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The Origin of the Solar System The Great Chain of Origins —a review of the origin of matter John Dobson said, “Three ingredients of the universe are hydrogen, helium, and dust of exploded stars.” The Big Bang created protons, neutrons, and electrons, which later combined to form hydrogen and helium. Nucleosynthesis inside stars accounts for the elements heavier than helium up to iron; elements heavier than iron were created in supermassive stars and in supernovae. Supernovae are the primary means by which heavy elements are dispersed into interstellar space, making possible the formation of rocky planets like Earth. Recently, dark matter and dark energy have become the mysterious ingredients of the universe that seem to far outweigh the visible matter. The Origin of the Solar System Early Hypotheses 1) Catastrophic hypotheses Example: passing star hypothesis: Star passing near the Sun tore material out of the Sun, from which planets could form Prediction: Only few stars should have planets! 2) Evolutionary hypotheses Example: Laplace’s nebular hypothesis: Rings of material separate from the spinning cloud, carrying away angular momentum of the cloud cloud could contract further (forming the sun) Prediction: Most stars should have planets! The Solar Nebula Hypothesis Basis of modern theory of planet formation. Planets form at about the same time from the same cloud as the star. Planet formation sites observed today as dust disks. Sun and our solar system formed ~ 5 billion years ago. Evidence for Ongoing Planet Formation Many young stars in the Orion Nebula are surrounded by dust disks: Probably sites of planet formation right now! Dust Disks around Forming Stars Dust disks around some T Tauri stars can be imaged directly (HST). The Story of Planet Building Planets formed from the same protostellar material as the Sun, still found in the Sun’s atmosphere. Rocky planet material formed from clumping together of dust grains in the protostellar cloud. Mass of less than ~ 15 Earth masses: Mass of more than ~ 15 Earth masses: Planets can not grow by gravitational collapse Planets can grow by gravitationally attracting material from the protostellar cloud Earthlike planets Jovian planets (gas giants) The Condensation of Solids To compare densities of planets, compensate for compression due to the planet’s gravity: Only condensed materials could stick together to form planets Temperature in the protostellar cloud decreased outward. Further out Protostellar cloud cooler Metals with lower melting point condensed Variation of chemical composition throughout solar system Formation and Growth of Planetesimals Planet formation starts with clumping together of grains of solid matter: Planetesimals (few cm to km in size) Planetesimal growth through condensation and accretion Gravitational instabilities may have helped in the growth of planetesimals into protoplanets The Growth of Protoplanets Simplest form of planet growth: Unchanged composition of accreted matter over time As rocks melted, heavier elements sink to the center differentiation This also produces a secondary atmosphere outgassing Solar Nebula Theory wrap-up Collisions Condensation Solar Nebula Coalescence Accretion Planetesimals Dust grains (< a few cm) Planets cm km Collisions Violent event (high pressures & Metallic core? temperatures) Dwarf Planets Small SolarSystem Bodies Asteroids – Meteoroids – Meteorites Comets – Meteors Extrasolar Planets = planets orbiting around other stars Extrasolar planets can not be imaged directly Detection using same methods as in binary star systems (spectroscopic binary): Look for “wobbling” motion of the star around the common center of mass. (Indirect) Detection of Extrasolar Planets Observing periodic Doppler shifts of stars with no visible companion Evidence for the wobbling motion of the star around the common center of mass of a planetary system Over 150 extrasolar planets detected so far. Survey of the Solar System Relative Sizes of the Planets Planets and their orbits Note: orbits and bodies are not to the same scale Planetary Orbits Mercury Venus Earth All planets in almost circular (elliptical) orbits around the sun, in approx. the same plane (ecliptic). Sense of revolution: counter-clockwise Sense of rotation: counter-clockwise (Exceptions: Venus and Uranus) (Distances and times reproduced roughly to scale) Orbits generally inclined by no more than 3.4o Exception: Mercury (7o) Two Kinds of Planets Terrestrial planets Jovian planets Mercury, Venus, Earth, Mars Jupiter, Saturn, Uranus, Neptune Terrestrial Planets Four inner planets of the solar system Relatively small in size and mass (Earth is the largest and most massive) Rocky (solid) surface Relatively dense: 3.3 – 5.5 g/cm3 Surface of Venus can not be seen directly from Earth because of its dense cloud cover. Craters on Planets’ Surfaces Craters (like on our moon’s surface) are common throughout the solar system Not seen on Jovian planets because they don’t have a solid surface The Jovian Planets Much larger in mass and size than terrestrial planets Low average density: 0.7 – 1.7 g/cm3 Dense atmosphere Mostly gas; no solid surface All have rings (not just Saturn!) Space Debris In addition to planets, small bodies orbit the sun: asteroids, comets, meteoroids Asteroid Eros, imaged by the NEAR spacecraft Asteroids • Asteroids are small generally rocky bodies that orbit the Sun, mostly in the asteroid belt. • They range tremendously in size, from Ceres (~1000 km) down to kilometer-sized bodies and even smaller. • Probably billions are house-sized rocks. Asteroid Belt Distribution of 21,785 asteroids Note: Making the plotted points large enough to see causes them to appear far more closely packed than they really are. Comets (“icy mud balls”) Meteoroids Visible as streaks of light: meteors The Oort Cloud and the Kuiper Belt Approx. 100 AU Orbits and bodies are not to scale Satellites • The number of planetary satellites changes frequently as more are discovered! – Jupiter 62 – Saturn 31 – Uranus 27 – Neptune 13 – Mars 2 – Earth 1 – Mercury and Venus are moonless 27 Dating the Solar System Sun and planets should have about the same age. Ages of rocks are measured by radioactive dating: Measure abundance of a radioactively decaying element to find the time since formation of the rock. Dating of rocks on Earth, on the moon, and meteorites all give ages of ~ 4.6 billion years. Dwarf Planets • Pluto and similar objects fail to fit into either family • Recently, scientists have discovered more than 200 similar objects orbiting the Sun at the same distance as Pluto • In 2006, a new family was introduced – the dwarf planets – Massive enough to pull themselves spherical – Orbits have not been swept clear of debris 30