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Suppose you find a rock originally
made of potassium-40, half of which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Suppose you find a rock originally
made of potassium-40, half of which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!