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AST 111

AST 111. Exoplanets I. Exoplanets. Exoplanets : Planets orbiting stars other than the Sun. Exoplanets. Remember: 99.9% of our Solar System is in the Sun Stars are so far that it is very difficult to image their planets

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AST 111

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  1. AST 111 Exoplanets I

  2. Exoplanets Exoplanets: Planets orbiting stars other than the Sun

  3. Exoplanets • Remember: 99.9% of our Solar System is in the Sun • Stars are so far that it is very difficult to image their planets • Light from stars 1,000,000,000x stronger than from planets, and the starlight gets blurred • Only 10 have been directly imaged

  4. A Picture • From the book: • 1 to 10 billion scale: • The Sun is a grapefruit • Earth is the head of a pin 15 meters away • Jupiter is a marble 80 meters away • Distance to nearest star is distance across U.S. • You’re in San Francisco looking at Washington D.C. • The grapefruit is hard enough to find! Good luck with the pinhead!

  5. Exoplanets • But technology now moves at an astounding rate. As I was writing this, I received this!! Credit: ESO

  6. Exoplanets • YOU CAN OBSERVE EXOPLANETS. • A 10” scope with a CCD camera is all that’s needed. ($4k) • It’s even a homework problem in the book: Chapter 13, Problem 54

  7. Exoplanets • Before 1990’s, all we knew about planets was our own Solar System • 1200 exoplanets (and more every day!) have been counted

  8. Lots of Earths!

  9. 51 Pegasi • First exoplanet discovered (1995) • Surface temp of 1340 oF

  10. How do we find them? • Exoplanets are indirectly observed by: • Gravitational wobble • Astrometric technique • Doppler technique • Transits and eclipses • Direct detection

  11. Gravitational Wobble • Objects in a solar system orbit the center of mass • The star appears to “wobble”

  12. Gravitational Wobble • Larger mass planets cause more wobble • More planets add their own wobbles to the star • Our Sun: • Jupiter creates a wobble • Saturn’s effect is perceptible • Other planets’ effects hard to see

  13. Gravitational Wobble (Astrometric) • Astrometric method: • Precisely measure stellar position • 10 LY away: • Large planet at 5AU from Sun-like star causes the star to move “the width of a human hair… seen at 3 miles” • Best for massive planets orbiting close stars • Takes many years of observing– ONE exoplanet found with this method

  14. Gravitational Wobble (Doppler) • 51 Pegasi discovered by alternating blue and red shifts • Recall spectra: • Blueshifts if moving toward us • Redshifts if moving away

  15. Gravitational Wobble (Doppler)

  16. Gravitational Wobble (Doppler) • Doppler technique accurate to 1 m/s • Period of graph is the planet’s orbital period • Kepler’s laws give distance from star

  17. Gravitational Wobble (Doppler) • Can determine orbital shape • More elliptical orbit, more “skewed” graph

  18. Gravitational Wobble (Doppler) • The Doppler technique does not work from bird’s eye view! • Recall that there must be relative motion toward or away from observer.

  19. Gravitational Wobble (Doppler) • Works best with more massive, closer stars • More gravitational pull • More wobble • Less time wait for doppler shifts

  20. Mass Estimate • Conservation of Momentum: Mstarvstar = Mplanetvplanet Mplanet = Mstarvstar vplanet • Doppler technique gives vstar. Know Mstar. _____________

  21. Mass Estimate • Kepler’s Laws give the radius of orbit • Doppler effect gives period of planet’s orbit • Also, vplanet = 2paplanet pplanet Mplanet = Mstarvstarpplanet 2paplanet ___________ ____________________

  22. Mass Estimate • 51 Pegasi: • 2.12 x 1030 kg (star’s mass) • 57 m/s (star’s velocity) • Radius of orbit: 7.82 x 109 m • Planet’s orbital period: 3.65 x 105 seconds • Mass of 51 Pegasi works out to 0.47 MJupiter

  23. Transits and Eclipses • Transit: Planet passes in front of star • Eclipse: Planet passes behind the star • Can’t actually see the dot moving across the star • Measure brightness changes

  24. Transits and Eclipses • Requires correct alignment (edge-on)

  25. Transits and Eclipses • Amount of “dip” in brightness gives planet’s radius • Have mass from Kepler’s Laws • Can get density: • Looks like terrestrial? • Looks like Jovian?

  26. Transits and Eclipses • Can show composition of upper atmosphere • During transit, starlight passes through the planet’s atmosphere

  27. Planet Size • From brightness measurements: Fraction of light blocked = area of planet’s disk = pr2planet area of star’s disk pr2star = r2planet r2star ________________________________ ________________ __________

  28. Example: HD209458 • rplanet = rstar x (fraction of light blocked)1/2 • Star HD209458: • Radius of 800,000 km • Planet blocks 1.7% of light (0.017) • rplanet = 800,000 km x (0.017)1/2 • rplanet =100,000 km

  29. Transits and Eclipses • One can observe the system in infrared during eclipse • Planets glow in infrared • Decrease in IR part of spectrum shows how much IR the planet gives • Can calculate planet’s temperature • Can identify greenhouse gases

  30. Direct Detection • Works for very large planets • Far from parent star

  31. Direct Detection

  32. Direct Detection

  33. Exoplanets • From the book: Although it is too soon to know for sure,it seems ever more likely that our MilkyWay Galaxy is home to billions of planetarysystems.

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