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OU ASTR 1514 - Planets Around other Stars

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ASTR 1514 1st Edition Lecture 13Planets around Other Stars Chapter 5.6- 1,889 extrasolar planets have been confirmed with an additional 3,186 candidates.- First definitive detection of extrasolar planet was in 1995. There are different ways to find extrasolar planets: Direct Imaging method, Transit Method,Radial Velocity Method, and Gravitational Lensing. 1. Direct Imaging- Seeing a planet in an image after blocking light from the star in some way.- Very few planets have been detected with this method because the star is too bright and the planets are too small.2. Transit Method- A planet passing in front of a star can decrease the total brightness of the star.- Best bet for finding Earth-like planets, however you need to watch for a long timebecause the transit will only occur once per year. 3. Radial Velocity (wobble) Method- Center of mass “balance point between two masses” Ex: Two kids on a teeter totter, one heavier than the other, center of mass would be the point where the kidsare completely balanced, it will be closer to the heavier kid. - The Center of Mass is always closer to the more massive object. - The ratio of the distances from the planets to the center of mass is related to the ratio of masses: (RadiusA)(MassA)=(RadiusB)(MassB).- If a binary system has very massive stars and the separation between the two starsis small then the stars will be moving fast orbiting around their center of mass, with the outer star moving the fastest. - Planets will also cause the star to move, however, planets are much smaller than the second star so the star doesn’t move much. - With Radial Velocity we detect an extrasolar planet through the motion of a star.- If the star is very far away then we detect it’s motion by spectroscopic radial velocity: Stars and planets orbit one another by gravity, motion can be detected byDoppler Shifts- Example of Doppler Shifts with Sound Waves: The pitch of an ambulances siren as it approaches you, then going away from you. - Doppler Effect: Motion of a light source toward or away from us changes the wavelength of the waves reaching us. a) As the source of waves approaches the observer the wavelengths become shorter and the frequency higherb) As the source of waves moves away from the observer the wavelength becomes longer and the frequency lower. Easiest Ways to Detect Planets1) It is Easier to detect more massive planets with Radial Velocity Method because a massive planet will cause the star to move faster, causing larger more detectable Doppler Shift. 2) Easier to detect a more massive planet with transit method because it will cause a larger dip in the brightness of the star.3) Easier to detect a closer planet with radial velocity method because it will exert a larger gravitational force on the star, causing it to move faster, making a more easily detected Doppler shift.4) Easier to detect closer planets with transit method because they will have a shorter orbitalperiod, so you don’t have to wait as long to see multiple


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