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AS 100 Notes (Lecture 31-38)

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CAS AS 100
Michael Mendillo

AS 100 Lecture 3105072014Section 131Detecting planets around other starsChallenges 1 Context of our scale model solar system distance2 A sunlike star is a billion times brighter than the light reflected from any of its planets the glare scattered starlight generally overwhelms any small spots of planetary lightWays to learn about distant objectsDirectly by obtaining images or spectra of the objects themselvesAlways preferable Ex imaging of visible light reflected by the planets or of infrared light that the planets emit or obtaining visible or infrared spectra of the planetsIndirectly inferring existence of properties without actually seeing the objects under studyTwo major approaches to indirectly find planets1 Observing the motion of a star to detect the subtle gravitational effects of orbiting planetsearliest discoveries from this method relies on observing stars in order to detect motion that we can attribute to gravitational tugs from orbiting planets 2 Observing changes to a stars brightness that occur when one of its planets passes in front of the star as viewed from EarthNASAs Kepler mission Astrometric Method Uses very precise measurements of stellar positions in the sky to look for the sight motion caused by orbiting planets Used to identity binary star systems bc two orbiting stars move periodically around their center of massUnavoidable Constraints 1 The further away the star is the smaller its sidetoside movement will appearThis method works best for detecting the gravitational effects of massive planets around relatively nearby stars2 For a planet of any particular mass a greater orbital distance means a larger astronomical effect on its star but it also means that the effect takes longer to measure Doppler Method Uses Doppler effect which allows us to measure changes in a stars velocity toward or away from us that are caused by orbiting planets Looks for back and forth motion that can be measured from Doppler shifts in a stars spectrum Blueshift moving towards us Redshift moving away from us Alternating redshifts and blueshifts indicates orbital motion around a center of mass 51 Pegasihalf mass of Jupiter combination of planets high mass with its closeness to its star makes it a HOT JUPITERMore than 700 planets have been detected with the Doppler Method Limitations Best suited for identifying massive planets that orbit relatively close to their star bc being closer means a stronger gravitational tug and hence greater velocity for the star as it orbits the center of massRequires observing long enough to see a stars orbit around the center of mass which means the Doppler Method is biased toward finding closein planets with short orbital periodsRequires a fairly large telescope in order to obtain spectra with a high enough resolutionHow can changes in a stars brightness reveal the presence of planetsTransit method used by Kepler mission to search for planets around other starsA planets orbital tilt inclination is aligned with the direction of the earth is called a transit
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