ASTR100 Class 01 - University of Maryland Department of
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Transcript ASTR100 Class 01 - University of Maryland Department of
ASTR100 (Spring 2008)
Introduction to Astronomy
Other Planetary Systems
Prof. D.C. Richardson
Sections 0101-0106
But first…
When did the planets form?
We cannot find the age of a planet, but
we can find the ages of the rocks that
make it up.
We can determine the age of a rock by
analyzing the proportions of various
atoms and isotopes within it.
The decay of radioactive elements
into other elements is a key tool in
finding the ages of rocks.
Age dating of
meteorites that are
unchanged since
they condensed
and accreted tell
us that the solar
system is about
4.6 billion years
old.
Thought Question
Suppose you find a rock originally
made of potassium-40, half of which
decays into argon-40 every 1.25 billion
years. You open the rock and find 3
atoms of argon-40 for every 1 atom of
potassium-40. How old is the rock?
A. 1.25 billion years.
B. 2.5 billion years.
C. 5 billion years.
D. It is impossible to determine.
Thought Question
Suppose you find a rock originally
made of potassium-40, half of which
decays into argon-40 every 1.25 billion
years. You open the rock and find 3
atoms of argon-40 for every 1 atom of
potassium-40. How old is the rock?
A. 1.25 billion years.
B. 2.5 billion years.
C. 5 billion years.
D. It is impossible to determine.
How do we detect planets
around other stars?
Planet Detection
Direct: Pictures or spectra of the
planets themselves.
Indirect: Measurements of stellar
properties revealing the effects of
orbiting planets.
Gravitational Tugs
The Sun and Jupiter
orbit around their
common center of
mass.
The Sun therefore
wobbles around that
center of mass with
the same period as
Jupiter.
Gravitational Tugs
Sun’s motion around
solar system center
of mass depends on
tugs from all the
planets.
(as seen from 10 ly)
Astronomers who
measure this motion
around other stars
can determine
masses and orbits of
all the planets.
Astrometric Technique
We can detect
planets by
measuring the
change in a star’s
position in the sky.
However, these tiny
motions are very
difficult to measure
(~0.001 arcsec).
(as seen from 10 ly)
Doppler Technique
Measuring a star’s
Doppler shift can tell
us its motion toward
and away from us.
Current techniques
can measure
motions as small as
1 m/s (walking
speed!).
First Extrasolar Planet Detected
Doppler shifts of star
51 Pegasi indirectly
reveal planet with 4day orbital period.
Short period means
small orbital
distance.
First extrasolar
planet to be
discovered (1995).
First Extrasolar Planet Detected
The planet around 51 Pegasi has a mass
similar to Jupiter’s, despite its small orbital
distance.
Thought Question
Suppose you found a star with the
same mass as the Sun moving back
and forth with a period of 16 months.
What could you conclude?
A. It has a planet orbiting inside 1 AU.
B. It has a planet orbiting outside 1 AU.
C. It has a planet orbiting at 1 AU.
D. It has a planet, but we do not have
enough information to know its
orbital distance.
Thought Question
Suppose you found a star with the
same mass as the Sun moving back
and forth with a period of 16 months.
What could you conclude?
A. It has a planet orbiting inside 1 AU.
B. It has a planet orbiting > 1 AU.
C. It has a planet orbiting at 1 AU.
D. It has a planet, but we do not have
enough information to know its
orbital distance.
Transits and Eclipses
A transit is when a planet passes in front of a star.
The resulting eclipse reduces the star’s apparent
brightness and tells us the planet’s radius.
When there is no orbital tilt, an accurate measurement
of planet mass can be obtained.
Direct Detection
Special techniques for concentrating or
eliminating bright starlight are enabling the
direct detection of planets.
How do extrasolar planets
compare with those in our own
solar system?
Measurable Properties
Orbital period, distance, and shape.
Planet mass, size, and density.
Composition.
Orbits of Extrasolar Planets
Most of the detected
planets have smaller
orbits than Jupiter.
