Extra Solar Planets

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Extra Solar Planets
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Just some introductory materials. A very fast
moving field.
My favorite website http//www.exoplanets.org
Touch on masses of stars/planets Some of the
results concerning exoplanet discovery Several
techniques for searching, Kepler the new
King Also have star system/planet building
webpages http//curriculum.calstatela.edu/courses
/builders/lessons/less/les1/choose.html
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Binary Stars
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More than 50 of all stars in our Milky Way are
not single stars, but belong to binaries
Pairs or multiple systems of stars which orbit
their common center of mass.
If we can measure and understand their orbital
motion, we can estimate the stellar masses.
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The Center of Mass
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center of mass balance point of the system.
Both masses equal gt center of mass is in the
middle, rA rB.
The more unequal the masses are, the more it
shifts toward the more massive star.
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Estimating Stellar Masses
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Recall Keplers 3rd Law Py2 aAU3
Valid for the solar system star with 1 solar
mass in the center.
We find almost the same law for binary stars with
masses MA and MB different from 1 solar mass
aAU3
____
MA MB
Py2
(MA and MB in units of solar masses)
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Examples
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a) Binary system with period of P 32 years and
separation of a 16 AU
163
____
MA MB 4 solar masses.
322
b) Any binary system with a combination of period
P and separation a that obeys Keplers 3. Law
must have a total mass of 1 solar mass.
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Visual Binaries
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The ideal case
Both stars can be seen directly, and their
separation and relative motion can be followed
directly.
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Spectroscopic Binaries
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Usually, binary separation a can not be measured
directly because the stars are too close to each
other.
A limit on the separation and thus the masses can
be inferred in the most common case
Spectroscopic Binaries
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Spectroscopic Binaries (II)
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The approaching star produces blueshifted lines
the receding star produces redshifted lines in
the spectrum.
Doppler shift ? Measurement of radial velocities
? Estimate of separation a
? Estimate of masses
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Spectroscopic Binaries (III)
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Typical sequence of spectra from a spectroscopic
binary system
Time
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Eclipsing Binaries
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Usually, inclination angle of binary systems is
unknown ? uncertainty in mass estimates.
Special case Eclipsing Binaries
Here, we know that we are looking at the system
edge-on!
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Eclipsing Binaries (II)
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Peculiar double-dip light curve
Example VW Cephei
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Extra-Solar Planets

• Hard to see faint planet right next to very
  bright star
• Two main indirect techniques available(Like a
  binary star system but where 2nd star has
  extremely low mass)
• Watch for Doppler wobble in position/spectrum
  of star
• Watch for transit of planet which slightly dims
  light from star
• More than 700 planets discovered since 1996
• See http//exoplanets.org/ or several other sites
• Initially tended to be big (?Jupiter) and very
  close to star (easier to see), but starting to
  find others now.

51 Peg the first extra-solar planet discovered
HD 209458 Transit of planet across star
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Radial Velocity or Wobble Method

• 51 Peg back in 1996, followed by hundreds of
  others, primarily from Geoff Marcys group out of
  California (Lick and Keck Observatories). Marcy
  went on Letterman wearing a Hawaiian shirt we
  both bought in Konatried mine on and its a
  little too small now 15 years later. Hmmm.
• Depends on techniques to get ultra high spectral
  resolution (meters per second) via iodine cells
  and other tricks
• Need stars closer to edge on, has mass
  uncertainties because of unknown viewing angle
• Works, but need long time, long surveys, mostly
  one target at a time.

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First Extrasolar Planet

• Doppler shifts of the star 51 Pegasi indirectly
  revealed a planet with 4-day orbital period.
• This short period means that the planet has a
  small orbital distance.
• This was the first extrasolar planet to be
  discovered (1995).

Insert TCP 6e Figure 13.4a unannotated
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First Extrasolar Planet
Insert TCP 6e Figure 13.4b

• The planet around 51 Pegasi has a mass similar to
  Jupiters, despite its small orbital distance.

