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Neil F. Comins • William J. Kaufmann III Discovering the Universe Eighth Edition CHAPTER 5 Formation of the Solar System Essay Questions for this Lecture What are the major similarities and differences between inner and outer planets? How did the solar system form? How does the condensation sequence theory explain the similarities and differences between Jovian and Terrestrial planets? How do we detect other planetary systems around distant stars? What are the strengths and limitations of each method? WHAT DO YOU THINK? 1. 2. 3. How old is the Earth? How do we know??? How old are the Sun and other planets? Were they created during the “Big Bang”? How do we know? Have any Earthlike planets been discovered orbiting Sunlike stars? How can we tell? In this chapter you will discover… how the solar system formed why the early solar system was much more violent than it is today how astronomers define various types of objects in the solar system how the planets are “grouped” In this chapter you will discover… how moons formed throughout the solar system the “debris” in the solar system that disks of gas and dust, as well as planets, have been observed around a growing number of stars that newly forming stars & planetary systems are being observed The solar system exhibits clear patterns of composition and motion. These patterns are far more important and interesting than numbers, names, and other trivia. Planets are very tiny compared to distances between them. Sun • Over 99.9% of solar system’s mass • Made mostly of H/He gas (plasma) • Converts 4 million tons of mass into energy each second Mercury • Made of metal and rock; large iron core • Desolate, cratered; long, tall, steep cliffs • Very hot and very cold: 425°C (day), –170°C (night) Venus • Nearly identical in size to Earth; surface hidden by clouds • Hellish conditions due to an extreme greenhouse effect • Even hotter than Mercury: 470°C, day and night Earth Earth and Moon to scale • An oasis of life • The only surface liquid water in the solar system • A surprisingly large moon Mars • Looks almost Earth-like, but don’t go without a spacesuit! • Giant volcanoes, a huge canyon, polar caps, and more • Water flowed in the distant past; could there have been life? Jupiter Much farther from Sun than inner planets Mostly H/He; no solid surface 300 times more massive than Earth Many moons, rings Jupiter’s moons can be as interesting as planets themselves, especially Jupiter’s four Galilean moons • Io (shown here): Active volcanoes all over • Europa: Possible subsurface ocean • Ganymede: Largest moon in solar system • Callisto: A large, cratered “ice ball” Saturn Giant and gaseous like Jupiter Spectacular rings Many moons, including cloudy Titan Cassini spacecraft currently studying it Rings are NOT solid; they are made of countless small chunks of ice and rock, each orbiting like a tiny moon. Artist’s conception The Rings of Saturn Cassini probe arrived July 2004. (Launched in 1997) Uranus Smaller than Jupiter/Saturn; much larger than Earth Made of H/He gas and hydrogen compounds (H2O, NH3, CH4) Extreme axis tilt Moons and rings Neptune Similar to Uranus (except for axis tilt) Many moons (including Triton) Pluto and Eris Much smaller than other planets Icy, comet-like composition Pluto’s moon Charon is similar in size to Pluto Space isn’t empty… We know space is filled with gas and dust – the raw materials from which planetary systems form! …and its composition changes Spectra of exploding and old stars shows heavier elements being ejected, too Clues to Formation of our Solar System? Clue #1: Motion of Large Bodies All large bodies in solar system orbit in same direction & in nearly same plane. Most also rotate in that direction. Clue #2: Two Major Planet Types Terrestrial planets are rocky, relatively small, & close to Sun. Jovian planets are gaseous, larger, & farther from Sun. Clue #3: Swarms of Smaller Bodies Many icy comets, small “dwarf planets”, & rocky asteroids populate solar system in 3 areas Notable Exceptions Several exceptions to normal patterns need to be explained. What theory best explains the features of our solar system? According to the nebular theory, our solar system formed from a giant cloud of interstellar gas. (nebula = cloud) Why are there two major types of planets? The Laws of Physics 1. As gravity causes the cloud to contract, it heats up. (Conservation of Energy!) 2. As it contracts, it spins faster. (Conservation of Angular Momentum!) 3. As it spins and contracts, it flattens. 