ASTA01 @ UTSC – Lecture 12
Chapter 12
The Origin of the Solar System
-Architecture of the solar system
- Planets
-Asteroids
-Comets
-Meteors and meteoroids
1
A Survey of the Planets
• To explore consequences of the solar
nebula theory, astronomers search the
present solar system for evidence of its
past.
• You should begin with the most general view
of the solar system.
• It is almost entirely empty space.
2
A Survey of the Planets
• Imagine that you reduce the solar system
until Earth is the size of a grain of table
salt – about 0.3 mm in diameter.
• The Sun is the size of a small plum 4 m from
Earth.
• Jupiter is an apple seed 20 m from the Sun.
• Neptune, at the edge of the solar system, is a
large grain of sand located 120 m from the
central plum.
3
A Survey of the Planets
• You can see from the rescaled models of
the solar system that planets are tiny
specks of matter scattered around the
Sun, the last significant remains of the
solar nebula.
4
Revolution and Rotation
• The planets revolve around the Sun in
orbits that lie close to a common plane.
• The orbit around the Sun of Mercury, the
planet closest to the Sun, is tipped 7.0°to
Earth’s orbit.
• The rest of the planets’ orbital planes are inclined
by no more than 3.4°.
• The solar system is basically ‘flat’ and
disk-shaped.
5
Revolution and Rotation
• The rotation of the Sun and planets on their
axes also seems related to the same overall
direction of motion.
• The Sun rotates with its equator inclined 7.2° to ecliptic
• Most planets’ equators are tipped less than 30°.
6
Revolution and Rotation
• However, the rotations of Venus and Uranus
are peculiar.
• Compared with the other planets, Venus rotates backward.
• Uranus rotates on its sides, with the equator almost
perpendicular to its orbit.
7
Revolution and Rotation
• The preferred direction of motion in the
solar system (counterclockwise as seen
from the north) is related to the rotation of
a disk of material that became the planets.
• All the planets revolve around the Sun in that
direction.
8
Revolution and Rotation
• Furthermore, nearly all the moons in the solar
system, including Earth’s moon, orbit around
their planets counterclockwise.
• With only a few exceptions, most of which are
understood, revolution and rotation in the solar
system follow a common theme.
Triton
Neptune & Triton
9
Two Kinds of Planets
1. Four small Earth-like worlds, called terrestrial planets
10
Two Kinds of Planets
2. Four giant Jupiter-like worlds.
11
Two Kinds of Planets
• The difference is so dramatic that you are led to
say,
• “Aha, this must
mean something!”
12
Two Kinds of Planets
• There are three important points to note
about these categories.
Terrestrial
Jovian
13
Two Kinds of Planets
• One, they are distinguished by their
location.
• The four inner planets are
quite different from the
outer four.
14
Two Kinds of Planets
• Two, almost every solid surface in the solar
system is covered with craters:
• Terrestrial planets and moons have solid
surfaces, while giant planets don’t have a
visible surface (in fact, they don’t have
surfaces)
15
Two Kinds of Planets
• Three, the planets are distinguished by
properties such as composition, rings, and
moons.
Rocky
Gaseous
16
Two Kinds of Planets
• The division of the planets into two
families is a clue to how our solar system
formed.
• The present properties of individual planets,
however, don’t reveal everything you need to
know about their origins.
• The planets have all evolved since they
formed.
• For further clues, you can look at smaller
objects that have remained largely unchanged
since the birth of the solar system.
17
Asteroids
• The first asteroid (gr.: asteroidos = star-like) was Ceres,
~500 km diameter body discovered in 1801
• It is covered by ice and clay, and may have
liquid water between the rocky core and icy
crust
Ceres filled an empty slot for m=3 in the so-called TitiusBode law of planetary distances, but now is not considered
a natural law any more.
• This rule can be expressed as
•
a = (4 + 3 ⋅ 2m) AU /10
for m = -∞, 0, 1, 2, 3, 4, 5, 6..
