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Lecture 11

ASTA01H3 Lecture Notes - Lecture 11: Exoplanet, Zircon, Zodiacal Light


Department
Astronomy
Course Code
ASTA01H3
Professor
Kristen Menou
Lecture
11

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Chapter 12: The Origin of the Solar System
Lecture 11
The History of the Atoms in Your Body
-by the time the universe was three minutes old, the protons, neutrons, and electrons
now in your body had come into existence
-although those particles formed quickly, they were not linked together to form the
atoms that are common today
-most of the matter was hydrogen (almost 75%), and about 25% was helium
-very few (<1%) of the heavier atoms were made in the big bang
-the big bang also formed the so-called dark matter, which permeates the planetary
systems at very low density, but has very little interaction with them
-during the first few hundred million years after the big bang, matter collected to form
galaxies containing billions of stars. Our galaxy was built ~8.8 Gyr ago (so-called thin
disk)
-nuclear reactions inside stars combine low-mass atoms, such as hydrogen, to make
heavier atoms
-generation of stars combined the original particles, fusing them into atoms such as
carbon, nitrogen and oxygen: CNO
-most of the iron in your body was produced by:
-carbon fusion in the explosions of stars called supernovae, and
-decay of radioactive atoms in the expanding matter ejected by supernovae
-atoms heavier than iron, such as iodine, were created by rapid nuclear reactions that
can occur only during supernova explosions
-our galaxy contains at least 100 billion stars, of which the Sun is one
-the Sun formed from a cloud of gas and dust about 4.56 billion years ago (4.56 Gyr
ago)
The Origin of the Solar System
-the solar nebula theory supposes that planets form in the rotating disks of gas and
dust around young stars
-our own planetary system formed in such a disk-shaped cloud around the Sun
!1
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-when the Sun became luminous enough, the remaining gas and dust were blown
away into space by the solar wind (a stream of charged particles from the Sun),
and UV (ultraviolet) radiation, leaving the planets orbiting the Sun
-according to the solar nebula hypothesis, Earth and the other planets of the solar
system formed 4.56 billions of years ago as the Sun condensed from the
interstellar medium
-Kant-Laplace nebula
-Immanuel Kant (1724-1804)
-Pierre-Simon de Laplace (1749-1824)
-dust and stones
-gas rings
-the theory predicts that most stars should have planets because planet formation is a
natural part of star formation
-therefore, planets should be very common in the universe — probably more
common than stars
-these young stars form at the centre of a rotating cloud of gas and dust that stars to
contract due to gravity
-collisions between particles in the rotating cloud tend to flatten the cloud into a disk
shape
-in addition, as the cloud shrinks it spins faster due to the conservation of angular
momentum
-most of the material in the spinning disk forms a star in the centre, while the
remaining material forms the planets and other bodies such as asteroids and
comets
-there is clear evidence that disks of gas and dust are common around young stars
-the idea is so comprehensive and explains so many observations that it can be
considered to have ‘graduated’ from being just a hypothesis to being properly
called a theory
-bipolar flows from protostars were the first evidence of such disks
Lecture 12
Revolution and Rotation
-the planets revolve around the Sun in orbits that lie close to a common plane
!2
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-the orbit around the Sun of Mercury, the planet closest to the Sun, is tipped
7.0degrees to Earth’s orbit
-the rest of planets’ orbital planes are inclined by no more than 3.4degrees
-the solar system is basically ‘flat’ and disk-shaped
-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 7.2degrees to ecliptic
-most planets’ equators are tipped less than 30degrees
-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
-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
-furthermore, nearly all the Moons in the solar system, including Earth’s moon, orbit
around their planets counterclockwise
Two Kinds of Planets
-1. four small Earth-like worlds, called terrestrial planets
-2. four giant Jupiter-like worlds, called jovian planets
-three important points to note about these categories:
-1. they are distinguished by their location
-the four inner planets are quite different from the outer four
-2. 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) — rocky vs gaseous
-3. the planets are distinguished by properties such as composition, rings, and
moons
-the division of the planets into two families is a clue to how our solar system formed
Asteroids
-the first asteroid was Ceres, ~500km diameter body discovered in 1801
-it is covered by ice and clay, and may have liquid water between the rocky core and
icy crust
!3
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