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Extrasolar planets Finding planets Finding planets around other stars is hard! need to look for something very faint very close to something that is MUCH brighter! In the past 10-15 years, astronomers have gotten very clever and it has paid off! I. Finding planets indirectly by gravity – Orbit can’t be seen directly – But the motion can be detected by the Doppler shift if it is oriented correctly relative to our line of sight Doppler Measurements Yield Planet Mass Mass = 4.6 M II. Finding planets indirectly by light • If the orbit of a planet lines up just right from our perspective, it might pass in front of its parent star • This makes a sort of eclipse called a transit Transit Yields Planet Radius 10b Size Radius = 1.4 R Transits and Kepler mission • Problem: dimming is small, determined by relative size of planet and star – Very small for Earth sized planets! • Has been detected for large planets • Kepler mission is currently searching for Earth-sized planets! NASA’s Kepler Mission Determining the frequency of Earth-size and larger planets in the habitable zone of Sun-like stars Extrasolar planet detections • Lots of planets (>500!) have been detected, all in the last 10-15 years • For most of these, we don’t know what the planets are made of, but for the few we can tell, they are probably gas planets like Jupiter • At least, until this week …. Kepler 10-b • Just announced this week! • Earth-sized planet • Very close to its parent star, so very hot Transit and Doppler Measurements Yield Density + 10b Size Density Mass = 8.8 g/cm3 Volume Composition of Kepler-10b Summary • Kepler-10b is orbiting a star very much like our own Sun, but with an age greater than 8 billion years, at a distance of 560 light years. • Kepler-10b is the smallest exoplanet discovered to date and the first unquestionably rocky planet orbiting a star outside our Solar System Gliese 581 • Multiple planet system with a planet in a habitable zone? – Low mass M star, so habitable planets are close – Seven planets have been suggested! – Gliese 581g has a period of 37 days and is predicted to be in the habitable zone! – Gl 581g is “tidally locked” to star, meaning same side always faces the stars