Astronomy 340 Fall 2007

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?

12. Direct Imaging

13. Radial Velocity Variation

14. Radial Velocity Variations

15. Exoplanets circa 2000

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

18. 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

19. 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

20. (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