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Astronomy190 - Topics in Astronomy

Astronomy190 - Topics in Astronomy. Astronomy and Astrobiology Lecture 19 : Extrasolar Planets Ty Robinson. Questions of the Day. What techniques are used to study extrasolar planets? What is the difference between direct and indirect detection of a planet?

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Astronomy190 - Topics in Astronomy

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  1. Astronomy190 - Topics in Astronomy Astronomy and Astrobiology Lecture 19 : Extrasolar Planets Ty Robinson

  2. Questions of the Day • What techniques are used to study extrasolar planets? • What is the difference between direct and indirect detection of a planet? • What are the known extrasolar planets like?

  3. A Multitude of Worlds • 365 Planets • 308 Planetary Systems • 39 Multiple Planet Systems • Most are Jovians • 10 SuperEarths (2-10 Earth masses) Not bad for not being able to see anything!

  4. Indirect Detection of Extrasolar Planets These techniques use changes in the position or brightness of a star to infer the existence of a planet

  5. Planet and Star orbit around a common center of mass. This can make the star appear to ‘dance’, even when the planet can’t be seen.

  6. The Doppler Technique http://planetquest.jpl.nasa.gov

  7. Detection of Radial Velocity via Spectral Shift Na @ 589.3nm • Radial velocity changes in the star being observed are detected via the shift in the wavelengths of known absorption features in the spectrum of the star. Redshifted if the source is moving away from the observer, blueshifted if the source is moving towards the observer. Source not moving Observer sees Blueshift Observer sees Redshift Source redshifted Na @ 591.5nm

  8. What We Can Observe • If there is no planet, then there is no strong, periodic change in a star’s apparent radial velocity. • Stars with planets show periodic changes in RV No Planet: RV changes small And random Planet: Periodic RV changes

  9. Period Period Period and Orbital Eccentricity

  10. The Radial Velocity (Doppler) Technique • uses spectroscopic measurements of starlight to detect the redshift or blueshift from the radial velocity component of a star’s motion • most successful technique to date • most sensitive to big planets close to their star • can be used to infer planetary mass, albeit with a sine i ambiguity due to the (usually) unknown inclination of the planetary system to the observer’s line of sight • can determine planetary period and eccentricity

  11. Transit

  12. Change in the Star’s Brightness From http://astro.u-strasbg.fr/goutelas/g2005/chap03-moutou.pdf

  13. T. Brown and D. Charbonneau The Kepler Mission Measures stellar brightness changes caused by transiting terrestrial planets. Monitoring 100,000 stars for 4 years! Transit Telescope Launched 2009

  14. Transit Technique • detects the faint drop in a star’s brightness due to a planet passing across the face of the star • sensitive to a large planet/star area ratio (so more sensitive to large planets overall, but has improved sensitivity to smaller planets around smaller stars) • more sensitive to planets close to their star because these are more likely to be seen in transit • can be used to determine planetary size, if the star’s size can be estimated (transit does not give mass!) • provides planetary orbital period and distance • limitation: the probability of a transit is low for any given planetary system, so transits are rare

  15. Direct Detection of Extrasolar Planets • Directly detects photons from the planet itself. • We can either suppress the light from the parent star • Or cleverly subtract the star’s light

  16. allows us to directly study a planet Extremely difficult! star-planet contrast is large star and planet often can’t be separated on the sky (angular resolution) star-planet contrast is typically less in the infrared 109 106 Direct Detection

  17. Removing the Light from the Parent Star • direct detection techniques also require some means to separate or remove the light from the parent star so that the light from the smaller, fainter planet can be seen • subtraction techniques: light from both planet and star, and star alone, are measured and subtracted to reveal the planet’s radiation • suppression or cancellation techniques: light from the parent star is cancelled (nulled) or blocked out (suppressed) using a special type of instrument

  18. Extrasolar Planet Properties: Mass • Rise towards lower masses (smallest are 3.5Mearth) • In this sample, only 13/138 (10%) have Mp > 5Mj

  19. Hot Jupiters very easy to detect using transit, radial velocity techniques form beyond frost line, migrate inwards habitable moons?

  20. Questions of the Day • What techniques are used to study extrasolar planets? • What is the difference between direct and indirect detection of a planet? • What are the known extrasolar planets like?

  21. Quiz 1 - Describe one way that we constrain the inner edge of the HZ around the Sun and one way that we constrain the outer edge. 2 - Describe how “Hot Jupiters” form. 3 - What is one thing you did not understand from today’s lecture?

  22. Challenges: Angular Separation from Parent Star • angular resolution improves with shorter wavelength and/or bigger optics (lens or mirror)

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