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. First exoplanet discovered (1995)
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”
CM just outside Sun’s visible surface.

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.
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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
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22. Mass Estimate 51 Pegasi: Mass of 51 Pegasi works out to 0.47 MJupiter
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
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________________
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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 Milky Way Galaxy is home to billions of planetary systems.