Extra Solar Planets

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 Also have star system/planet building webpages: http://curriculum.calstatela.edu/courses/builders/lessons/less/les1/choose.html

0 Binary Stars 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.

0 The Center of Mass center of mass = balance point of the system. 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.

0 Estimating Stellar Masses Recall Kepler’s 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)

0 Examples: 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 Kepler’s 3. Law must have a total mass of 1 solar mass.

0 Visual Binaries The ideal case: Both stars can be seen directly, and their separation and relative motion can be followed directly.

0 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:

0 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) 0 Typical sequence of spectra from a spectroscopic binary system Time

0 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) 0 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 discovered HD 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 transitis 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 Keck adaptive optic image showing planets orbiting HR 8799. http://apod.nasa.gov/apod/ap081117.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

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) • FIELD IS CHANGING FAST –CHECK THE WEB/APPS!

Extra Solar Planets