4. The temperature varies by distance Inner parts of the disk are hotter than outer parts. Rock can be solid at much higher temperatures than ice. Temperature Distribution of the Disk and the Frost Line Fig 9.5 Inside the frost line: Too hot for hydrogen compounds to form ices Outside the frost line: Cold enough for ices to form Formation of Terrestrial Planets • Small particles of rock and metal were present inside the frost line. • Planetesimals of rock and metal built up as these particles collided. • Gravity eventually assembled these planetesimals into terrestrial planets. Tiny solid particles stick to form planetesimals. Summary of the Condensates in the Protoplanetary Disk Gravity draws planetesimals together to form planets. This process of assembly is called accretion. Summary of the Condensates in the Protoplanetary Disk Accretion of Planetesimals • Many smaller objects collected into just a few large ones. Formation of Jovian Planets • Ice could also form small particles outside the frost line. • Larger planetesimals and planets were able to form. • The gravity of these larger planets was able to draw in surrounding H and He gases. The gravity of rock and ice in jovian planets draws in H and He gases. Nebular Capture and the Formation of the Jovian Planets Moons of jovian planets form in miniature disks. Summary of Key Ideas Formation of the Solar System Hydrogen, helium, and traces of lithium, the three lightest elements, were formed shortly after the creation of the universe. The heavier elements were produced much later by stars and are cast into space when stars die. By mass, 98% of the observed matter in the universe is hydrogen and helium. The solar system formed 4.6 billion years ago from a swirling, disk-shaped cloud of gas, ice, and dust, called the solar nebula. The four inner planets formed through the accretion of dust particles into planetesimals and then into larger protoplanets. The four outer planets probably formed through the runaway accretion of gas and ice onto rocky protoplanetary cores over millions of years, but possibly by gravitational collapse in under 100,000 years. Formation of the Solar System The Sun formed at the center of the solar nebula. After about 100 million years, the temperature at the protosun’s center was high enough to ignite thermonuclear fusion reactions. For 800 million years after the Sun formed, impacts of asteroid-like objects on the young planets dominated the history of the solar system. Comparative Planetology The four inner planets of the solar system share many characteristics and are distinctly different from the four giant outer planets. The four inner, terrestrial planets are relatively small, have high average densities, and are composed primarily of rock and metal. Jupiter and Saturn have large diameters and low densities and are composed primarily of hydrogen and helium. Uranus and Neptune have large quantities of water as well as much hydrogen and helium. Comparative Planetology Pluto, once considered the smallest planet, has a size, density, and composition consistent with the known Kuiper Belt Objects (KBOs). Asteroids are rocky and metallic debris in the solar system, larger than about a kilometer in diameter, and found primarily between the orbits of Mars and Jupiter. Meteoroids are smaller pieces of such debris. Comets are debris that contain both ice and rock. Planets Outside Our Solar System Astronomers have observed disks of gas and dust orbiting young stars. At least 250 extrasolar planets have been discovered orbiting other stars. Most of the extrasolar planets that have been discovered have masses roughly the mass of Jupiter. Extrasolar planets are discovered indirectly as a result of their effects on the stars they orbit. Key Terms accretion albedo asteroid asteroid belt average density comet core-accretion model crater gravitational instability model meteoroid microlensing moon (natural satellites) orbital inclination planet planetesimal protoplanet protoplanetary disks (proplyds) protosun solar nebula solar system terrestrial planet WHAT DID YOU THINK? Were the Sun and planets among the first generation of objects created in the universe? No. All matter and energy were created by the Big Bang. However, much of the material that exists in our solar system was processed inside stars that evolved before the solar system existed. The solar system formed billions of years after the Big Bang occurred. WHAT DID YOU THINK? How long has Earth existed, and how do we know this? Earth formed along with the rest of the solar system, about 4.6 billion years ago. The age is determined from the amount of radioactive decay that has occurred in it. WHAT DID YOU THINK? What typical shape(s) do moons have, and why? Although some moons are spherical, most look roughly like potatoes. Those that are spherical are held together by the force of gravity, pulling down high regions. Those that are potato-shaped are held together by the electromagnetic interaction between atoms, just like rocks. These latter moons are too small to be reshaped by gravity. WHAT DID YOU THINK? Have any Earthlike planets been discovered orbiting Sunlike stars? Not really. Most extrasolar planets are Jupiter-like gas giants. The planets similar in mass and size to Earth are either orbiting remnants of stars that exploded or, in the case of Gliese 581, a star much less massive and much cooler than the Sun.