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24 The Solar System The study of the solar system is the most local aspect of astronomy, which includes: the eight traditional planets ―dwarf planets‖ and ―plutoids‖ smaller icy planetesimals beyond Neptune (Kuiper Belt objects) planets’ moons and rings comets asteroids meteoroids the Sun—Sol The ―sol‖ar system contains only one star. It does not include any other stars or astronomical phenomena. General Description of Solar System: The sun is the dominating influence, both gravitationally and radiatively. All major bodies orbit in the same approximate plane—all planets within 7 from ecliptic. All planets and most major bodies orbit in the same direction—CCW as seen from north. 25 Most planets rotate in the same direction (CCW as seen from north), with a broad range of axial tilts. Solar system is fairly dirty—billions of asteroids, meteoroids, and comets. The eight traditional planets fit into one of two categories based on their general characteristics. Terrestrial planets: ―Earth-like‖ Small planets—mostly solid High densities (4 – 5.5 gm/cm3) All have rocky, solid surfaces Closely spaced orbits (< 1.5 AU from sun) Jovian planets: ―Jupiter-like‖ Large gaseous worlds—―gas giants‖ Low densities (0.7 – 1.7 gm/cm3) Have no solid surfaces Extensive moon and ring systems Widely spaced orbits (5.2 – 30 AU) Note that Pluto does not fit into either category 26 These descriptions give us clues to the origin of the Solar System… Any theory (model) for the SS formation must explain: CCW rotation and revolution of planets Coplanar orbits of planets Difference between terrestrials and Jovians Nebular Theory: Solar system formed from a contracting ―cloud‖ of interstellar gas and dust. Many molecular clouds distributed throughout plane of Milky Way Dense, cool parts of clouds can be triggered to collapse by density or shock wave. As cloud contracts under its own gravity, the conservation of angular momentum flattens it into a disk Original spin was CCW—planets revolve CCW As nebula contracts, temperature rises—hottest at core Eventually temperature and density are high enough in core of nebula for nuclear fusion. Sun is “born” Not all material in disk is incorporated into Sun. Why not? 27 Leftover material begins to cool and condense. Distance from newly formed sun determines which molecules condense first—condensation sequence. Molecules that can condense at high temps (metals and rocks) make up the majority of the composition of the terrestrials. Molecules that can only condense at low temps (ices) had to be several AUs from the sun to condense. As molecules condensed and collided with one another in the nebula, they fused together— accretion. Within a relatively short time span (100,000 years, according to computer simulations) trillions of planetesimals were formed by accretion. Planetesimals near center of forming solar system were rocky and metallic, those farther out (> several AUs) were icy. As planetesimals continued to collide and grow, gravity began to play a dominant role in accretion— spacing. Eventually, the four terrestrial planets we see today ―won out‖ in the inner solar system. The cores of the Jovian planets were formed from icy planetesimals in the outer solar system. Meanwhile, the newly formed sun had driven all remaining gas into the outer solar system. Once the icy cores of the Jovians grew large enough, 28 they could gravitationally attract and hold gases (H and He). Formation process of Jovians created mini ―solar‖ systems of moons around planets. After approximately 100 million years from initial contraction, the solar nebula had condensed and accreted into the sun and 8 major planets. Still billions of planetesimals left over. Where are they? The solar nebula theory predicts that planet formation should be a common byproduct of star formation. Over 300 “extra-solar” planets discovered so far! Most discovered via the radial velocity method: uses the Doppler Effect to detect unseen planets