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Search for Extra-Solar Planets
Stephen Eikenberry
1 November 2012
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?
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Kepler’s Laws
• Kepler described 3 “laws” of planetary motion (for our
solar system)
• Kepler did not have a physical basis for the laws (i.e.
Newton’s 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 Kepler’s rules for planet motion worked
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Kepler’s First Law
•
•
•
•
•
Planet orbits are ELLIPSES (what’s 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 eccentricity=0 (both foci at center)
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Kepler’s 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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Kepler’s Third Law
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•
•
•
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 can’t 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-hunting-radmethod.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 4.23day 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 >10 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, 228 861 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
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