heat would trigger massive forest fires
around the world.
• Soot from such fires has been found in the final
Cretaceous clay layers.
8
Continuing Bombardment of the Planets
• Once the firestorms are cooled, the
remaining dust in the atmosphere would
block sunlight and produce deep darkness
for a year or more, killing off most plant life.
(so-called Nuclear winter model, since
similar effect would result from nuclear war)
• Other effects, such as acid rain and
enormous tsunamis (tidal waves), are also
predicted by the models.
9
Continuing Bombardment of the Planets
• Geologists have located a crater at least 150 km in
diameter centred near the village of Chicxulub in the
northern Yucatán region of Mexico.
Continuing Bombardment of the Planets
• Although the crater is completely covered by sediments,
mineral samples show that it contains shocked quartz typical
of impact sites and that it is the right age.
Continuing Bombardment of the Planets
• The impact of an object 10 to 14 km in
diameter formed the crater about 65 million
years ago, just when the dinosaurs and many
other species died out.
• Most Earth scientists now believe that this is the
scar of the impact that ended the Cretaceous
period.
• We still don’t fully know why some animal families
survived while other died.
• It took 3 Myr for marine ecosystems to recover
12
Continuing Bombardment of the Planets: SL9
• Earthlings watched in awe during six days
in the summer of 1994 as 20 or more
fragments from the head of comet
Shoemaker-Levy 9 slammed into Jupiter.
• This produced impacts equalling millions of
megatons of TNT.
• On Earth it would cause an almost C-T
(cretecious-tertiary) type extinction
13
Continuing Bombardment of the Planets: SL 9
• Each impact created a fireball of hot gases and left
behind dark smudges that remained visible for months
afterward.
14
Continuing Bombardment of the Planets: SL 9
15
Continuing Bombardment of the Planets
• The chance that a major impact will occur
during your lifetime is so small that it is hard
to estimate
• However, the consequences of such an
impact are so severe that humanity should
be preparing.
• One way to prepare is to find those NEOs (Near
Earth Objects) that could hit this planet, map
their orbits in detail, and identify any that are
dangerous.
• E.g., project Spacewatch
16
Explaining the Characteristics of the Solar System
• Now, you have learnt enough to put all the
pieces of the puzzle together and explain
the distinguishing characteristics of the
solar system in the table.
Explaining the Characteristics of the Solar System
1. The orbits of the planets lie in the same plane because
the rotating solar nebula collapsed into a disk, and the
planets formed in that disk. Objects are co-eval (4.)
2. The division into small inner and giant outer planets rests
upon the amount of solid material (mainly water ice and
silicate rock) available in the inner/outer part of the disk.
• There was more material beyond ice boundary
• There was not enough material near the Earth for
accretion of planets to proceed to the final stage quickly,
in the runaway mode. Near Jupiter, protoplanets of
several Earth masses could form quickly, before the gas
dissipated. Reaching 10 Earth masses triggered gas flow.
Explaining the Characteristics of the Solar System
• The solar nebula hypothesis calls on
continuing evolutionary processes to
gradually build the planets.
• Scientists call this type of explanation an
evolutionary theory.
• In contrast, a catastrophic theory invokes special,
sudden, even violent, events.
20
Explaining the Characteristics of the Solar System
• Uranus rotates on its side and Venus
rotates backward.
• Both these peculiarities could have been
caused by off-centre impacts of massive
planetesimals [when] they were forming.
• This is an explanation of the catastrophic type.
21
Explaining the Characteristics of the Solar System
• Similarly, Earth’s unusually large moon
could also be a result of a giant impact
that stripped debris from the Earth, which
accreted to form the Moon.
22
Explaining the Characteristics of the Solar System
• The heat of formation – the energy
released by infalling matter – was
tremendous for all planets.
• The Earth had an ocean of lava
• Jupiter must have grown hot enough to
glow with a luminosity of about 1 percent
that of the present sun.
23
Explaining the Characteristics of the Solar System
• However, because it never got hot enough to start
nuclear fusion as a star would, it never generated
its own energy though fusion of hydrogen into
helium.
• Jupiter is still hot inside.
• In fact, both Jupiter and Saturn radiate more
heat than they absorb from the Sun.
• So, they are evidently still cooling.
24
Explaining the Characteristics of the Solar System
• A glance at the solar system suggests that
you should expect to find a planet between
Mars and Jupiter at the present location of
the asteroid belt. But it never formed there!
Explaining the Characteristics of the Solar System
• The bodies that should have formed
a planet between Mars and Jupiter were
broken up, thrown into the Sun,
or ejected from the solar system.
