Astronomy - 4/22/11 Quiz Ques?on #1 Most of our...

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Unformatted text preview: 4/22/11 Quiz Ques?on #1 Most of our observa?onal evidence for exoplanets A.  comes directly from pictures of the planets. B.  is based on electromagne?c waves received directly from the planets. C.  comes indirectly from observa?ons of stars, and thus is uncertain or even doubJul. D.  comes indirectly from observa?ons of stars, but is compelling or even decisive. Astro 109 Lecture 26: Extrasolar Planets April 22 Apr. 22 Quiz Ques?on #2 Key Concepts A surprising result from the search for planets around other stars is that A.  every planetary system found has been very similar to our own solar system. B.  planets as massive as Jupiter have been found in orbits smaller than Mercury’s. C.  no giant planets have been found, only small ones like Earth. D.  alien life forms have been detected on several planets. E.  no planets can be found orbi?ng stars other than our own. Apr. 22 •  Doppler technique •  transit technique •  characteris?cs of exoplanets •  upda?ng our theory of planet forma?on –  radial migra?on –  scaTering Apr. 22 Are we alone? •  The Drake equa?on: N = R∗ fp ne f￿ fi fc L How can we find planets around other stars? Frank Drake •  •  •  •  •  •  •  R* = rate of star forma?on fp = frac?on of stars that have planets ne = average number of habitable planets per star fl = frac?on of those that develop life fi = frac?on of those that develop intelligent life fc = frac?on of those with communica?on technology L = length of ?me civiliza?ons transmit signals Apr. 22 Apr. 22 1 4/22/11 From Lecture 8 Center of Mass Modify Kepler’s 1st law •  Newton’s 3rd law: if Sun pulls on Earth, then Earth also pulls on Sun. F = ma •  Both move! a= ⇔ F m •  Effect is small in Solar System. •  One of the ways we detect extrasolar planets. Apr. 22 Apr. 22 Center of Mass r1 M1 r2 M2 r 1 M1 = r2 M 2 r1 r2 = M2 M1 v1 v2 ⇒ = M2 M1 Apr. 22 Apr. 22 Example Example The semimajor axis of Jupiter’s orbit is 5.2 AU = 7.8×1011 m. Jupiter’s mass is 1.9×1027 kg, while the Sun’s mass is 2.0×1030 kg. What is the radius of the Sun’s “wobble” due to Jupiter? How big is that compared with the size of the Sun (radius 7.0×108 m)? How fast does the Sun move as it “wobbles”? The period of Jupiter’s orbit is 11.86 yr = 3.7×108 s. r1 r2 = M2 M1 r1 = r2 × v M2 M1 = 7.8 × 1011 m × = 7.4 × 108 m Apr. 22 1.9 × 1027 kg 2.0 × 1030 kg distance time 2π r1 = P 2π × (7.4 × 108 m) = 3.7 × 108 s = 12 m/s = = 45 km/hr Apr. 22 2 4/22/11 Example Suppose we were observing the Sun’s wobble from D = 10 light ­years ≈ 1017 m away. What would the angular size of the wobble be? (Use radius instead of diameter.) From Lecture 2 = r1 × 206265 arcsec D = α 7.4 × 108 m × 206265 arcsec 1017 m = 0.002 arcsec Apr. 22 Apr. 22 Thought Ques?on Doppler method Could we actually observe the Sun’s wobble from a distance of 10 ly, using the op?cal telescopes currently available today? •  we measure the star •  we infer the planet Apr. 22 Apr. 22 Discussion Ques?on The orbital period of an unseen planet A.  is the same as the period of the star’s Doppler curve. B.  is longer than the star’s period. C.  is shorter than the star’s period. Apr. 22 Discussion Ques?on Suppose you see a star similar to the Sun moving back and forth with a period of 2 years. What can you conclude? A.  B.  C.  D.  