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AST101 Lecture 18
Extra Solar Planets
Finding Planets
1. Direct Imaging
HR 8799
Direct Searches
Direct searches are difficult because stars are so bright.
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
How Bright are Planets?
You gain by going to long wavelengths,
where the Sun is relatively faint, and
the planet is relatively bright.
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.
HR 8799
(A5V)
Finding Planets
2. Transits
Finding Planets
2. Transits
Transits
Artist’s Conception
Transits requires an edge-on orbit.
• Jupiter blocks 2% of the Sun's light
• the Earth blocks about 0.01%.
Venus, 8 June 2004
Finding Planets
3. Astrometric
Wobble
Finding Planets
4. Doppler Wobble
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
R 1 M1 = R 2 M 2 ;
a = R 1 + R2
This is Newton’s law of equal and opposite reactions.
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⊕ = 2 cm/s; VJ = 3 m/s
51 Pegasi b
Doppler Wobble: Gliese 876
The three planets of Gl 876:
masses = 2.5 MJ, 0.8 MJ, and 7.5 M⊕
Gliese 876
M4V star
3 planets, including the least massive known (0.75 M⊕)
Extrasolar Planetary Systems
55 Cancri (G5V): 5 planets
• 1 MU 0.4 AU
• 1 MJ 0.15 AU
• 1 Ms 0.25 AU
• 0.5 MJ 0.8 AU
• 4 MJ 5 AU
Biases and Limits
Extrasolar Planets
Planets are preferentially found around metal-rich stars
- mostly younger than the Sun.
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.
Faint Young Sun Problem