Extra Solar Planets Just some introductory materials. A very fast moving field. My favorite website: http://www.exoplane ts.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
0. Extra Solar Planets. 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 - PowerPoint PPT Presentation
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Extra Solar PlanetsJust 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
Both masses equal => center of mass is in the middle, rA = rB.
The more unequal the masses are, the more
it shifts toward the more massive star.
Estimating Stellar MassesRecall Kepler’s 3rd Law:
Py2 = aAU
3
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:
MA + MB = aAU
3 ____ Py
2
(MA and MB in units of solar masses)
Examples:a) Binary system with period of P = 32 years
and separation of a = 16 AU:
MA + MB = = 4 solar masses.163____322
b) Any binary system with a combination of period P and separation a that obeys Kepler’s
3. Law must have a total mass of 1 solar mass.
Visual Binaries
The ideal case:
Both stars can be seen directly, and
their separation and relative motion can be followed directly.
Spectroscopic Binaries
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:
Spectroscopic Binaries (II)
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
Spectroscopic Binaries (III)
Tim
e
Typical sequence of spectra from a spectroscopic binary system
Eclipsing Binaries
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!
Eclipsing Binaries (II)
Peculiar “double-dip” light curve
Example: VW Cephei
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 discoveredHD 209458 – Transit of planet across star
Radial Velocity or “Wobble” Method
• 51 Peg back in 1996, followed by hundreds of others, primarily from Geoff Marcy’s group out of California (Lick and Keck Observatories). Marcy went on Letterman wearing a Hawaiian shirt we both bought in Kona…tried mine on and it’s 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.
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
First Extrasolar Planet
• The planet around 51 Pegasi has a mass similar to Jupiter’s, despite its small orbital distance.
Insert TCP 6e Figure 13.4b
Other Extrasolar Planets
• Doppler shift data tell us about a planet’s mass and the shape of its orbit.
Doppler Technique
• Measuring a star’s 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!).
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.
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.
Transit Missions
• NASA’s 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.
Transits and Eclipses
• A transit is when a planet crosses in front of a star.• The resulting eclipse reduces the star’s apparent
brightness and tells us planet’s radius.• No orbital tilt: accurate measurement of planet mass
Direct Imaging Problem:Brightness 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.
Direct Imaging
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
What have we learned about extrasolar planets?
Orbits of Extrasolar Planets
• Most of the detected planets have orbits smaller than Jupiter’s.
• Planets at greater distances are harder to detect with the Doppler technique.
Orbits of Extrasolar Planets
• Orbits of some extrasolar planets are much more elongated (have a greater eccentricity) than those in our solar system.
Multiple-Planet Systems
• Some stars have more than one detected planet.
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.
Hot Jupiters
Revisiting the Nebular Theory
• The nebular theory predicts that massive Jupiter-like planets should not form inside the frost line (at << 5 AU).
• The discovery of hot Jupiters has forced reexamination of nebular theory.
• Planetary migration or gravitational encounters may explain hot Jupiters.
Planetary Migration
• A young planet’s 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.
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
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)