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Download Time From the Perspective of a Particle Physicist
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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 (little dust grains) protoplanets by accretion: collisions, gravity smaller objects to stick together PHYS 162 8 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 9 PHYS 162 10 PHYS 162 11 Planets in other Star Systems • test out how planets are formed with more examples • first extrasolar planet observed in 1995. In Jan 2000, 28 observed and now >4500 candidates about 1000 confirmed (11/2013). 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” • Planet orbits obey Kepler’s laws. 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 Observe Directly block out star in telescope optics Do if found exoplanet by other means PHYS 162 13 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 14 47 Ursae Majoris (one of first discovered) Doppler shift 2 large planets PHYS 162 15 55 Cancri (one of first discovered) Doppler shift very complicated. One close large planet plus 3-4 more? PHYS 162 16 Summary: some of the exoplanets found by Doppler shift - easiest to find big planets close to a star PHYS 162 17 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 18 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) • Has discovered over 3000 possible planets, ~600 confirmed (Jan 2014), rest available for analysis. Candidates often binary star systems • Database at www.planethunters.org • 2013: two motors fail, can no longer “point”. PHYS 162 19 Kepler telescope • orbit field of view (Cygnus + Lyra) PHYS 162 20 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 21 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 22 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 23 Exoplanet Highlights Jan 2014 • Smallest: Kepler-37b. About the size of our Moon • Earthiest: Kepler-78b has earthlike mass and 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. Bit 3 star system with “C” being a M1 class with 1% of Sun’s luminosity poor for “intelligent” life PHYS 162 24 Planetary Atmospheres • composition of a planet’s atmosphere depends on • • • • • Surface Gravity Temperature light atoms/molecules move faster than heavy molecules if velocity = escape velocity gas leaves planet Mercury, Moon: all escape Earth: lightest (H,He) escape Jupiter: none escape PHYS 162 25 Familiar Molecules molecule mass H2 hydrogen 2 He helium 4 CH4 methane 16 NH3 ammonia 17 H20 water 18 N2 nitrogen 28 O2 oxygen 32 CO carbon monoxide 28 CO2 carbon dioxide 44 PHYS 162 26 Atmosphere of Venus vs Earth 96.5% CO2, 3% N2 runaway greenhouse effect 78% N2, 21% O2, 0.04% CO2, ~1% H20. most CO2 absorbed by oceans PHYS 162 27 Atmosphere of Jupiter and Saturn ammonia, sulfuric acid, water interiors are helium and hydrogen, core of ice/rock Titan, moon of Saturn, has ~90% nitrogen rest mostly methane and argon; pressure similar to Earth PHYS 162 28
