2. AST101 Lecture 18 Extra Solar Planets

4. Finding Planets
Direct Imaging
HR 8799

5. HR 8799 (A5V)

6. Direct Searches
Direct searches are difficult because stars are so bright.

7. How Bright are Planets? Planets shine by reflected light.
The amount reflected is the amount received
(the solar constant)
- Times the area of the planet
- Times the albedo (reflected), or
- Times (1-albedo) (emitted)
Lp = L*/4πd2 a πRp2 ~ L* (Rp/d)2
For the Earth, (Rp/d)2 ~5 x 108
For Jupiter, (Rp/d)2 ~108

8. How Bright are Planets? You gain by going to long wavelengths,
where the Sun is relatively faint, and the planet is relatively bright.

9. How Far are Planets from Stars?
By parallax, 1 AU = 1“ at 1 pc
1 pc (parsec) = 3.26 light years
1“ (arcsec) = 1/3600 degree
As seen from α Centauri (4.3 LY):
Earth is 0.75 arcsec from Sol
Jupiter is 4 arcsec from Sol
Can we see this?
Yes, but it takes special
techniques, and is not easy.

10. Finding Planets
2. Transits

11. Finding Planets
2. Transits

12. Transits Transits requires an edge-on orbit.
Venus, 8 June 2004
Artist’s Conception
Transits requires an edge-on orbit.
Jupiter blocks 2% of the Sun's light
the Earth blocks about 0.01%.

13. Finding Planets
3. Astrometric Wobble

14. Finding Planets
4. Doppler Wobble

15. Orbits
Planets do not orbit the Sun - they both orbit the center of mass.
The radius of the orbit is inversely proportional to the mass
The radius of the Sun’s orbit with respect to the Earth is 1/300,000 AU, or 500 km
R1 M1 = R2 M2; a = R1 + R2
This is Newton’s law of equal and opposite reactions.

16. Orbital Velocity V = 2πr/P r is the radius of the orbit
P is the orbital period
V is the orbital velocity
How fast does the star “wobble”?
Kepler’s 3rd law: P2 = a3
a ~ rp (M* >> Mp)
r* = mp/m* rp (center of mass)
V* = 2π mp/m* /(rp)1/2
VE = 2 cm/s; VJ = 3 m/s

17. 51 Pegasi b

18. Doppler Wobble: Gliese 876
The three planets of Gl 876:
masses = 2.5 MJ, 0.8 MJ, and 7.5 M⊕

19. Gliese 876
M4V star
3 planets, including the least massive known (0.75 ME)

20. Extrasolar Planetary Systems
55 Cancri (G5V): 5 planets
1 MU 0.4 AU
1 MJ AU
1 Ms AU
0.5 MJ 0.8 AU
4 MJ 5 AU

21. Biases and Limits

23. Extrasolar Planets
Planets are preferentially found around metal-rich stars
- mostly younger than the Sun.

25. Habitable Zones Refer back to our discussion of the Greenhouse Effect.
Tp ~ (L*/D2)0.25
The habitable zone is the region where the temperature
is between 0 and 100 C (273 and 373 K), and water can
be liquid.
Tp depends on both the solar luminosity L* and the
distance D.
D does not change, but L* does, as the star evolves.

26. Faint Young Sun Problem