FORMATION OF THE SOLAR SYSTEM

When, What, How?

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WHEN: AGES VIA RADIOACTIVE DECAY

Recall that

  • Many heavy elements have UNSTABLE NUCLEAR ISOTOPES.
  • Such nuclei can FISSION into lighter ones.
  • Common is loss of He-4 nuclei -- alpha particles.
  • Other PARENT NUCLEI lose neutrons, protons, electrons or positrons
  • Typically the resulting DAUGHTER NUCLEI are still unstable, so
    there is a DECAY CHAIN to a final, STABLE NUCLEAR ISOTOPE.

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    HALF-LIFE

    N(t) = N_0 2^{-t/t_{1/2}}

    e.g., 1/2 gone after 1 half-life; 3/4 gone after 2 half-lives;
    7/8 gone after 3 half-lives.

    A few important NUCLEOCHRONOMETERS:

    U-235, U-238, K-40.

    These lead to oldest:

  • earth rocks of 3.9 Gyr
  • moon rocks of 4.4 Gyr
  • meteorites of 4.55 Gyr.
  • THE AGE OF THE SOLAR SYSTEM IS 4.55 Gyr
  • This value agrees with calculations for the evolution of a star
    with the mass and composition of the Sun.

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    WHAT MUST BE EXPLAINED?

  • 99.9% of SS MASS in the SUN
  • 99% of SS ANGULAR MOMENTUM in the Planets
  • PLANETS are relatively ISOLATED
  • ECCENTRICITIES of planetary ORBITS are SMALL
  • INCLINATIONS also SMALL
  • Planetary ORBITS are ALL PROGRADE
  • MOST planetary SPINS are PROGRADE
  • MOST MOONS have PROGRADE, nearly EQUATORIAL ORBITS
  • TERRESTRIAL vs. JOVIAN DICHOTOMY
  • ASTEROIDS are like pieces of planets
  • KUIPER BELT contains asteriods and short period comet nuclei
  • COMETS are PRIMITIVE, ICY & DISTANT, most in the Oort cloud

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    HOW CAN ALL THIS BE UNDERSTOOD?

    CONDENSATION WITHIN A NEBULA!

    Start with a COLLAPSING, ROTATING CLOUD of GAS and DUST.

    Gravity pulls in faster along rotation axis.

    Protostar forms at the center, a flattened nebula holds less of the mass,
    more of the angular momentum.

    Dust grains stick, settle towards disk plane.

    Both Size and Gravity enforce Republicanism: the rich get richer

    PLANETESIMALS grow via ACCRETION of gas and dust.

    Then they COLLIDE: typically bigger ones grow, smaller are smashed;
    some of these fragments are accreted later, others escape.

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    Condensation in a NEBULA naturally explains:

  • More mass to the center (Sun)
  • More angular momentum to the outside
  • Gaps between planets
  • Prograde orbits, low inclinations and low eccentricities
  • Collisions USUALLY give prograde spins
    (but can give exceptions, like Venus and Uranus)

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    TEMPERATURE GRADIENT YIELDS DIFFERENTIATION

  • Hottest near new star; only refractory elements condense
  • Inner cores stay rocky
  • Outer cores accrete more gas with prograde spin
    -- yields many outer planet moons
  • T Tauri wind blows gas from inner SS
  • -> INNER and OUTER SS differences
  • Also moon distributions and orbits
  • To some degree, nature of asteroids and comets

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    PLANETS AROUND OTHER STARS

    First evidence: DUST DISKS seen around nearby young stars.

    Such disks often had holes in the center: likely areas cleared out by planets.

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    Reasonably direct evidence:

    Only since 1994 (51 Pegasi)

    Very accurate RADIAL VELOCITIES (< 50 m/s) of stars indicate tugs from
    much less massive objects with periods of days to years.

    Very precise ASTROMETRY (0.002 arcsec) indicate wobbles in stars' paths over
    years which are also due to companions

    As of 2002, some 110 good indirect planet detections.

  • Several stars have multiple planets detected.

    Both techniques: Much easier to detect big, multi-Jupiter mass planets

  • SO, most are more massive than Jupiter
  • A few as light as Saturn have been estimated
  • SURPRISE: Hot Jupiters often found w/in 1 AU
    Formed in situ; would have to survive heat and T Tauri wind.
    OR, Migrated inward through nebular disk?
    Are these the typical plantary system or just a selection effect?
    Is our solar system the weird one?
  • One planet's transit has been seen after orbit known from radial velocity curves, confirming it for sure!

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    Future searches:

    Direct evidence preferred

    Imaging via blocking out light of star (IR much better than visible)

  • Interferometry: Space Interferometry Mission;
  • Terrestrial Planet Finder
  • Search for Extra Terrestrial Intelligence:
  • radio signals, optical (laser) signals, visits???

    If you're really interested, take the Perspectives 2002 course on this subject.