1. Exoplanets: Indirect Search Methods
24 October 2016
© 2014 Pearson Education, Inc.

2. Nearly 4000 ESPs have been found since then
KEY IDEAS
Planets around other stars are called Extrasolar planets (ESPs), or exoplanets
They are common
First direct evidence of an exoplanet was found in 1995 around 51 Pegasi
Nearly 4000 ESPs have been found since then

3. } Direct methods: Indirect methods:
ESP Detection Methods
Direct methods:
Detection of visible or IR light from planet.
Indirect methods:
Astrometry.
Doppler shift.
Transit. }
to detect
star’s
“wobble”

4. Chapter Opener
Images of 4 planets
© 2014 Pearson Education, Inc.

5. Model spectrum of the sun
and planets as seen from a distance
comparable to that of a nearby star (10
pc), shown in physical units.

6. Detecting Extrasolar Planets
dwarf star,
not planet
Cannot yet use telescopes to take visible pictures of ESPs:
Planets are too close to their stars to image, even with the best telescopes.
Most planets are too dim to image in visible light: Sun-like star is ~109 x brighter than its planets.
Recent attempts to “null” out the starlight.
In IR light, star is 106 x brighter than planet

7. Another success for Newton’s Laws Unnumbered Figure Page 368
© 2014 Pearson Education, Inc.

8. A big planet gravitationally tugs on its star.
Stellar "Wobble"
A big planet gravitationally tugs on its star.
A star and planet actually orbit around the center of mass of the two.
The visible star appears to wobble slightly (“reflex motion”).

10. Figure 13.1
© 2014 Pearson Education, Inc.

11. Sun is tugged by the planets (mostly Jupiter & Saturn).
Wobble of Our Sun
Sun is tugged by the planets (mostly Jupiter & Saturn).
Astrometry (measurement of star positions) is not precise enough to detect most stellar wobble.
0.0002”
Sun from 30 light-years
(Hubble ≈ 0.1”)

12. Figure 13.3
© 2014 Pearson Education, Inc.

13. A big planet gravitationally tugs on its star.
Stellar Wobble
A big planet gravitationally tugs on its star.
A star and planet actually orbit around the center of mass of the two.
The visible star appears to wobble slightly (“reflex motion”).

14. Detecting Stellar "Wobble"
Stellar wobble can be detected by observing the Doppler shift of light from the star.
Doppler shift: Light appears bluer when star moves toward us, and redder when star moves away.
Works best for edge-on orbits; does not work for face-on orbits.

15. The Doppler Shift
Doppler shift.
Sound waves.

16. What we actually observe:
The Doppler Shift
What we actually observe:
Absorption lines in star’s spectrum are shifted toward the red or blue.

17. Figure 13.4
© 2014 Pearson Education, Inc.

18. Figure 13.8b
© 2014 Pearson Education, Inc.

19. Figure
© 2014 Pearson Education, Inc.

22. Unnumbered Figure 2 Page 392
© 2014 Pearson Education, Inc.

23. Kepler Spacecraft
© 2014 Pearson Education, Inc.

24. © 2014 Pearson Education, Inc.

25. © 2014 Pearson Education, Inc.

26. Figure 13.5
© 2014 Pearson Education, Inc.

27. Figure 13.11
© 2014 Pearson Education, Inc.

28. Figure 13.9
© 2014 Pearson Education, Inc.

29. Figure
© 2014 Pearson Education, Inc.

30. © 2014 Pearson Education, Inc.

31. © 2014 Pearson Education, Inc.

32. Kepler
After 150,000 stars and 4 years of observation

33. Kepler (all planets)
After 150,000 stars and 4 years of observation

34. Kepler (in “Habitable Zone”)

35. Kepler (extrapolation of “Earth-like planets”)

36. in our galaxy!

37. Figure 13.4b
© 2014 Pearson Education, Inc.

38. Kepler 186f is like Earth
© 2014 Pearson Education, Inc.

39. © 2014 Pearson Education, Inc.

40. Hot Jupiters Giant planets near the star!
Easy to find because their gravity pulls hard on the star and they orbit quickly
Not like anything in our Solar System

41. There is a well-established correlation between a star’s metallicity and its likelihood to host hot Jupiters

42. Figure 13.13
© 2014 Pearson Education, Inc.

43. Table 13.1
© 2014 Pearson Education, Inc.

44. Table 13.2
© 2014 Pearson Education, Inc.

45. Summary: Direct Methods
In principle, it is possible to see exoplanets by the light they reflect, but the brightness difference is too large: like a moth in a searchlight. Only a few cases found…
Infra-red light (IR) is more promising, because the planets are only a million times dimmer
Shades, coronagraphs, adaptive optics, interferometry improve this

46. Summary: Indirect Methods
A planet causes the star to execute a small orbit, that can be detected by
measuring the star position (astrometry) or
by measuring its speed (Doppler spectroscopy of radial velocity
A planet can be detected by occultation (transit method)
Each method has advantages for detecting some planets
© 2014 Pearson Education, Inc.