• This was due to the gravitational influence
of massive Jupiter which formed first.
• It induced high, destructive, collision speeds
• This arrested the accumulation process
26
Explaining the Characteristics of the Solar System
3. All 4 Jovian worlds have ring systems.
• You can understand this by considering
the large mass of these worlds and their
remote location in the solar system.
• A large mass makes it easier for a planet
to hold onto orbiting ring particles.
• Also, being farther from the Sun, the ring
particles are not as easily swept away by
the pressure of sunlight and the solar
wind.
Explaining the Characteristics of the Solar Systems
• Terrestrial planets – low-mass worlds located
near the Sun – have no planetary rings. They
are dynamically unstable due to closeness to
the sun.
• New research shows that we should not expect
to find rings around ‘hot jupiter’ exoplanets
• Probably no moons either
28
Growth of Protoplanets
• Icy planetesimals have formed in the outer parts of
the solar nebula.
• They have been scattered by encounters with
the Jovian planets.
• Most escaped from the heliocentric orbit and now
wander through the Galaxy
• Some remained barely attached as Oort cloud of
comets
• Others were never strongly scattered and now form
the Kuiper belt of comet-like bodies
29
Debris Disks: Other planetary systems
• What you have learnt about the solar
system: the formation of Kuiper belt of icy
planetesimals, collisions between bodies,
production of meteoroids and dust
has now been observationally confirmed in
other planetary systems!
• The story of their discovery precedes the
discovery of exoplanets. We have first found
the disks of dust, derived from planetesimal
collisions, and later full-fledged planets.
30
Debris Disks
• Infrared astronomers in 1984 have accidentally
spotted very cold, low-density dust disks around
stars such as Vega, beta Pictoris, and epsilon
Eridani.
• Although much younger than the Sun, these
stars are on the main sequence and have
completed their formation, sometime long time ago.
They are usually 20-200 Myr old.
• So, they are clearly in a later stage than the
newborn stars in Orion.
31
1993
1984
Beta Pic: comparison of visible and IR data yields a high
brightness or reflectivity (albedo), like Saturn’s rings.
However, the particles are olivines & pyroxenes, not ice.
32
33
Debris Disks = exo-zodiacal disks = dusty disks
• These low-density disks dominated by dust not gas
generally have innermost zones with even lowerdensity places where planets may have formed.
β Pic
ε Eri
34
Debris Disks = exo-zodiacal disks = dusty disks
• Such tenuous dust disks are sometimes called
debris disks.
• This is because they are understood to be debris
released in collisions and evaporation among
small bodies such as comets and asteroids.
35
Debris Disks = exo-zodiacal disks = dusty disks
• Zodiacal light: sunlight scattered by interplanetary dust
particles (IDP) in our solar system
36
Debris Disks: Kuiper belt
• Astronomers believe the Sun has an extensive
debris disk of cold dust extending far beyond
the orbits of the planets
•
Gerard P. Kuiper (1905-1973)
But the Kuiper belt dust has not
yet been detected
37
Debris Disks
• Notice the difference between the two kinds of
disks that astronomers have found.
• The low-density dust disks such as the one
around Beta Pictoris are produced by dust from
collisions among comets, asteroids, and Kuiper
belt objects.
• Such disks are evidence that planetary systems
have already formed (age = 10-1000 Myr)
• The dense disks of gas and dust such as those
seen around the stars in Orion are sites where
planets could be forming right now (age < 10 Myr)
38
FEB = Falling Evaporating Bodies in Beta Pictoris
FEB
star
H & K calcium absorption lines
absorption line(s)39
that
move on the time scale
Beta Pictoris: IR disk and how the sky would look like
Infrared image analysis
(Lagage & Pantin 1994)
40
Debris Disks
• If planetesimals are there, then you can
expect that there are also planets orbiting
those stars.
• A planet is a 1000+ km body which clears
a space where it orbits a star.
• Many of the debris disks have details of
structure and shape that are probably caused
by the gravity of planets orbiting within or at
the edges of the debris.
41
HD 1415969
Observations by Hubble Space Telescope
(NICMOS near-IR camera).
Age ~ 5 Myr,
a transitional disk
Gap-opening PLANET ?
So far out?
Only if migrated outward
R_gap~350Ad
R ~ 0.1 R_gap
42
HD 14169A disk gap confirmed by new observations
(HST/ACS)
43
Alpha Pisces
Austrini
(α PsA)
Fomalhaut
A disk of a
bright
southern star
44
45