18
Titius-Bode rule: 1766
• Astronomy professor Johann Daniel Dietz (Titius) from
Wittenberg (Germany) inserted his observation on
planetary distances into a German translation of Charles
Bonnet's book Contemplation de la Nature
• Johann E. Bode reformulated and popularized Titius rule
in his textbook on astronomy in 1772
19
Titius-Bode rule and its role in planet discoveries
• T-B rule was considered interesting but not important
until Uranus was accidentaly discovered in 1781, and it
fit the well!
• Predictions of new
planets were made
Soon Ceres was found.
G. Piazzi (1801)
-
W. Herschel (1781)
20
Titius-Bode rule and its role in planet discoveries
• T-B rule was considered interesting but not important
until Uranus was discovered in 1781, and it fit the law
rather precisely!
• Predictions of new
planets were made
G. Piazzi (1801)
-
W. Herschel (1781)
21
Asteroids
• Why Titius-Bode law eventually fell out of favor and
is now called a “Titius-Bode rule”: Neptune and Pluto
fail to obey it.
• However, the distribution of
• Satellite systems and extrasolar planets resembles
the power-law form of Titius rule to some extent : a ~
cn, where c=const different for different systems
22
T-B rule in exoplanetary (i.e., extrasolar) systems?
• However, the distribution of many satellite systems and
extrasolar planets resembles somewhat a power-law form
of Titius rule to some extent : a ~ cn, where c=const
different for different systems.
• Why? We don’t fully know.
• There are many exceptions.
23
Space Debris: Planet Building Blocks
• The solar system is littered with three kinds
contains four kinds of space debris (officially
known as minor bodies):
•
•
•
•
Asteroids,
Comets, and
Meteoroids
Planetoids (dwarf planets) like Pluto, Eris,
Sedna, Quaoar and other large, round bodies
• Although these objects represent a tiny fraction of the
mass of the system, they are a rich source of
information about the origin of the planets.
24
Asteroids - minor planets
Small rocky worlds.
• Most of them orbit the Sun in a belt between the
orbits of Mars and Jupiter.
• Roughly 20 000 asteroids
have been catalogued
• Most in asteroid belt
• They are smaller than
the Moon
Vesta 525 km diameter
Ceres 950 km
Moon 3470 km
asteroid belt (white)
25
Asteroids do have moons: Ida and its moon Dactyl
26
Asteroids belong to orbital families:
Ida is in the Koronis family, a group of 300 minor bodies
all of which have a = 2.86…2.89 AU , e=0.01…0.09, i=1o…3o
Asteroids in a family come
from a disruption of a
common ancestor asteroids
Florentina
Lacrimosa (41 km)
Elvira
This happened when two
asteroids collided in the
asteroid belt between Mars
and Jupiter >2 Gyr ago
Ida
Urda (40 km)
Koronis
Nassovia
Asteroids are as old as
planets, actually planets
formed from asteroid-like
primitive bodies called
planetesimals
27
Vesta
• Spacecraft Dawn has orbited in Jul 2011- Sep 2012
asteroid Vesta; a=2.36 AU, e=0.088, P=3.63 yr,
diameter 525 km, surface gravity acceleration =
(1/40) g
28
Asteroids
• About 2000 follow orbits that bring them
into the inner solar system, where they
can occasionally collide with a planet.
• Earth has been struck many times in its
history.
29
Asteroids
• Other asteroids share Jupiter’s orbit: the Greeks and
Trojans
• Some other have
been found beyond
the orbit of Saturn.
30
Asteroids
• About 200 asteroids are more than
100 km in diameter.
• Tens of thousands are estimated to be more
than 10 km in diameter.
• There are probably a million or more that are larger
than 1 km and billions that are smaller.
31
Asteroids
• As even the largest are only a few
hundred kilometres in diameter, Earthbased telescopes can detect no details on
their surfaces.
• The Hubble Space Telescope can image only
the largest features.
32
Asteroid Eros
• Photos returned by robotic spacecraft such as NEAR
Shoemaker in 2000-2001 (which landed as a 1st on
asteroid surface) and space telescopes show that
asteroids are generally irregular in shape and
battered by impact cratering.
