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Extra-Solar Planet Populations Stephen Eikenberry 4 November 2010 AST 2037 1 Radial Velocity Planet Searches • So … need a speedometer to measure star velocity versus time • To a precision of a few meters per second! • Across distances of many light years!!! • How? Doppler shift of spectral lines 2 51 Pegasi • In 1995, Mayor & Queloz announce the discovery of an orbital signature with amplitude = 50 m/s in a 4.23day period around star 51 Pegasi • Mass = 0.5 MJUP First extra-solar planet 3 51 Pegasi: Hot Jupiter? • At that location, expected temperature is VERY high (about 2000K or higher!) • So … Jupiter-like planet, but closer than Mercury “Hot Jupiter” • How do you make something like that???? 4 Planet Bonanza • Geoff Marcy & Paul Butler quickly confirmed 51 Pegasi • They had lots of archival data from searches for Jupiter-type planets (periods >10 years, so they were still “in progress”) • No on even thought to look for short-period MASSIVE planets (why would they be easier?) • Found many “Hot Jupiters” – most extra-solar planets known today are Hot Jupters 5 ES-Planet Population • As of this morning, 228 861 planets are now known to orbit other stars (!!) • All of this has happened in about 15 years – someone currently finds a new planet every couple of days or less • These planets are NOT generally like our Solar System objects – WHY? 6 7 8 9 Upsilon Andromedae • First multiple planet system 10 Upsilon Andromedae • First multiple planet system 11 12 HD 209458 • Another Hot Jupiter 13 HD 209458 • What is a “transit”? 14 HD 209458 • A transit observed • Note: only ~1% dip 15 HD 209458 - Results • Just at the entry moment into transit, for a brief instant, only the upper atmosphere of the planet absorbs any starlight • With a powerful enough spectrograph, we can look for absorption lines at this instant • Result: COMPOSITION of the planet atmosphere • HD 20948b contains – WATER!! 16 Gliese 581c • Low-mass planet, with mass 5 Mearth • Orbit semi-major axis 0.07 AU • Low-mass star 17 Gliese 581c • Just inside Habitable Zone • Gliese 581d just outside HZ 18 Eccentricity 19 Circumbinary Planets 20 Transiting Planets in the HZ 21 Orosz et al. 2012 Image Credits: NASA Borucki et al. 20 22 Planets in Habitable Zones • Many planets are currently known in the Habitable Zone around their parent stars • Most of these are gas giants no solid surface • But … gas giants in our Solar System have lots of moons • What happens to Europa if you move it/Jupiter to a distance of 1 AU? 23 Planets in Habitable Zones • What happens to Europa if you move it/Jupiter to a distance of 1 AU? • Really? • Europa mass is closer to our moon’s mass – why no water there? • So … need giant planets in the HZ with giant moons … 24 Our Solar System is Not Typical • Sun is an unusually massive star • Our solar system has no planets with orbital periods less than Mercury • Our inner solar system does not have a super-Earth-size to Neptune-size planet • Most planetary systems don’t have a Jupiter analog → Planet formation theories should typically produce planetary systems unlike our own 25 Borucki+ 2011; Batalha+ 2012; Howard+ 2012; Fressin+ 2013; Burke+ Terrestrial Planet Finder • Ultra-high-contrast imager satellite • Capable of finding Earth-mass planets in HZ around nearby stars 26 Summary • We have found hundreds of planets around other stars • Overwhelming majority are massive gas giants, many close to their parent star • This is because they are easiest to find with the Doppler technique; but Kepler mission (transits) is changing that • Have found: multiple planet systems (20+); planet atmospheres; some low-mass (probably solid) planets • Eccentricity seems more common than circular orbits; problems for life • So far, only a few gas giants in the HZ; first possibly-solid planets; maybe moons could host life (??) • Future searches will be sensitive to Earth-mass planets in the HZ 27 Exoplanet Zoo: Examples • We have found hundreds of planets around other stars 28 Exoplanet Zoo: Examples • Multiple-planet systems 29 Exoplanet Zoo: Examples • OGLE-2005-BLG-390Lb – 5.5 Earth-mass, far from star (T=-360 ˚F) • COROT-7b: 4.8 Earth-mass, density = Earth-density (!); close to star (T ~1300K) • Gliese 581g: 3-4 Earth-mass; T ~- 25 ˚F to +160 ˚F; in the Habitable Zone (!), but tidally locked 30