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Planet Formation Phil Armitage, University of Colorado XIII Ciclo de Cursos Especiais Theory Initial conditions? Very detailed: only one system Solar System observations How do planets form? Many systems: limited individual information Extrasolar planet observations XIII Ciclo de Cursos Especiais Historically: observation that the planets orbit the Sun in (approximately) the same orbital plane Nebula Hypothesis: planets formed from a rotating disk of gas and dust orbiting the proto-Sun (Kant, Laplace in the 18th century) …fundamentally correct concept Quantitatively: Terrestrial planet formation: Victor Safronov - “Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets” (1969) Giant planet formation: Hiroshi Mizuno (~1980) building on many earlier ideas XIII Ciclo de Cursos Especiais New developments 1. Observations of protoplanetary disks (initial conditions) 2. Discovery of the Solar System’s Kuiper Belt 3. Discovery of extrasolar planetary systems …partially confirm earlier ideas, but also point to the unexpected importance of planetary system evolution and reveal a great diversity of planetary systems XIII Ciclo de Cursos Especiais Outline 1. 2. 3. 4. 5. Observations of planetary systems Protoplanetary disks Formation of planetesimals (km-scale bodies) Formation of terrestrial and giant planets Evolution and stability of planetary systems Today: mostly introductory Generally, mix of basic ideas + open questions Feel free to ask questions at any time! XIII Ciclo de Cursos Especiais Solar System Observations Terrestrial planets Low mass (up to 1 Earth mass = 6 x 1027 g), mostly rocky objects. Found in the inner Solar System (Mercury 0.39 AU, Mars 1.52 AU) XIII Ciclo de Cursos Especiais Giant planets - found only in the outer Solar System Gas giants: primarily gaseous objects but not made of the same composition as the Sun… enriched in heavy elements Jupiter: ~10-3 Msun, ~300 MEarth Ice giants: ~10 Mearth of rock and ice, plus large (several Earth masses) contributions from gas 2 (or maybe 3) classes of planet that need to be explained… XIII Ciclo de Cursos Especiais Comparison of the multipoles of gravity field (J2 - J6) with internal structure models High density EOS for H / He (c.f. Militzer & Hubbard 2008 work) Could do much better on the observations: JUNO mission Tristan Guillot XIII Ciclo de Cursos Especiais Integrated properties Mass: planets are ~0.2% of the mass of the Sun Sun is ~2% “metals” (not H / He) - most of the heavier elements are also in the Sun… planet formation need not be 100% efficient Angular momentum: 50 2 -1 Jupiter’s orbit LJ = M J GM sun aJ " 2 #10 g cm s 2 Solar rotation Lsun = k 2 M sun Rsun "sun ~ 3 #10 48 g cm2 s-1 ! ..segregation of mass from angular momentum during the formation of the Solar System ! XIII Ciclo de Cursos Especiais Minimum mass Solar Nebula How much mass was needed to form the planets? 1. Take mass of heavy elements in each planet 2. Augment the mass with enough H / He to restore Solar composition 3. Spread the mass into an annulus around each orbit Jupiter’s orbit spread Jupiter’s augmented mass (~3 x real mass) across this annulus to yield a surface density XIII Ciclo de Cursos Especiais Minimum mass Solar Nebula (Weidenschilling 1977) *3 2 $ ' r -2 " gas (r) = 1.7 #10 3 & ) g cm %1 AU ( ! Integrated mass out to 30 AU = 0.01 Msun Comparable to the masses of disks measured around other stars BUT… this is at best a lower limit - could have been more gas / could have been a different radial profile… XIII Ciclo de Cursos Especiais Minor bodies in the Solar System Asteroids, Kuiper Belt objects, comets… Dynamical clues as to the early evolution of the Solar System Most stable orbits in the Solar System are populated with minor bodies XIII Ciclo de Cursos Especiais a rperi = a(1" e) rapo = a(1+ e) Distribution of Kuiper Belt Objects beyond Neptune: ! 1. Population in 3:2 resonance with Neptune P1 i = P2 j …with i, j integers. “Plutinos”, include Pluto itself. Who ordered that!? 2. Apparent edge at ~47 AU (not just selection function) ! XIII Ciclo de Cursos Especiais The puzzle of Earth’s water… Liquid water is stable on the Earth today because the temperature at atmospheric pressure is 0 C < T < 100 C XIII Ciclo de Cursos Especiais BUT - when minerals that later formed the Earth condensed, pressure in the disk was very low. Water would be vapor, would not form water-rich rocks… XIII Ciclo de Cursos Especiais Chemical evidence: Ratio of deuterium to hydrogen: Earth’s water: 153 parts per million Meteorites known as carbonaceous chondrites: 159 ppm Comets: 309 ppm These meteorites appear to originate from the outer asteroid belt (beyond about 2.7 AU) Evidence for radial transport - mass dynamically negligible but critical for life How common is water on planets in the “habitable zone”? XIII Ciclo de Cursos Especiais Extrasolar Planets Difficult to directly image extrasolar planets Contrast ratio in reflected light: # "R p2 & *10 f =% A ~ 1.4 )10 ( 2 4 " a $ ' …for Earth Astronomical