33
Asteroids
• Some asteroids appear to be rubble piles
of broken fragments.
• A few are known to be double objects or to
have small moons in orbit around them.
• These are understood to be evidence of multiple
collisions among the asteroids.
34
Asteroids
• A few larger asteroids show signs of
volcanic activity on their surfaces that may
have happened when the asteroid
was young.
35
Asteroids
• Astronomers recognize the asteroids as
debris left over by a planet that failed to
form at a distance of about 2.5-3 AU from
the Sun.
• A good theory should explain why a planet
failed to form there, leaving behind a belt of
construction material.
36
Kirkwood gaps in asteroid belt
• Daniel Kirkwood (1857)
37
Comets
• In contrast to the rocky asteroids, the
brightest comets are impressively beautiful
objects. Most comets are faint and are
difficult to locate
even at their
brightest.
• But in 2013 a very
bright comet is
expected
(mV= -14m !)
Comets
• A comet may take months to sweep through the inner
solar system.
• During this time, it appears as a glowing head with an
extended tail of gas and dust.
Comet Hartley 2, visited in Nov. 2010 by EPOXI
• The beautiful tail of a comet can be longer than 1 AU.
• However, it is produced by an icy nucleus only a few
tens of kilometres in diameter. Nucleus is covered
with a dark crust, which breaks at places allowing gas
(H2O +CO+…) + dust + sand + stones, to escape in
jets.
Comets: Halley’s comet
The nucleus remains frozen and inactive
while it is far from the Sun.
• As the nucleus moves along its elliptical orbit into
the inner solar system, the Sun’s
heat begins to vapourize the ices,
releasing gas and dust.
41
Comets
• The pressure of sunlight and the solar wind push the
gas and dust away, forming a long tail. Peter Apian’s
drawing from 1532 shows that the tail always points
away from the sun. (He drew comet Halley.)
42
Comets
• The gas and dust respond differently to the
forces acting on them.
• So, they often separate into two separate sub-tails.
Comet Hale-Bopp in 1995
Comets
• Comet nuclei contain ices of water and
other volatile compounds such as carbon
dioxide, methane, and ammonia.
• These ices are the kinds of compounds that
should have condensed from the outer solar
nebula. The comets never fully melted.
• That makes astronomers think that comets are
ancient samples of the gases and dust from which
the outer planets formed.
44
Comets
• Five spacecraft flew past the nucleus of
Comet Halley when it visited the inner
solar system in 1985 and 1986.
• Since then, spacecraft have visited the nuclei
of several other comets.
• Images show that comet nuclei are irregular in
shape and very dark, with jets of gas and dust
spewing from active regions on the nuclei.
45
Comets
Comets
• In general, crusts of these nuclei are
darker than a lump of coal.
• This suggests that they have composition
similar to certain dark, water- and carbon-rich
meteorites.
Comets
• Since 1992, astronomers have discovered
roughly a thousand small, dark, icy bodies
orbiting in the outer fringes of the solar
system beyond Neptune.
48
Comets
• This collection of objects is called the
Kuiper belt.
• It is named after the Dutch-American
astronomer Gerard Kuiper, who predicted
their existence in the 1950s.
49
Comets
• There are probably 100 million bodies
larger than 1 km in the Kuiper belt.
• Any successful theory should explain how
they came to be where they are.
• We will return to them later in this course
50
Comets
• Astronomers believe that some comets,
those with the shortest orbital periods
and orbits in the plane of the
solar system, come from the Kuiper belt.
51
Comets
• The longer-period comets do not always
orbit in the plane of the solar system, but
can approach the Sun from random
directions.
• They are believed to originate from the Oort
cloud, a roughly spherical cloud of comets
that lies much farther away at a distance of
almost one light-year from the Sun.
• The solar nebula theory can explain these two
different sources of comets.
52
Kuiper belt and Oort cloud
53
Comets
• American astronomer Carolyn Shoemaker
holds the record for being the most
successful comet hunter alive, although,
remarkably, she only took up astronomy at
age 51.
