New paper in Science last Friday: Pluto

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New paper in Science last Friday Plutos moon
Charon may also have formed by a giant impact
Simulations by Robin Canup and collaborators (who
also simulated the impact that formed the Earths
moon)
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Astronomical Searches for Extrasolar Planets
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Until recently, we knew of only one planetary
system
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Planets are not easy to see next to
starsJupiter is a billion times fainter than
the Sun!like trying to see a firefly next to a
search-light
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Star and Planet orbit each other Astrometry
-wobble on the sky
Tiny changes in position on the sky --difficult
to measure from the ground
Sun as seen from 10 pc away
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Star and Planet orbit each other Radial Velocity
or Doppler method
Star also wobbles toward and away from us
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Most successful current method for finding
planets around Sun-like stars
Iodine cell in telescope beam provides many,
sharp absorption lines against which very slight
Doppler shifts of the stellar spectrum become
visible
wavelength
Spectral lines wavelength
slight Doppler shift
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How the radial velocity method works
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Radial velocity method only gives Minimum mass
(orbit inclination uncertain) Orbit (period,
orbit size, eccentricity)
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First planet found around a Sun-like star (in
1995) 51 Peg
Discovered in 1995 4 day period Min mass 0.5
MJupiter 0.05 AU from star
Biases of the Doppler method?
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Upsilon Andromedae Multiple planets Discovered
in 1999
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Many close-in hot Jupiters
A few multi-planet systems
A few with gas giants in the same place as ours
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How do giant planets get so close to the star?
Planet migration
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Most known exoplanets are large gas giants
J
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Stars with more heavy elements seem more likely
to have planets
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Why do stars with more heavy elements have more
planets?
Explanations 1. This is a result of the
selection of stars for planet searches and has
nothing to do with which stars form planets
more easily (selection bias) 2. More heavy
elements means more solid material in the nebula
to build planets with so the chances of
forming a 15 Earth-mass core for a gas
giant are higher 3. Stars may look enriched in
heavy elements if they eat planets and if
theyre not mixed by convection
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Planet transits
Giant exoplanet
Venus
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Note Jupiters radius is 1/10th of the Suns
so its area is 1/100th of the Suns Earths
radius is 1/100th so its area is 1/10,000th of
the Suns
1 change
Biases of the transit method?
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• Transit of a planet around the star HD 209458

Brian Lee (UofT grad student) searches for
transiting planets
Derive radius for the planet as well as
inclination of orbit Combined with mass from RV,
calculate planets density So far, 6 planets have
been found this way
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Direct imaging
Young planets are hotter and brighter --easier to
detect
Adaptive Optics (to get sharper images) and
Coronagraph (to block out starlight)
Parandis Khavari
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Also, interferometers can null out stars
light, leaving planet
Interferometer setup Fringe
pattern on sky Image
with star blacked
out
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Microlensing
another star
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Pulsar Timing First planets detected in 1992
around pulsars, not regular stars!
Pulse arrival times Timing error
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Pulsar 125712
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READING UP TO MIDTERM
Goldsmith Owen The Search for Life in the
Universe
READING 1st WEEK Ch. 1, 2 7 (p.
163-166) READING 2nd WEEK Ch. 6 3 READING
3rd WEEK Ch. 4 5 READING 4th WEEK Ch. 11
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TAs Parandis Khavari khavari_at_astro
A-C Duy Nguyen nguyen_at_astro D-La Tim
Rothwell rothwell_at_astro Le-R Marija
Stankovic stankovic_at_astro S-Z Format
for e-mail subject line ltLastnamegt,
ltFirstnamegt AST 251 ltsubjectgt
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Upcoming Dates
Paper topics due in class TODAY Topics
approved by Feb 7 pick up TA comments at
midterm Modifications finalized by Feb 14 Paper
outlines due in class Mon Feb 21 Midterm
test Mon Feb 7 4pm Sidney Smith Hall
2102 A-La Sidney Smith Hall 2118
Le-Z Midterm review sessions by TAs Tue Feb
1 3-4pm McLennan 15th floor conference
room Fri Feb 4 12-1pm McLennan 15th floor
conference room
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Instructions for Paper Outlines

• Turn in outline on paper (typed) in class on (or
  before) Feb 21
• DO NOT SUBMIT PAPER OUTLINE BY EMAIL
• Must include
• 1. Your name and student ID
• 2. Email address
• 3. (Approved) Paper topic
• 4. Thesis statement
• 5. Outline of main points in the paper
• -- one line/sentence per paragraph that you
  intend to write
• -- references indicated for where the supporting
  information will come from
• 6. Bibliography, more or less complete
• 10 penalty for each days delay after Feb 21
  zero after one weeks delay
• One letter grade deduction for not handing in
  paper outline
• Response from TAs by Feb 28 Final paper due
  March 14

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Paper writing advice and guidance
Strongly recommend the Writing Centres web
site http//www.utoronto.ca/writing/index.html
Especially the guidelines on Writing about
Physics http//www.utoronto.ca/writing/physguide.
html (linked from Supplementary Resources on
the course web site) Also feel free to consult
with your TA as you make progress --by email or
in person by appointment.