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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) L p = L * /4πd 2 a πR p 2 ~ L * (R p /d) 2 For the Earth, (R p /d) 2 ~5 x 10 8 For Jupiter, (R p /d) 2 ~10 8

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 Transits requires an edge-on orbit. Jupiter blocks 2% of the Sun's light the Earth blocks about 0.01%. Artist’s Conception 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 M 1 = R 2 M 2 ; a = R 1 + R 2 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 3 rd law: P 2 = a 3 a ~ r p (M * >> M p ) r * = m p /m * r p (center of mass) V * = 2π m p /m * /(r p ) 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 M J, 0.8 M J, 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 M U 0.4 AU 1 M J 0.15 AU 1 M s 0.25 AU 0.5 M J 0.8 AU 4 M J 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. T p ~ (L * /D 2 ) 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. T p 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