units: 24-25 magnitudes ! Contrast at the peak of the planet’s thermal emission is less: about f ~ 10-6 at 20 µm for the Earth XIII Ciclo de Cursos Especiais Tinetti et al. (2006) Imaging planets would allow measurement of atmospheric spectra - biomarkers such as oxygen / ozone… XIII Ciclo de Cursos Especiais Tinnetti et al. (2006) Future goal: NASA’s Terrestrial Planet Finder / ESA’s Darwin proposals All detections of extrasolar planets to date are via indirect methods XIII Ciclo de Cursos Especiais Radial velocity searches Observable: time dependent radial velocity of star (via Doppler shift of spectral lines), due to perturbation from orbiting planet For planet on circular orbit: v K = GM* a Linear momentum conservation: M*v* = M p v K ! M Observable: K = v* sini = p M* GM* sini a ! XIII Ciclo de Cursos Especiais ! Mp K = v* sini = M* GM* sini a Observable quantity: ! is ~ m s-1 best precision Usually unknown: derive lower limit on mass Massive planets are easier to detect Planets at small a easier to detect Jupiter: 12.5 m s-1 Earth: 9 cm s-1 very challenging observationally, but achievable… XIII Ciclo de Cursos Especiais High resolution astronomical spectrographs: R ~ 105 (3 km s-1) How can we detect m s-1 shifts? Consider limit to radial velocity measurement from a single pixel, assuming perfect calibration: "N ph = dN ph dv "v # "v min $ N 1ph2 dN ph /dv Can detect small RV shift if (a) high S/N and (b) spectrum has plenty!of structure (limited by thermal broadening) XIII Ciclo de Cursos Especiais Estimate S/N = 100, thermal broadening means lines have width of ~10 km s-1 Then spectrum with Npix “effective” pixels yields an RV measurement that could be as good as: "v shot 100 m s -1 ~ N 1pix2 Can measure very small RV shifts against shot noise if calibration is stable - need only resolve the lines… ! Actual noise sources include: • stellar activity • stellar oscillations (the signal for helioseismology) Sub-m s-1 very challenging XIII Ciclo de Cursos Especiais Mp K = v* sini = M* GM* sini a If a survey could detect K > Kmin for some sample of stars: log Mp sin i ! P = Psurvey Detectable Undetectable log a In fact no survey is anything like this simple… but basic selection function is of this form with Kmin ~ 20 m s-1 for complete samples… XIII Ciclo de Cursos Especiais Example of real data, measure: • orbital period • MP sin i (with stellar mass) • orbital eccentricity e …all that is known for most extrasolar planets XIII Ciclo de Cursos Especiais Multiple planet system Interesting degeneracies and statistical questions concerning survey biases - be careful! XIII Ciclo de Cursos Especiais Results #1: eccentricity vs semi-major axis XIII Ciclo de Cursos Especiais Results #2: mass vs semi-major axis XIII Ciclo de Cursos Especiais Results #3: eccentricity vs mass XIII Ciclo de Cursos Especiais Summary of radial velocity findings 1. Planet frequency among “Solar type” stars is at least 7% 2. “Hot Jupiters” - massive planets at a < 0.1 AU 3. Typical planet is eccentric: <e> = 0.27 #2, #3 are different from Solar System expectations 4. Mass function favors low mass planets, radial distribution increases to large orbital radius Note: only very limited information on planet population with M and a similar to that of Jupiter… XIII Ciclo de Cursos Especiais Fischer & Valenti (2005) Abundance of (detected) planets is a strongly increasing function of the metallicity of the host star measured from the spectrum Giant planet formation process “knows” about the trace abundance of heavy elements (~1-2%) XIII Ciclo de Cursos Especiais Transits Detection of planet via photometric monitoring of host star " Rp %2 Fractional decrement f = $ ' ( 0.01 (Jupiter) # R* & during transit Probability of ! observing a transit OK from ground = 8.4 )10*5 (Earth) space only Ptransit R* + R p = a About 10% for hot Jupiters, 0.5% for Earth in Earth’s orbit XIII Ciclo de Cursos Especiais Ground based data quality (TrES project) XIII Ciclo de Cursos Especiais Space based data quality (COROT mission results) Giant planets: direct measurement of planetary radius - confirms that these are gas giant planets - limited information on structure Terrestrial planets: Kepler mission should be sensitive to planets with Earth radius XIII Ciclo de Cursos Especiais Also possible to observe the secondary eclipse / phase modulation in the infra-red (Spitzer): Measure of temperature on the day / night side of the planet Harrington et al. (2006) XIII Ciclo de Cursos Especiais A radius mystery Measured Rp are not a one parameter family with planet mass What is the additional physics at work in setting the radius? Torres et al. (2008) • planetary structure? • dynamics (heating due to tides)? XIII Ciclo de Cursos Especiais What we need to explain How do terrestrial and gas giant planets form? How can we understand their orbits: • in the Solar System? • in extrasolar planetary systems? Hope is that this will inform questions such as: • how typical is the Solar System? • how common are habitable planets? XIII Ciclo de Cursos Especiais