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UT AST 301 - Class 22 - Topics

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AST 301Introduction to AstronomyJohn LacyRLM [email protected] LiRLM [email protected] JeonRLM [email protected] site: www.as.utexas.eduGo to Department of Astronomy courses,AST 301 (Lacy), course websiteTopics for this weekArea and volumeFinding extrasolar planetsImagingAstrometric techniqueDoppler techniqueTransit techniqueWhat is foundMassesOrbitsDensities and compositionsFormation of extrasolar planetsDoppler techniqueMeasuring a motion in the sky of 0.001 arcseconds isalmost as difficult as taking a picture of a planet.It turns out that it is easier to measure the Doppler shift ofthe star than it is to measure its change in position.On your homework you will calculate the Doppler shift ofthe Sun caused by the pull of Jupiter.It is small, but with some effort we can measure it.Doppler shift of a star caused by a planetWhat can we calculate?The period of the planet’s orbit is the time from one peak ofthe velocity curve to the next.If we know the mass of the star, we can use Kepler’s 3rdlaw to calculate the radius of the planet’s orbit.The distance the star travels in that time is the period timesthe speed, which is given by the maximum Doppler shift.d = vtThis is the circumference of the star’s orbit, and the radiusof the star’s orbit = circumference / 2π.The ratio of the mass of the planet to the mass of the starequals the ratio of the radius of the star’s orbit to theradius of the planet’s orbit.So we can learn both the mass of the planet and the size ofits orbit.A complicationThe Doppler shift also depends on how we view the orbit ofthe planet and its star.We can’t know this from just the Doppler shift.Velocities derived fromDoppler shiftsThe original data has theDoppler shifts caused bythe three planetscombined.They show them separatelyin this figure.Transit techniqueThere is yet another technique for finding planets aroundother stars.If the planet’s orbit happens to make it pass between usand the star, it can block some of the starlight and makethe star appear dimmer.And if the planet passes behind the star, the planet’s light(either reflected starlight or emitted infrared light) can beblocked by the star.If you were observing the Sun from far away and Jupiterpassed between you and the Sun, how much of theSun’s light would be blocked?Hint: Jupiter’s diameter is 1/10 the Sun’s diameter.Transit techniqueHow large is this planet?How bright is its surface compared to its star’s surface?Composition of a planet’s atmosphereWhat sorts of planets have we found?What are their orbits


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UT AST 301 - Class 22 - Topics

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