Chapter 13 13.1 Detecting Extrasolar Planets Other Planetary Systems The New Science of Distant Worlds • Our goals for learning • Why is it so difficult to detect planets around other stars? • How do we detect planets around other stars? Why is it so difficult to detect Brightness Difference planets around other stars? • A Sun-like star is about a billion times brighter than the sunlight reflected from its planets • Like being in San Francisco and trying to see a pinhead 15 meters from a grapefruit in Washington, D. C. Special Topic: How did we learn How do we detect planets around other stars are Suns? other stars? • Ancient observers didn’t think stars were like the Sun because Sun is so much brighter. • Christian Huygens (1629-1695) used holes drilled in a brass plate to estimate the angular sizes of stars. • His results showed that, if stars were like Sun, they must be at great distances, consistent with the lack of observed parallax. 1
Gravitational Tugs Planet Detection • Sun and Jupiter orbit • Direct: Pictures or spectra of the planets around their common center of themselves mass • Indirect: Measurements of stellar • Sun therefore properties revealing the effects of orbiting wobbles around that planets center of mass with same period as Jupiter Gravitational Tugs Astrometric Technique • Sun’s motion around • We can detect planets solar system’s center by measuring the of mass depends on change in a star’s tugs from all the position on sky planets • However, these tiny • Astronomers around motions are very other stars that difficult to measure measured this motion (~0.001 arcsecond) could determine masses and orbits of all the planets Doppler Technique First Extrasolar Planet • Measuring a star’s • Doppler shifts of star Doppler shift can tell 51 Pegasi indirectly us its motion toward reveal a planet with and away from us 4-day orbital period • Current techniques • Short period means can measure motions small orbital distance as small as 1 m/s (walking speed!) • First extrasolar planet to be discovered (1995) 2
First Extrasolar Planet Other Extrasolar Planets Large planet mass Highly eccentric orbit • Planet around 51 Pegasi has a mass similar to • Doppler data curve tells us about a planet’s mass and Jupiter’s, despite its small orbital distance the shape of its orbit Planet Mass and Orbit Tilt Transits and Eclipses • We cannot measure an exact mass for a planet without • A transit is when a planet crosses in front of a star knowing the tilt of its orbit, because Doppler shift tells • The resulting eclipse reduces the star’s apparent us only the velocity toward or away from us brightness and tells us planet’s radius • Doppler data gives us lower limits on masses • No orbital tilt: accurate measurement of planet mass Spectrum during Transit Direct Detection • Change in spectrum during transit tells us about • Special techniques can eliminate light from brighter composition of planet’s atmosphere objects • These techniques are enabling direct planet detection 3
What have we learned? Other Planet-Hunting Strategies • Why is it so difficult to detect planets around other stars? • Gravitational Lensing: Mass bends light in – Direct starlight is billions of times brighter than starlight reflected from planets a special way when a star with planets • How do we detect planets around other passes in front of another star. stars? – A star’s periodic motion (detected through • Features in Dust Disks: Gaps, waves, or Doppler shifts) tells us about its planets ripples in disks of dusty gas around stars – Transiting planets periodically reduce a star’s can indicate presence of planets. brightness – Direct detection is possible if we can block the star’s bright light What have we learned about 13.2 The Nature of Extrasolar Planets extrasolar planets? • Our goals for learning • What have we learned about extrasolar planets? • How do extrasolar planets compare with those in our solar system? Orbits of Extrasolar Planets Measurable Properties • Most of the detected planets have orbits • Orbital Period, Distance, and Shape smaller than Jupiter’s • Planet Mass, Size, and Density • Composition • Planets at greater distances are harder to detect with Doppler technique 4
Orbits of Extrasolar Planets Multiple-Planet Systems • Orbits of some • Some stars extrasolar planets are have more much more elongated than one (greater eccentricity) detected than those in our planet solar system Multiple-Planet Systems 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 • Special techniques can eliminate light from brighter objects • These techniques are enabling direct planet detection How do extrasolar planets compare Surprising Characteristics with those in our solar system? • Some extrasolar planets have highly elliptical orbits • Some massive planets orbit very close to their stars: “Hot Jupiters” 5
What have we learned? Hot Jupiters • What have we learned about extrasolar planets? – Detected planets are all much more massive than Earth – They tend to have orbital distances smaller than Jupiter’s – Some have highly elliptical orbits • How do extrasolar planets compare with those in our solar system? – Some “Hot Jupiters” have been found Can we explain the surprising 13.3 The Formation of Other Solar Systems orbits of many extrasolar planets? • Our goals for learning • Can we explain the surprising orbits of many extrasolar planets? • Do we need to modify our theory of solar system formation? Planetary Migration Revisiting the Nebular Theory • A young planet’s motion can create • Nebular theory predicts that massive waves in a planet- Jupiter-like planets should not form inside forming disk the frost line (at << 5 AU) • Discovery of “hot Jupiters” has forced • Models show that reexamination of nebular theory matter in these waves can tug on a planet, • “Planetary migration” or gravitational causing its orbit to encounters may explain “hot Jupiters” migrate inward 6
Orbital Resonances Gravitational Encounters • Resonances between planets can also cause • Close gravitational encounters between two their orbits to become massive planets can eject one planet while more elliptical flinging the other into a highly elliptical orbit • Multiple close encounters with smaller planetesimals can also cause inward migration Do we need to modify our theory Modifying the Nebular Theory of solar system formation? • Observations of extrasolar planets have shown that nebular theory was incomplete • Effects like planet migration and gravitational encounters might be more important than previously thought What have we learned? Planets: Common or Rare? • Can we explain the surprising orbits of many extrasolar planets? • One in ten stars examined so far have – Original nebular theory cannot account for turned out to have planets “hot Jupiters” – Planetary migration or gravitational encounters may explain how Jupiter-like • The others may still have smaller (Earth- planets moved inward sized) planets that current techniques cannot • Do we need to modify our theory of solar detect system formation? – Migration and encounters may play a larger role than previously thought 7
How will we search for Earth-like 13.4 Finding New Worlds planets? • Our goals for learning • How will we search for Earth-like planets? Transit Missions Astrometric Missions • NASA’s Kepler mission is scheduled • GAIA: A European mission planned for 2010 that to begin looking for will use interferometry to measure precise motions transiting planets in of a billion stars 2008 • SIM: A NASA mission planned for 2011 that will • It is designed to use interferometry to measure star motions even measure the 0.008% more precisely (to 10 -6 arcseconds) decline in brightness when an Earth-mass planet eclipses a Sun- like star What have we learned? Direct Detection • How will we search for Earth-like planets? • Determining whether – Transit missions will be capable of finding Earth-mass planets Earth-like planets that cross in front of their are really Earth-like stars ( Kepler to launch in 2008) requires direct – Astrometric missions will be capable of detection measuring the “wobble” of a star caused by an orbiting Earth-like planet • Missions capable of – Missions for direct detection of an Earth-like blocking enough planet will need to use special techniques (like starlight to measure interferometry) for blocking starlight Mission concept for NASA’s the spectrum of an Terrestrial Planet Finder (TPF) Earth-like planet are being planned 8
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