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Planets Beyond the Solar System: Is our Solar System Unusual or Normal? Eric Jensen Swarthmore College Alumni Weekend, 06/08/2007 The past ~ 4000 BCE to 1995 How many planets are there? • ~ 4000 BCE: 5 (Mercury, Venus, Mars, Jupiter, Saturn) • ~ 1610: 6 (Earth) • 1781: 7 (Uranus) • 1846: 8 (Neptune) • 1930: 9 (Pluto) • Mid-1995: Still 9! • June 8, 2007: 237 How many planets are there? • June 8, 2007: 237 – 229 extrasolar planets around 195 stars – 8 planets in our solar system This and similar graphs are created from data in the Extrasolar Planets Encyclopedia, http://exoplanet.eu/ Known extrasolar planets, Dec. 1997 Currently known extrasolar planets Image from the California and Carnegie Planet Search, http://exoplanets.org/ The present 1995 to now The era of Doppler-shift detections The giant-planet era Techniques for planet discovery • Detecting stellar “wobbles” caused by planets: – Doppler shift: measuring velocity changes – Astrometry: measuring position changes • Detecting eclipses of starlight by orbiting planets • Direct imaging of planets Techniques for planet discovery • Detecting stellar “wobbles” caused by planets: – Doppler shift: measuring velocity changes – Astrometry: measuring position changes • Detecting eclipses of starlight by orbiting planets • Direct imaging of planets Successful search strategies to date The Sun moves as the planets orbit it Sun’s diameter Figure from Planet Quest, © 1997 by Ken Croswell Detecting the wobble • The Sun moves at about 12 meters/second (25 mph), mostly due to Jupiter. • The size of this path is about 0.001 arcsec (less than a millionth of a degree) as seen from 10 parsecs (33 light years) away. Doppler Shift: • Motion toward or away from us shifts the wavelength of light received. • Side-to-side motion has no effect. Image from http://threadless.com/ Example: velocity vs. time for 51 Peg Doppler shift surveys • Current surveys are monitoring ~ 2000 nearby stars. • Results to date show that about 6% of Sun-like stars surveyed have Jupiter-like planets closer than 5 AU. • These surveys cannot detect Earth-like planets; Saturns (at Saturn’s distance from the star) are marginal. Photometry of planet eclipses, a.k.a. ‘transits’ Image © Hans Deeg Artist’s conception of a transit © 1999 Lynette Cook; used by permission of the artist Transits measure a planet’s radius: The amount of light blocked by the planet is 2 Planet's area πRPlanet = 2 Star's area πRStar € We know Rstar, so measuring the dimming gives us Rplanet! 1999: Transits detected! Charbonneau et al. 1999 Hubble Space Telescope data from Brown et al. 2001 Transits across the star HD 209458 • Transits tell us that Rplanet = 1.5 RJupiter • Doppler shift tells us that Mplanet = 0.7 MJupiter. • Combined, we find that the density is 0.23 g/cm3. (Earth is 5.5 g/cm3, Jupiter is 1.3 g/cm3) • This is clearly a gaseous, Jupiter-like planet! Charbonneau et al. 2000 The era of transiting planet detections is just starting Secondary eclipses can reveal planet surface structure Planet getting brighter as it orbits Model map of planet surface brightness Knutson et al., Nature, May 2007 What have we learned so far? Not all stars are created equal Abundance of ‘heavy’ elements matters <3% of stars observed have detected planets Sun’s abundance 25% of stars observed have detected planets Log fraction of elements > helium, relative to Sun The more ‘heavy’ elements the parent star has, the more likely it is to host a planet Plot courtesy of Greg Laughlin Low-mass planets are more common Planet mass (Jupiter masses) Planets found so far are Jupiter-like, not rocky Planets Stars Saturn Bakos et al. 2007 Jupiter Planets have weather! Simulations of HD 185269 b Temperature Range: 900K to 1760K Simulations by Jonathan Langton Limitations of current techniques for finding planets They are largely sensitive only to close-in, Jupiter-like planets. Limitations of Doppler shift • Convection on stars’ surfaces, and stellar pulsations, cause radial velocity variations of ~ 1 meter/ second. Convection on Sun’s surface • Motion of Sun due to Earth is 0.1 meter/second. Limitations of Doppler shift Radial velocity variations from stellar pulsations Bouchy et al. 2005 Limitations of photometry • Even the “quietest” stars have some random variability due to, e.g., sunspots. • An Earth-mass planet dims a Sun-like star by about 0.005%. • Only about 1 Earth-like planet in 100 would be oriented so that an eclipse would be visible to us. Limitations of photometry: small planets are hard to see! Data: first planet detected by the COROT mission, May 2007 Figure: Greg Laughlin Rotation plus active regions can mimic transits The future The era of Earth-like planet detection? • Photometric (transit) detection: NASA’s Kepler mission (launch November 2008) • Astrometric detection? NASA’s Space Interferometer Mission • Direct imaging? NASA’s Terrestrial Planet Finder (launch 2014?) Are we typical? Are we alone? • We don’t know yet! But we’ll find out soon... • Current techniques are biased toward close-in, giant planets • Detecting planets like our giant planets takes a long time (Jupiter: 12 years; Saturn: 29 years) • Detecting Earth-like planets is hard! But Kepler mission has a chance in next five years. The universe is infinitely wide. Its vastness hold innumerable atoms, Beyond all count, beyond all possibility of number Flying along their everlasting ways. So it must be unthinkable that Our sky and our round world are precious and unique… Out beyond our world there are, elsewhere, Other assemblages of matter making other worlds. Ours is not the only one in air’s embrace… You’ll never find one single thing, Completely different from all the rest Alone, apart, unique, Sole product, single specimen of its kind… There are other worlds, more than one race of people, And many kinds of animals. Lucretius, 70 BCE What about Pluto? Image from http://threadless.com/ The newest dwarf planet: Eris, larger than Pluto Image from Mike Brown Pluto shares many properties with other Kuiper Belt Objects 2:1 5:3 3:2 4:3 50 Orbital Inclination [deg] 40 30 20 10 N < 1 Opposition N ! 1 Opposition 0 30 Image from Dave Jewitt 35 40 Semi-Major Axis [AU] 45 50