• Canadian amateur astronomer David Levy is
also one of the most successful comet
discoverers in history.
• He is the first person to have discovered comets
using visual, photographic, and electronic
methods.
54
Comets
• Carolyn and Eugene Shoemaker, David Levy, and
Philippe Bendjoya were co-discoverers of Comet
Shoemaker–Levy 9, which flew by Jupiter,
disintegrated under tidal force in 1992, and in
1994 collided with Jupiter
55
Meteoroids, Meteors, and Meteorites
• Unlike the stately comets, meteors flash across
the sky in momentary streaks of light.
• They are commonly called “shooting stars.”
56
Meteoroids, Meteors, and Meteorites
• They are not stars but small bits of rock and
metal falling fast into Earth’s atmosphere.
• They burst into incandescent vapour about 80
km above the ground because of friction with
the air.
• This hot vapour condenses
to form dust, which settles
slowly to the ground, adding
~40 000 tons per year to the
planet’s mass.
~~
57
Meteoroids, Meteors, and Meteorites
• Technically, the word meteor refers to the streak of
light in the sky. In space, before its fiery plunge,
the object is called a meteoroid.
• Some meteoroids hitting Earth are large (60
m body hit over Siberia in 1908 and
disintegrated in the air – no meteor found on
the ground - causing
• Tunguska catastrophe
58
Meteoroids, Meteors, and Meteorites
• Most meteoroids are specks of dust, grains of sand, or
tiny pebbles.
• Almost all the meteors you see in the sky are produced
by meteoroids that weigh less than 1 g.
• Only rarely is one massive enough and strong enough to
survive its plunge, and reach Earth’s
surface.
• Such a rock is called a meteorite (this one was found in
Canada)
59
Meteoroids, Meteors, and Meteorites
• Meteorites can be divided into three broad
categories.
• Iron,
• Stony, and
• Stony-iron.
60
Meteoroids, Meteors, and Meteorites
• Iron meteorites are solid chunks of iron and nickel.
• Stony meteorites are silicate masses that resemble
Earth rocks.
• Stony-iron meteorites are iron-stone mixtures.
Meteoroids, Meteors, and Meteorites
• One type of stony meteorite called
carbonaceous chondrites has a chemical
composition that resembles a cooled lump
of the Sun with the hydrogen and helium
removed.
Meteoroids, Meteors, and Meteorites
• These meteorites generally contain
abundant volatile compounds including
significant amounts of carbon and water.
• They may have similar composition to comet
nuclei.
• Allende meteorite
found in Mexico
Meteoroids, Meteors, and Meteorites
• Heating would have modified and driven
off these fragile compounds.
• So, carbonaceous chondrites must not have
been heated since they formed.
• Astronomers conclude that carbonaceous
chondrites, unlike the planets, have not evolved
and thus give direct information about the early
solar system.
• Ca+Al inclusion in a chondrite
• Sign of original heating:
• Lightnings in the protoplanetary
disk?
64
Meteoroids, Meteors, and Meteorites
• You can find evidence of the origin of
meteors through one of the most pleasant
observations in astronomy.
• You can watch a meteor shower, a display of
meteors that are clearly related by a
common origin.
Meteoroids, Meteors, and Meteorites
• For example, the Perseid meteor shower
occurs each year in August.
• During the height of the shower, you might
see as many as 40 meteors per hour.
• The shower is so named because all its meteors
appear to come from a point in the constellation
Perseus.
66
Meteoroids, Meteors, and Meteorites
• Meteor showers are seen when Earth
passes near the orbit of a comet.
• The Eta Aquariids meteor shower, best
viewed from the southern hemisphere, is
caused by the Earth passing through dust
released by Halley’s Comet.
Meteoroids, Meteors, and Meteorites
• The meteors in meteor showers must be
produced by dust and debris released
from the icy head of the comet.
• In contrast, the orbits of some meteorites
have been calculated to lead back into the
asteroid belt.