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Other Extrasolar Planets

• Doppler shift data tell us about a planets mass
  and the shape of its orbit.

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Doppler Technique

• Measuring a stars Doppler shift can tell us its
  motion toward and away from us.
• Current techniques can measure motions as small
  as 1 m/s (walking speed!).

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Planet Mass and Orbit Tilt

• We cannot measure an exact mass for a planet
  without knowing the tilt of its orbit, because
  Doppler shift tells us only the velocity toward
  or away from us.
• Doppler data give us lower limits on masses.

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Transit Method

• Astronomers do photometry well and can detect
  small, periodic changes in light level. Small
  telescopes can do this.
• Need very close to edge-on systems, usually
  within a degree given planet sizes, separations,
  and geometry.
• More than a thousand candidates here or coming
  (Kepler mission!), dozens confirmed.
• Can detect Earth-like planets, but needs long
  timescales to see planets far out from their
  suns.

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Transit Missions

• NASAs Kepler mission was launched in 2008 to
  begin looking for transiting planets.
• It is designed to measure the 0.008 decline in
  brightness when an Earth-mass planet eclipses a
  Sun-like star.

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Transits and Eclipses

• A transit is when a planet crosses in front of a
  star.
• The resulting eclipse reduces the stars apparent
  brightness and tells us planets radius.
• No orbital tilt accurate measurement of planet
  mass

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Direct Imaging ProblemBrightness Difference

• A Sun-like star is about a billion times brighter
  than the light reflected from its planets.
• This is like being in San Francisco and trying to
  see a pinhead 15 meters from a grapefruit in
  Washington, D.C.

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Direct Imaging
Keck adaptive optic image showing planets
orbiting HR 8799. http//apod.nasa.gov/apod/ap0811
17.html
A VLT infrared image of a hot young planet around
a brown dwarf star.
An HST coronograph image of a planet around
Fomalhaut. http//apod.nasa.gov/apod/ap081114.html

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What have we learned about extrasolar planets?
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Orbits of Extrasolar Planets

• Most of the detected planets have orbits smaller
  than Jupiters.
• Planets at greater distances are harder to detect
  with the Doppler technique.

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Orbits of Extrasolar Planets

• Orbits of some extrasolar planets are much more
  elongated (have a greater eccentricity) than
  those in our solar system.

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Multiple-Planet Systems

• Some stars have more than one detected planet.

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Orbits of Extrasolar Planets

• Most of the detected planets have greater mass
  than Jupiter.
• Planets with smaller masses are harder to detect
  with Doppler technique.

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Hot Jupiters
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Revisiting the Nebular Theory

• The nebular theory predicts that massive
  Jupiter-like planets should not form inside the
  frost line (at ltlt 5 AU).
• The discovery of hot Jupiters has forced
  reexamination of nebular theory.
• Planetary migration or gravitational encounters
  may explain hot Jupiters.

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Planetary Migration

• A young planets motion can create waves in a
  planet-forming disk.
• Models show that matter in these waves can tug on
  a planet, causing its orbit to migrate inward.

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Planets Common or Rare?

• One in ten stars examined so far have turned out
  to have planets.
• The others may still have smaller (Earth-sized)
  planets that current techniques cannot detect.
• Kepler seems to indicate COMMON

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Take Aways

• Very likely all stars, or nearly all stars, have
  planets based on our current detection rates,
  keeping in mind our limitations.
• At least a few percent of systems with planets,
  and likely more, have Earth-like planets. Worst
  case scenario tens of millions in the Milky Way.
• A little early to say if our Solar System is
  typical, but there exists quite a range out there
  different from our own http//www.space.com/7916-
  strange-zoo-worlds.html
• Hot Jupiters
• Big planets farther out, Cthonian worlds, water
  worlds, super Earths, rogues
• Some highly eccentric orbits
• Tatooine planets in binary star systems
  (which are common)
• FIELD IS CHANGING FAST CHECK THE WEB/APPS!