Transcript c12exoplanets_lec1_2008

2nd Midterm:
Difficult Exam . . .
Common Mistakes
1. Greenhouse Effect:
• CO2 absorbs IR light (not UV).
• 2. Geological activity on Jupiter’s moons is due to
their composition of ice: Allows melting and flow at
low temperatures in the outer solar system.
• 3. Titan retained its atmosphere because it’s cold:
molecules don’t move fast enough to escape.
Announcements
• Homework due a week from Friday (May 2):
• Read Chapter 13:
Planets Around Other Stars
• Final Exam - comprehensive: May 20 @ 8am:
237 Hearst Gym
• Telescope Observations:
- Observing Starts: 8pm !
- Sketch 2 Objects & Star Chart (3 obs of Mars):
- Due Friday, May 2 (w/ HW) !
"The Dawn of Creation:
The First Two Billion Years"
A Special Lecture by
Steven Beckwith
Wednesday, April 23, 5:30pm
Chevron Auditorium International House
Be There !
Are there Other Planetary Systems ?
Are any of them
like our Solar System ?
February 5, 1996
Now Exiting our Solar System to …
.
. .
Now Leaving The
our Solar
. . ..
MilkySystem
Way Galaxy
.
.
.
.
.. .
. .
Democritus:
Greek philosopher
(460 - 370 BC).
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“ There are innumerable worlds of
different sizes. These worlds are
at irregular distances, more in one
direction and less in another, and
some are flourishing, others
declining. Here they come into
being, there they die, and they are
destroyed by collision with one
another. Some of the worlds have
no animal or vegetable life nor any
water. “
Epicurus (341-270 B.C.)
Greek philosopher in Athens where he opened a school of philosophy
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“There are infinite worlds both
like and unlike this world of
ours ... we must believe that in
all worlds there are living
creatures and plants and other
things we see in this world…”
http://www.gospelcom.net/chi/DAILYF/2002/02/daily-02-17-2002.shtml
Giordano Bruno
1584
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Giordano Bruno lost the thread of truth
and the Roman Church burned him for it.
Wrote; ‘
De l'infinito, universo e mondi
(The Infinity, the Universe and Its Worlds)
• Bruno was enthusiastic
about the Copernican
theory that the planets
circle the sun and he
believed there are many
other planets around other
stars, some of them with
life.
Burned at the Stake
February 17, 1600
By Catholic Church
Stars are a
billion times
brighter…
…than the planet
hidden in
the glare.
Detecting Earths:
Like detecting
a firefly
next to a
nuclear explosion
Light Reflected off Planet
Cross Sectional Area of planet:  r2
r
Fraction of Star light
That hits planet:
R
a)  r2 / 4  R2
Area of Sphere: 4  R2
For Earth: Fraction is 10 -10
Planet Detection
The Wobble of a Star:
Gravitational pull
by the planet
Star moves slower, due to
conservation of momentum: mass x velocity
MSTAR VSTAR = Mplanet Vplanet
Wobble
Velocity
1/2 mPLVPL2 = G mPLMStar / r
VPL ~ 10 km/s
Momentum:
( )V
VStar =
MPL
MSTAR
PL
VSTAR ~ 10 m/s
x (MPL /MJUP)
Detecting a Star’s Wobble:
Doppler Effect
Spectrum
starlight
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Detecting the wobble of a Star,
pulled around by its planet.
(Meters/sec)
12 years . . . Jupiter
Need Telescope . . .
Quiz
The period of the sine wave is:
a)
b)
c)
d)
2 day
10 day
20 day
40 day
Starlight
High Resolution ``Echelle”
Spectrometer
Echelle
Spectrometer
From Telescope
CCD
Echelle
Grating
Collimator
Spectrum of Star:
Doppler Effect
Doppler Shift:
Dl/l = v / c
4096 Pixels
Claim of First Detected Extrasolar Planet:
51Pegasi
Orbital Period = 4.2 days !
Absurd
Michel Mayor & Didier Queloz
51 Peg
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One week Later . . .
4 Nights at Lick Observatory
October 11, 1995
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Determination of Orbital Distance
from Star to Planet
Period = 4.2 days
Kepler’s 3rd Law: P2 = a3
Units: P in years, a in AU
Solve for a:
a = 0.05 AU
Proximity: Temp = 1500 C
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Determination of Planet’s Mass
Conservation of Momentum:
momentum of star = momentum of planet
MSTAR VSTAR = Mplanet Vplanet
Solve for Mass of planet:
Mplanet = MSTAR VSTAR / Vplanet
MSTAR : Star Masses are known
(most are Sun-like)
VSTAR from Doppler shift: 55 m/s
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What is Vplanet ?
Vplanet = 2  a / P
You know “a” from Kepler’s 3rd Law: P2 = a3
Can Determine Mplanet
Quiz
From Doppler measurements, a
star exhibits the sine wave variation
In velocity. The orbital plane is oriented
edge-on to us at Earth, causing the
planet to block the star light.
b
a
c
d
At what point on the velocity plot
will the planet block the star light ?
Summary of Doppler Wobble Method:
Orbital Physics
Kepler’s 3rd Law and Conservation of Momentum
P2 = 4 2/[G(Mstar + Mplanet)] a3
P = orbital period a = orbital distance of planet from star
Simple Version of Kepler’s 3rd Law:
P2 = 4 2/GMstar a3 (planet’s mass is negligible compared to star.)
Measure P from Doppler periodicity: Can solve for a.
Circular Orbit:
Vplanet = 2  a / P
(Circumference/Time)
Momentum Conservation:
Mstar Vstar = MPlanet Vplanet
Mplanet = Mstar Vstar / VPlanet
Doppler Wobble Method:
Unknown Orbital Inclination
Tilt of Planet’s Orbital Plane: Unknown
• Edge-on Orbit: Full Doppler Effect
• Face-on Orbit: No Doppler Effect
• General Tilt Angle: True Wobble speed is
greater than Doppler indicates
True planet mass is GREATER than the
mass found by the Doppler method. (25% greater
on Avg.)
We measure M sin i
51 Pegasi
First Extrasolar Planet
Period = 4.2 days
Kepler’s 3rd Law: P2 = a3
Units: P in years, a in AU
Solve for a:
a = 0.05 AU
Proximity: Temp = 1500 C
Detecting Extrasolar Planets
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Kepler’s 3rd Law: P2 = a3
Momentum Conservation:
MSTAR VSTAR = Mplanet Vplanet
16 Cygni B
Mass = 1.7 MJUP
Not Sinusoidal
(Min)
Orbit Period
2.2 yr
Velocity
Wobble
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Velocity
Orbit of Planet around 16 Cygni
16 Cygni: Planet & Moon
Life on Gas Giant Planets ?
Earth-Like Moon
Floaters
Eccentric Orbit !
of Planet around 16 Cygni