Extrasolar Planets, Lebo, 8-1

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Transcript Extrasolar Planets, Lebo, 8-1

Search for Extra-Solar Planets
George Lebo
16 November 2012
AST 2037
Eclipsing (Transiting) Binary
Light Curve
Kepler Spacecraft – 2009
Earth Trailing Orbit
Kepler ExoPlanets
Solar Coronagraph
Vortex Coronagraph
Direct Imaging Using a Vortex
Coronagraph - HR8799
Beta Pictoris (Very Young Star)
Imaging With a Coronagraph
Animation URL
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
• In 1995, Michel Mayor & Didier
Queloz discover a planet around
51 Pegasi – how?
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
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 the center to the focus is c=ae where e is
the “eccentricity”
• Circles are ellipses with eccentricity=0 (both foci at center)
Kepler’s Second Law
• Planet motion sweeps out equal areas of the ellipse in equal
• Meaning … planet moves faster when it is closer to the star
and slower when it is farther away
Kepler’s 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)
Orbital Reflex Motion
• For a star/planet system, the planet does most of the
• Its low mass means it is farthest from the center of
• 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!
Barycenter (Center of Mass) of the Solar
Radial Velocity Planet Searches
• So … need a speedometer to measure star velocity versus
• To a precision of a few meters per second!
• Across distances of many light years!!!
• How? Doppler shift
of spectral lines
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
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
• Eccentricity
• Planet mass (note
• Really planet
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
51 Pegasi: Sky View
51 Pegasi: Hot Jupiter?
51 Peg period indicates a VERY small orbital radius
(P = 4.2 days, a = 0.05 AU)
51 Pegasi: Hot Jupiter?
• At that location, expected
temperature is VERY
high (about 2000K or
• So … Jupiter-like planet,
but closer than Mercury
 “Hot Jupiter”
• How do you make
something like that????
Planet Bonanza
• Geoff Marcy & Paul Butler quickly confirmed 51
• They had lots of archival data from searches for
Jupiter-type planets (periods >10 years, so they were
still “in progress”)
• No one 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
• “MARVELS” (Multi-object Apache Point Observatory
Radial Velocity Exoplanet Large-area Survey) is the UF’s
program launched in 2008 that promises to be one of the
most prolific of all exoplanet hunting programs.
ES-Planet Population
• As of November 14, 2012, there are 850 known extrasolar
planets(!!) and 611 extrasolar planetary systems.
• All of this has happened in about 20 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