Search for Extra-Solar Planets

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Search for Extra-Solar Planets

• Stephen Eikenberry
• 4 November 2010
• AST 2037

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Planets Around Other Stars

• None known prior to 1992 (!)
• In 1992, Alex Wolczan discovered 2 (now 3)
  planets around a neutron star
• But those seemed weird
• Probably supernova leftover bits
• In 1995, Michel Mayor Didier Queloz discover a
  planet around 51 Pegasi how?

3
Keplers Laws

• Kepler described 3 laws of planetary motion
  (for our solar system)
• Kepler did not have a physical basis for the laws
  (i.e. Newtons laws of motion)
• He just found patterns in the motions of planets
  and used them to develop 3 guidelines that
  provided a good matching description
• Newton later used his physical laws of motion to
  show WHY Keplers rules for planet motion worked

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Keplers First Law

• Planet orbits are ELLIPSES (whats that?)
• The sun/star is at one focus of the ellipse
• Both the planet and the star orbit the center of
  mass
• The distance from center to focus is the
  eccentricity
• Circles are ellipses with eccentricity0 (both
  foci at center)

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Keplers Second Law

• Planet motion sweeps out equal areas of the
  ellipse in equal time
• Meaning planet moves faster when it is closer
  to the star and slower when it is farther away

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Keplers Third Law

• a3/P2 Mtot
• a semi-major axis of the ellipse (AU)
• P period of the orbit (years)
• Mtot total mass of the system (solar masses)

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Orbital Reflex Motion

• For a star/planet system, the planet does most of
  the moving
• Its low mass means it is farthest from the center
  of mass
• Same period, larger distance means higher
  velocity (what is it for Earth? For Jupiter?)
• But you cant see it (too faint)
• Star moves VERY little
• High mass, means small distance from COM (what is
  it for Sun/Earth? Sun/Jupiter?)
• But we can SEE the star!

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Radial Velocity Planet Searches

• So need a speedometer to measure star velocity
  versus time
• To a precision of a few meters per second!
• Across distances of many light years!!!

• How? Doppler shift of spectral lines

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Radial Velocity Planet Searches

• How-to, with movie
• http//static.howstuffworks.com/flash/planet-hunti
  ng-rad-method.swf
• Take a spectrum with a big telescope and very
  precise (and STABLE) spectrograph

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Radial Velocity Information

• Jupiter has biggest reflex velocity effect on the
  Sun
• but this velocity is still small
• period is long

• Information we get
• Period (how?)
• Orbit distance (how?)
• Eccentricity
• Planet mass (note sin i uncertainty)

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Radial Velocity Information

• Jupiter has biggest reflex velocity effect on the
  Sun
• but this velocity is still small
• period is long

• Information we get
• Period (how?)
• Orbit distance (how?)
• Eccentricity
• Planet mass (note uncertainty!)
• Really planet MINIMUM mass!

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51 Pegasi

• In 1995, Mayor Queloz announce the discovery of
  an orbital signature with amplitude 50 m/s in a
  4.23-day period around star 51 Pegasi
• Mass 0.5 MJUP ? First extra-solar planet

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51 Pegasi Sky View
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51 Pegasi Hot Jupiter?

• 51 Peg period indicates a VERY small orbital
  radius (how small?)

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51 Pegasi Hot Jupiter?

• At that location, expected temperature is VERY
  high (about 2000K or higher!)
• So Jupiter-like planet, but closer than Mercury
  ? Hot Jupiter
• How do you make something like that????

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Planet Bonanza

• Geoff Marcy Paul Butler quickly confirmed 51
  Pegasi
• They had lots of archival data from searches for
  Jupiter-type planets (periods gt10 years, so they
  were still in progress)
• No on even thought to look for short-period
  MASSIVE planets (why would they be easier?)
• Found many Hot Jupiters most extra-solar
  planets known today are Hot Jupters

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ES-Planet Population

• As of this morning, 228 planets are now known to
  orbit other stars (!!)
• All of this has happened in about 10 years
  someone currently finds a new planet every couple
  of weeks or less
• These planets are NOT generally like our Solar
  System objects WHY?
• Next time properties of Extra-Solar Planets and
  implications for Life in the Universe