Week 15 (Chapter 13): Other Planetary Systems Nov. 30 Please pickup your grade sheet at front. We ll discuss at end of class. Other Solar Systems First one (51 Pegasi) discovered in 1995 Currently 268 exoplanets in >180 solar systems identified (61 added since last spring) Mostly bigger than Jupiter (M j ) Selection Effect f(mass, orbit) All very close to parent stars (<5AU) Selection Effect f(time) Detection based on extremely precise observations Detection Techniques Direct: Pictures or spectra of the planets themselves Indirect: Measurement of stellar properties revealing the effects of orbiting planets Astrometric measuring wobble Doppler measuring Doppler shift Direct Detection Brown Dwarf 2M1207 (blue object on right) and planetary companion. About 150 light years distant. Sun-like stars are ~billion times brighter than reflected light from planets Equivalent to standing in San Francisco and seeing a pinhead in Washington, D. C. illuminated by a grapefruitsized searchlight 15 meters away from the pinhead! Indirect: Astrometric REM: The Barycenter? Sun and Jupiter orbit around their common center of mass Sun therefore wobbles around that center of mass with same period as Jupiter Only 1 successful exoplanet detected by this method. Astrometric in Theory Sun s motion around solar system s center of mass depends on tugs from all the planets Astronomers around other stars could determine masses and orbits of all our planets by deconvolving combined effects 1
Indirect: Doppler Measuring a star s Doppler shift can tell us its motion toward and away from us Current techniques can measure motions as small as 1 m/s (walking speed) First Extrasolar Planet Doppler shifts of star 51 Pegasi indirectly reveal a planet with 4-day orbital period Short period means small orbital distance Works best for BIG planets with SHORT periods First Extrasolar Planet Other Extrasolar Planets Lower mass Higher mass Farther away from star Highly eccentric orbit Planet around 51 Pegasi has mass similar to Jupiter, M = 1M j, but very close to star 1 st Hot Jupiter Doppler data curve tells us about a planet s mass and the shape of its orbit Transits and Eclipses Direct Detection? 2M1207 A transit is when a planet crosses in front of a star The resulting eclipse reduces the star s apparent brightness and tells us planet s radius Special techniques can eliminate light from brighter objects These techniques are enabling direct planet detection 2
Known Orbits Most of the detected planets have orbits smaller than Jupiter s Planets at greater distances are harder to detect with Doppler technique (need more time) Selection Effects You will see what you are looking for in the time span you are looking. We have not been looking very long Our instruments can only detect Doppler shifts on planets parallel to the direction we are looking Hence, we are currently finding: Massive Planets Close in to their parent stars (fast orbits) Planets with highly eccentric orbits Orbits of Extrasolar Planets Orbits of known extrasolar planets are much more elongated (greater eccentricity) than those in our solar system An example of Selection Effect Semi- Major Axis values for 178 exoplanets Mass Distribution How do known extrasolar planets compare with our solar system? Most that have been detected are more massive than Jupiter Planets with smaller masses are harder to detect with Doppler technique 3
Surprising Characteristics Extrasolar Earths? Not a very good sample Many highly elliptical orbits Some very close to their stars: Hot Jupiters Blur the line between a large planet and a small star (Brown Dwarfs) Do not fit well with nebular hypothesis Goldilocks Zone Most-recent direct detection Oops. One more last week Missing Link? Corot-EXO-3B Supergiant Planet or a SubStellar Object (SSO) Between a Brown Dwarf and a planet Slightly smaller then Jupiter, but 21X the mass Twice the density of lead Orbital period 4 days, 6 hours 680 light years away 4