The star has a planet with an orbital radius < 1 AU. The star has a planet with an orbital radius of 1 AU. The star has a planet with an orbital radius > 1 AU. The star has a planet, but we can’t determine the orbital radius un?l we determine the planet’s mass. Apr. 22 3 4/22/11 51 Peg b Discussion Ques?on Which of the following cannot be determined with the Doppler technique alone? (Assume we know the size and mass of the star.) Radial Velocity (m/s) 150 A.  the mass of the planet B.  the orbital period of the planet C.  the size of the planet D.  the size of the planet’s orbit 100 51 Peg b, trend removed P = 4.2 d 50 0 −50 −100 −150 −0.2 exoplanets.org 0.0 0.2 0.4 0.6 Phase 1.0 1.2 current data original data 51 Peg b Mercury semimajor axis (AU) 0.0521 0.387 period (days) Apr. 22 0.8 4.23 88.0 Apr. 22 e=0.07 Inclina?on Eccentricity Compared to the true velocity, what we measure is … equal e=0.26 less than e=0.75 nothing! … always the same or smaller. We tend to underes1mate the velocity, and thus underes1mate the planet mass. Apr. 22 Apr. 22 Apr. 22 Apr. 22 hTp://exoplanets.org / hTp://exoplanets.org / 4 4/22/11 Mercury transits the Sun (2006) How can we measure the sizes of exoplanets? Apr. 22 Apr. 22 hTp://www.whatcomastronomy.org/mercury%20transit.html Venus transits the Sun (2004) Apr. 22 hTp://antwrp.gsfc.nasa.gov/apod/ap040609.html Transi?ng exoplanet Apr. 22 Discussion Ques?on Jupiter is about 1/10 the diameter (or radius) of the Sun. If it transited, by about how much would the Sun’s light dim? A.  B.  C.  D.  E.  0.1% (it would be about 99.9% its regular brightness) 1% (it would be about 99% its regular brightness) 10% (it would be 90% of its regular brightness) 30% (it would be about 70% of its regular brightness) 50% (it would be about half its regular brightness) Transit and planet size •  Area of star: 2 Astar = π Rstar •  Area of shadow: 2 Aplanet = π Rplanet •  Change in brightness: 2 2 2 Rplanet π Rstar − π Rplanet dimmed =1− = 2 2 regular π Rstar Rstar Apr. 22 Apr. 22 5 4/22/11 Discussion Ques?on Which of the following cannot be determined with the transit technique alone? (Assume we know the size and mass of the star.) A.  the mass of the planet B.  the orbital period of the planet C.  the size of the planet D.  the size of the planet’s orbit HD 209458b •  From Doppler shiv: M = 0.69MJ = 1.3 × 1027 kg •  From transit: •  Volume: V= •  Density: Apr. 22 R = 1.36RJ = 9.7 × 107 m M 1.3 × 1027 kg 3 = = 340 kg/m V 3.8 × 1024 m3 4 π R3 = 3.8 × 1024 m3 3 Apr. 22 Comparing densi?es Planet Mercury 5430 Venus 5243 Earth 5515 Mars 3934 Jupiter 1326 Hot Jupiters Density (kg/m3) Saturn 1318 Neptune 1638 HD 209458b ar1st’s concep1on •  (Not the only planets that exist – just the easiest to find.) 687 Uranus •  First discoveries were Jovian planets … very close to their stars! 340 Apr. 22 •  In the standard model, there is no way they could form where they are. How did they get there? Apr. 22 Kepler’s planet candidates Apr. 22 hTp://apod.nasa.gov/apod/ap110329.html hTp://kepler.nasa.gov/mul?media/Images/graphics/?ImageID=69 Apr. 22 6 4/22/11 Note: 15 confirmed to date Apr. 22 hTp://kepler.nasa.gov/mul?media/Images/graphics/mediatelecongraphics/?ImageID=125 Apr. 22 Kepler ­11 Apr. 22 hTp://kepler.nasa.gov/mul?media/Images/graphics/mediatelecongraphics/?ImageID=96 Apr. 22 hTp://kepler.nasa.gov/mul?media/Images/graphics/mediatelecongraphics/ Status •  Conrmed: hTp://exoplanet.eu/catalog.php –  544 planets –  455 planetary systems –  55 mul?ple ­planet systems •  Kepler: Characteris?cs of exoplanets hTp://kepler.nasa.gov/ –  1235 candidates –  15 confirmed Apr. 22 Apr. 22 7 4/22/11 Apr. 22 Apr. 22 Puzzles •  Why so close? •  Why so eccentric? Apr. 22 Apr. 22 Planet migra?on Apr. 22 Apr. 22 8 ...
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