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Exploring Detection Methods for Extrasolar Planets: Techniques and Challenges

This lecture focuses on the various detection methods used to identify extrasolar planets, emphasizing radial velocity variations, astrometry, and direct imaging. It discusses the challenges faced in detecting these distant worlds, including the overwhelming brightness of stars and the tiny separations between stars and their planets. The lecture also covers the first detection of a planet around a pulsar, transit methods, and the significance of timing in observations. Finally, it highlights recent discoveries and the evolution of our understanding of exoplanet populations and their characteristics.

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Exploring Detection Methods for Extrasolar Planets: Techniques and Challenges

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  1. Astronomy 340Fall 2007 4December 2007 Lecture #26

  2. Extrasolar Planets • Detection Methods • Radial velocity variation • Astrometry • Direct imaging • transients

  3. Imaging • Detection of “point source” image  reflected stellar light • Lp/L* = p(λ,α)(Rp/a)2 • α  angle between star and observer as seen from planet • p  geometric albedo • Ratio ~ 10-9 for Jupiter • Difficulties • Planets are overwhelmed by starlight • Separations are tiny  need space interferometry, adaptive optics

  4. Dynamical Perturbation • Motion of planet causes reflex circular motion in star about the center of mass of star/planet system • Observables:

  5. Dynamical Perturbation • Motion of planet causes reflex circular motion in star about the center of mass of star/planet system • Observables: • Radial velocity variations • Variations in position (astrometry) • Variation in the time of arrival of some reference signal (generally used for pulsars)

  6. Radial Velocity Variations • Just use Newton and Kepler….we’ll do this a bit later…

  7. Radial Velocity Variations • Just use Newton and Kepler…. • For Jupiter-Sun system  K = 12.5 m s-1 with a period of 11.9 years • For Earth-Sun system  K = 0.1 m s-1 • Only measure Mpsini, not Mp • All extrasolar planets were initially detected using radial velocity variations • Resolution of a few m s-1 are possible  but keep in mind the orbit times! • Might get down to 1 m s-1

  8. Astrometric Position • Star moves a bit as it orbits about the center of mass • Angular semi-major axis: • α = (Mp/M*) (a/d) • Units: a (AU), d (pc) • Jupiter-Sun system viewed from 10 pc away  500μas • Earth-Sun  0.3μas Need space interferometry  impossible from the ground

  9. Timing • 1st “planet” detected was around a pulsar  hard to believe! • Planet causes a tiny wobble which would affect timing of pular • Τp = 1.2 (Mpulsar/Mplanet)(P/1 year)2/3 ms • Discovery of few Earth mass sized objects around pulsar PSR 1257+12 • Where did they come from? • Survived the SNe? • Captured • Formed after the formation of the neutron star

  10. Transits/Reflections • How does planetary motion affect the apparent brightness of the star? • In suitable geometry, planet blocks out part of the star  2% for a Sun-Jupiter system • ΔL/L ~ (Rp/R*)2 • Tiny fractions for terrestrial planets  10-5 • Timing – transits are short! • Τ = (P/π)(R*cosδ + Rp)/a = 13(M*)-1/2(a)1/2(R*) h • In units of solar masses, solar radii, and AU • 25 hours for jupiter • 13 hours for Earth Maybe a large survey of large numbers of possible stars?

  11. Direct Imaging

  12. Radial Velocity Variation

  13. Radial Velocity Variations

  14. Exoplanets circa 2000

  15. Extrasolar Planets • Population  150 “exoplanets” • Most with masses: 100-5000 MEarth Santos, Benz, Mayor 2005

  16. Extrasolar Planets • Population  150 “exoplanets” • Most with masses: 100-5000 MEarth • 6 transits • Orbital periods 1.21-4.02 days • Planet radii: 1.0-1.35 RJ • Planet masses: 0.53-1.45 MJ • Mass-radius relation  ρ = 0.4-1.0

  17. Extrasolar Planets • Population  150 “exoplanets” • Most with masses: 100-5000 MEarth • 6 transits • Orbital periods 1.21-4.02 days • Planet radii: 1.0-1.35 RJ • Planet masses: 0.53-1.45 MJ • Mass-radius relation  ρ = 0.4-1.0 • Metallicity  correspondence between composition and presence of planets

  18. (Charbonneau et al. 2002) • Detection of 589.3nm feature from Na • Looking for deeper transit • Variation with time  “limb darkening” • “deeper” relative to other bands

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