Revisiting the Nebular Theory Nebular theory predicts that massive Jupiter-like planets should not form inside the frost line (at << 5 AU) Discovery of Hot Jupiters has forced reexamination of nebular theory Planetary migration, gravitational waves or close encounters may explain? Gravitational Encounters Close gravitational encounters between two massive planets can eject one planet while flinging the other into a highly elliptical orbit Multiple close encounters with smaller planetesimals can also cause inward migration Rogue Planets I AKA: planetars Lone planets not in orbit around any star. Normal creation process, then ejected by gravitational close encounters Or, accretion in cold gas/dust clouds Rogue Planets II Earth-sized rogue planets could have enough internal heat, from radioactive decay and tidal heating, if moon(s) present, to remain geologically active. Dynamicists suggest up to 5% of all planets? Modifying the Nebular Theory? Exoplanet observations, even if biased, show nebular theory is incomplete Effects like planet migration and gravitational encounters might be more important than previously thought Or we might be looking at another class of solar systems Planets: Common or Rare? Preliminary Conclusions: One in ten stars surveyed thus far have easily-detectable planets Should be about half Others may have smaller (Earth-sized) planets that current techniques cannot detect Are these the normal solar systems? 5
What s normal? Transit Missions NASA s Kepler mission is scheduled to begin looking for transiting planets in 2009. Designed to measure the 0.008% decline in brightness when an Earth-mass planet eclipses a Sun-like star Astrometric Missions GAIA: A European mission planned for 2010 that will use interferometry to measure precise motions of a billion stars SIM: A NASA mission planned for 2011 that will use interferometry to measure star motions even more precisely (to 10-6 arcseconds) Measures physical parallax of wobbling stars Terrestrial Planet Finder (TPF) Direct Detection Determining whether Earth-mass planets are really Earth-like requires direct detection Missions capable of blocking enough starlight to measure the spectrum of an Earth-like planet require interferometry NASA mission currently on hold due to lack of funding. Ad Astra My Other Vehicle is a Galaxy-class Starship Voyager 1 crossed terminator shock 2004 @ 94 AU But most instruments had failed by then IBEX: Interstellar Boundary Explorer Launched October 19 th 2008 to boldly go 6
Near-Term Orion CEV (back to the Moon) Shuttle retirement ~2010 DTO (Direct-to-Orbit) hypersonic aircraft, > Mach 15 Mars Direct & Mars Net What about people? Mars Net Continue robotic surface exploration Find water and map look for life Find landing sites with potential for sustainability 2030 timeframe Sample Return doubtful? Satellite Relay Constellation Safely land crew return vehicle w/ supplies Then send people Why? The Years Ahead Ongoing Spirit and Opportunity missions. Completion of Cassini mission at Saturn. Kepler, GAIA and SIM exoplanet missions. New Horizons 2015 KBO flyby. Luna GLOB polar rover looking for water ice. Project Constellation (LEO and return to the Moon) Multiple missions in 2018 to 2030 timeframe Mars robotic base construction Europa aquabot (Enceladus too?) Mars Direct (piloted mission) 2035? Titan Direct 2075? The Days Ahead Understanding your Grade Sheet information Open Q&A time for Exam #4 is now Wednesday, Dec. 9 th @ 6 PM is Exam #4 Exam #4 (the Final) will cover: Chapters 10, 11, 12, and 13. Class notes from Weeks 12, 13, 14, and 15. Exact questions from Term Exams 1-3. Objective Questions: 30 @ 2 points each. Essays: 4 @ 10 points each. (Semi-) Final Averages Column 1 is a real grade Columns 2 & 3 are speculative Column 1: Grade now w/ 4% is your class grade today without taking TE4. This is the walk away number. Even if you blow the final, your grade cannot get worse than this number. If you already have an A, please do not come to TE4 (the final exam). Column 2 is what your final grade would be if you make as much on the final as you did on your highest TE 1-3 grades. Column 3 is what your final grade would be if you made 100% on TE4 (the final). 7
Quick 1102 update No final exam Final grade will be based on your earned points divided by 1000. Plus a % curve to be calculated after 12/4. Last exercise still pending: Assignment 10 (Extrasolar Systems) due 12/4. All old exercises are open for partial credit. If you need em..go get em. 8