lecture21 - 1 Extraterrestrial Life Spring 2008...

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Unformatted text preview: 1 Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Lecture #21 Homework #5 is due Thursday (limited ofFce hours this afternoon, but I will be available all tomorrow afternoon if you have queries) Last time discussed radial velocity method for Fnding planets - very good for Fnding massive planets How can we Fnd habitable planets (Earth mass, in roughly Earth-like orbits)? Extraterrestrial Life: Spring 2008 Transits As seen from Earth, both Mercury and Venus occasionally transit the face of the Sun: Transits of Venus are rare: occur in pairs every ~200 years (next one in 2012) Extraterrestrial Life: Spring 2008 Transits If an extrasolar planet transits its host star as seen from Earth, cannot resolve the disk of the star and directly see the transit Instead, measure the reduction in the starlight caused by the transiting planet blocking light from a fraction of the star ’ s surface (i.e. see a very partial eclipse of the star). Extraterrestrial Life: Spring 2008 Magnitude of the transit signal Fraction of the starlight blocked is just the fraction of the stellar disk that is covered - i.e. the ratio of the area of the planet disk to the stellar disk. Planet has radius R p , star has radius R * ¡ractional drop is: f = " R p 2 " R * 2 = R p R * # $ % & ’ ( 2 Extraterrestrial Life: Spring 2008...
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lecture21 - 1 Extraterrestrial Life Spring 2008...

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