Planets at greater
distances are harder
to detect with the
Doppler technique.
Properties of Extrasolar Planets
Most of the detected
planets have larger
mass than Jupiter.
Planets with smaller
masses are harder to
detect with the
Doppler technique.
Planets: Common or Rare?
One in 10 stars so far have turned out
to have planets.
The others may still have smaller
(Earth-sized) planets that cannot be
detected using current techniques.
Surprising Characteristics
Some extrasolar planets have highly
elliptical orbits.
Some massive planets orbit very close
to their stars: “Hot Jupiters.”
Hot Jupiters
Do we need to modify our
theory of solar system
formation?
Revisiting the Nebular Theory
Nebular theory predicts massive
Jupiter-like planets should not form
inside the frost line (at << 5 AU).
The discovery of “hot Jupiters” has
forced a reexamination of the nebular
theory.
“Planetary migration” or gravitational
encounters may explain hot Jupiters.
Planetary Migration
A young planet’s
motion can create
waves in a planetforming disk.
Models show that
matter in these
waves can tug on a
planet, causing it to
migrate inward.
Gravitational Encounters
Close gravitational encounters between
two massive planets can eject one while
flinging the other into an elliptical orbit.
Multiple close encounters with smaller
planetesimals can also cause inward
migration.
Modifying the Nebular Theory
Observations of extrasolar planets
showed that the nebular theory was
incomplete.
Effects like planet migration and
gravitational encounters might be more
important than previously thought.
MIDTERM #1 REVIEW
Chapters 1–6
Chapter 1:
Our Place in the Universe
A. Our Modern View of the Universe
Planets, stars, galaxies, superclusters.
The speed of light: looking back in time.
B. The Scale of the Universe
Sizes & distances: planets, stars, galaxies.
The age of the universe.
C. Spaceship Earth
Our motion through the universe.
Chapter 2:
Discovering the Universe for Yourself
A. Patterns in the Night Sky
Constellations, celestial sphere, rise & set.
B. The Reason for Seasons
Axis tilt, equinoxes, precession.
C. The Moon, Our Constant Companion
Phases, eclipses.
D. The Ancient Mystery of the Planets
Geocentric vs. heliocentric.
Chapter 3:
The Science of Astronomy
A. The Ancient Roots of Science
Astronomy as the oldest science.
B. Ancient Greek Science
Birth of modern science.
C. The Copernican Revolution
Copernicus, Tycho Brahe, Kepler, Galileo.
D. The Nature of Science
Observe, hypothesize, experiment, predict.
Chapter 4:
Making Sense of the Universe:
Understanding Motion, Energy, & Gravity
A. Describing Motion: Examples from
Daily Life
Speed, velocity, acceleration, momentum,
force, mass, weight.
B. Newton’s Laws of Motion
Inertia, F = ma, equal and opposite force.
C. Conservation Laws in Astronomy
Energy and angular momentum.
D. The Force of Gravity
F = G M1 M2 / d2, tides.
Chapter 5:
Light: The Cosmic Messenger
A. Basic Properties of Light and Matter
EM spectrum, atoms, emission/absorption.
B. Learning from Light
Spectroscopy, thermal emission, Doppler.
C. Collecting Light with Telescopes
Bigger is better, space is clearer.
Chapter 6:
Our Solar System and its Origin
A. A Brief Tour of the Solar System
Sun, planets, moons, asteroids, comets.
B. Clues to the Formation of our Solar
System
Orderly motion, planet types, exceptions.
C. The Birth of the Solar System
The nebular hypothesis.
D. The Formation of Planets
Accretion, giant impacts, age.
E. Other Planetary Systems
Detection methods, challenges to theory.
Midterm Information
When: Tuesday March 4, 9:30 am
Where: here!
Bring pencil, student ID
No notes, no calculators, no mobiles!
Review: Monday March 3, 5-7 pm
Where: here!
Bring your questions and textbook!
Good Luck!