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Download Lecture 20: Formation of Planets, Exoplanets 3/30
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Planet Formation • contracting gas/dust cloud forms stars • swirling disk of material (H, He, C, O, heavier elements, molecules, “dust”) form planets • New born star heats up material, blows out of solar nebula planets (or at least protoplanets) need to form before material dissipates PHYS 162 1 PHYS 162 2 see disks around new stars in Orion nebula PHYS 162 3 Planet Formation II • material condensates if cool enough (gasliquid/solid) • heavier elements (metals, silicates) condense first, at higher temperatures, then molecules like water H and He remain gases • density and temperature falls with distance from star. Planet formation occurs when not too far and not too close • “snow line” separates type of planets being formed PHYS 162 4 Temperature in early Solar Nebula PHYS 162 5 “snow line” in early Solar Nebula Very, very simplistic PHYS 162 6 Planet Formation III • condensation starts, protoplanets grow in size -objects collide; stick together • over millions of years sweep out most smaller objects as collide with larger objects existing planets • only ~circular orbits won’t collide any further (asteroid belt between Mars and Jupiter) • Possible motion of planets to/from star may be critical PHYS 162 7 planetisimals (dust grains) protoplanets by accretion, collisions, gravity smaller objects stick together PHYS 162 8 Craters on Moon (and other planets’ moons) are fossil remnants of collisions in early solar system PHYS 162 9 Planet Formation IV • close to star: planets = heavy elements (iron,silicon) -water may be trapped at beginning in dust grains or come later from comets hitting surface??? • further from star: Gas Giants ices (water H2O, methane CH4) froze out early larger protoplanets more material to accrete • comets, meteors, asteroids give clues to composition of early solar system PHYS 162 10 PHYS 162 11 Planets in other Star Systems first extrasolar planet observed in 1995. In Jan 2000, 28 observed and now about 3700 confirmed. Many systems with 2 or more observed planets • difficult to observe directly • mostly look for impact on Star: wobbles due to gravity of planets or reduction of light due to “eclipse” • If multiple planets, will have to add effects of planets (our solar system: Jupiter with 12 year orbit, Earth with 1 year, etc) PHYS 162 12 Planets in other Star Systems PHYS 162 13 Observe Directly block out star in telescope optics Do if found exoplanet by other means. Can study atmosphere PHYS 162 14 Observe by Star’s Wobble: Doppler Shift or Proper Motion the larger the planet the larger the gravitational pull the smaller the orbit the larger the gravitational pull the smaller the orbit the more rapid is the wobble easiest to see large planets which are close to their stars PHYS 162 15 47 Ursae Majoris (one of first discovered) Doppler shift 2 large planets PHYS 162 16 55 Cancri (one of first discovered) Doppler shift very complicated. One close large planet plus 3-4 more? PHYS 162 17 Summary: some of the exoplanets found by Doppler shift - easiest to find big planets close to a star PHYS 162 18 Observe by planet eclipsing star WASP-4 Wide Angle Search for Planets Jupiter would reduce Sun’s light by 1%; Earth reduces by .01% “easy” (done by 7th grader at NIU Science Fair) once spotted can also analyze Doppler shift and try and observe atmosphere PHYS 162 19 Kepler telescope • launched in 2009, designed to detect Earth-sized (or smaller) planets by observing them eclipsing their stars • In orbit around the Sun….away from the Earth, points away from the Sun • Looked 150,000 main sequence stars (every 20 minutes) measures luminosity 20 parts per million (0.000002) • Database at www.planethunters.org • 2013: two motors fail, can no longer “point”. Now mostly just points away from Sun. PHYS 162 20 Kepler telescope • orbit original field of view (Cygnus + Lyra) PHYS 162 21 Kepler telescope after motor failure PHYS 162 22 Kepler Results: many Earth-like planets some in habitable zones (more later) Kepler 35 – binary star Planet orbiting 4-star (2 close binaries) PHYS 162 23 Kepler telescope • collected data from 2009-2013. pointing failed. Not sensitive to long periods like Jupiter’s 12 years • Sees a lot of planets between Earth and Neptune size. 603 in plot (1/14) PHYS 162 24 2325 found by Kepler 5/10/16 3586 found by everyone 3/2017 PHYS 162 25 Kepler telescope • Kepler data can estimate the number of Earth-like planets orbiting Sun-like stars. Didn’t take data long enough to do 1 year periods PHYS 162 26 Exoplanet Highlights 2014-2016 • Smallest: Kepler-37b. About the size of our Moon • Closest: Alpha CentauriB has Earth-sized but close to star and hot surface • Earthiest: Kepler-78b earthlike mass/diameter but 8.5 hour orbit and temp>2000 degrees C • Intriguing 1: Kepler-62e and 62f are 2 Earthlike planets in habitable zone. Both ~twice Earth size at .4 and .7 AU from star ~0.2 Sun’s luminosity • Intriguing 2: star Gliese667C has three planets in habitable zone. But 3 star system with “C” being a M1 class with 1% of Sun’s luminosity poor for “intelligent” PHYS 162 life 27 Extra Slide PHYS 162 28