1Class 16Extra-solar planets The radial-velocity technique for findingextrasolar planets Other techniques for finding extrasolarplanetsClass 15Formation of the Solar System What does a successful model needs to explain? The three key properties of the planetary system The basic properties of the Sun The Solar Nebula Model Age of the Solar System2I : What do we need to explain? Three basic properties of the planetary system… Size/composition of the terrestrial planets versus the Jovianplanets Directions and orientations of planetary orbits Sizes of terrestrial planet orbits versus Jovian planetaryorbits Even more fundamental fact that needs explaining… 99%of the angular momentum of the solar system isassociated with the planetary orbits (mostly Jupiter, infact) rather than the rotating Sun. Basic properties of the Sun… Contains 99.9% of the mass in the Solar System Slowly rotates (in same direction as planetary orbits); butcarries only tiny fraction of the solar system’s angularmomentum Composed mostly of hydrogen and helium3Composition of interstellar gas in our part of the GalaxyBigBangStars4 Brainstorm possible scientific ideas forhow solar system formed…5II : The Solar Nebula model Basics of the idea… Start with an interstellar gas cloud that starts collapsingunder the action of gravity Due to conservation of angular momentum, cloud flattensinto a rotating disk as it collapses Large quantities of gas flow inwards through thisaccretion disk, and form the Sun Planets form in the remaining disk… terrestrial planetsform in the hot inner part of the disk whereas Jovianplanets form in the cooler outer parts of the disk A mini-version of this scenario plays out around eachJovian planet, producing the satellite system6Likely temperature distribution in the Solar NebulaMount Fuji (Japan)7 Dependence of temperature on distance from proto-Sunis important for planet formation… Inner solar system (inside the “snow line”) Too warm for ices to freeze Only the “rock-forming” materials condense into solidparticles Successive collisions : dust ⇒ gravel ⇒ boulders ⇒planetesimals ⇒ planets8 Jovian planet formation (beyond “snow line”) Core accretion model: Due to freezing of ices, there’s a lot more solid materialavailable than in inner solar system Collisions built up a rock+ice object several times larger thanEarth (took several million years) Once core is large enough, gravitationally attracts thesurrounding gas to form thick atmospheres If correct, cannot form Neptune and Uranus in currentlocations… would need to have formed them closer in and bethrown out by Jupiter Disk instability model: Hypothesizes that the solar nebula was gravitationallyunstable… clumps could spontaneously form Once large clump formed, it gravitationally pulls in more gas… does not need an initial rock/ice core Uranus & Neptune could form in current location9Kuiper belt is a dusty/icy debris ring, gravitationally thrown tothe outer solar system by the massive planets (mainly Jupiter)10III : How long ago did theSolar System form? Current estimate: 4.56 billion years ago Determine this by radioactive dating of meteorites… Basics of radio-active dating: If the balance of protons-neutrons is wrong, an atomicnucleus is unstable In a given time (the half-life), half of these unstable nucleiwill split (decay) and form more stable nuclei87Rb → 87Sr (half-life 47.0 billion years)238U → 206Pb (half-life 4.5 billion years)14C → 14N (half-life 5730 years) Can estimate the age of a rock by how much of aparticular unstable isotope is still present Not a simple exercise… [discussion]I : Radial velocity technique Star does not remainabsolutely stationary asthe planet orbits Star and planet orbitabout their commoncenter of mass For each orbit of theplanet, the star makesone orbit about thecenter of mass We can usespectroscopy to detectthe motion of the star asit orbits its center ofmass… this uses theDoppler effect11Planets are easier todetect if they causestar to move withlarge velocities, andhave short periodscompared with thelength of theobserving campaign(~15 years)…So, you tend to detectmassive planets thatare close to their star712 day period12 As of now… 313 extrasolar planets have beendetected around 267 stars (from PlanetQuest) Most extrasolar planets found to date havebeen found with radial velocitiy technique… Some surprising properties… Finding many “hot Jupiters”… Massive planets; must be gas giants Orbiting close to star with short period E.g., first planet found (around 51-Pegasi in 1995) hasM>0.46MJup and orbits at 0.052AU (4.23 day period) Could not have possibly formed there… must haveformed further out and “migrated” in! Some planets have large eccentricities Much larger eccentricities than found in our solarsystem (see diagram on following slide)http://www.astro.lu.se/~danielm/13II : The Transit technique Consider special case when planet will passdirectly in front of its star… Can then watch planet block (eclipse) somefraction of the star light. Measure size of planet by depth of eclipse of star Measure composition of planet’s atmosphere usingabsorption lines during eclipse of star Measure temperature of planet by looking at drop ininfrared flux as star eclipses planet E.g. HD 209458… M=0.69MJup, but R=1.32RJup… very low density Orbits at 0.047AU… surface temperature 1130K! Spectrum shows H, C, O and Na (sodium) beingevaporated off surface of planet by starlight!14III : Other techniques… Direct imaging Currently have a very small number of cases Rationale behind TPF mission Astrometric technique Search for the motion of the star induced byplanetary orbits Rationale behind SIM mission Gravitational microlensing Use gravitational effects of planet on light formbackground star…1516Terrestrial Planet Finder InterferometerApparent motion of Sun due to planets (as seen from