11Formation of the Solar System andOther Planetary Systems2Questions to Ponder about Solar System1. Are all the other planets similar to Earth, or are they very different?2. Do other planets have moons like Earth’s Moon?3. How do astronomers know what the other planets are made of?4. Are all the planets made of basically the same material?5. What is the difference between an asteroid and a comet?6. Why are craters common on the Moon but rare on the Earth?7. Why do interplanetary spacecraft carry devices for measuring magnetic fields?8. Do all the planets have a common origin?3Questions to Ponder about Origins1. What must be included in a viable theory of the origin of the solar system?2. Why are some elements (like gold) quite rare, while others (like carbon) are more common?3. How do we know the age of the solar system?4. How do astronomers think the solar system formed?5. Did all of the planets form in the same way?6. Are there planets orbiting other stars? How do astronomers search for other planets?4There are two broad categories of planets:Earthlike (terrestrial) and Jupiterlike (jovian)• All of the planets orbit the Sun in the same direction and in almost the same plane• Most of the planets have nearly circular orbits5DensityVmD • The average density of any substance depends in part on its composition • An object sinks in a fluid if its average density is greater than that of the fluid, but rises if its average density is less than that of the fluid • The terrestrial (Earth-like) planets are made of rocky materials and have dense iron cores, which gives these planets high average densities• The Jovian (Jupiter-like) planets are composed primarily of light elements such as hydrogen and helium, which gives these planets low average densities6The Terrestrial Planets• The four innermost planets are called terrestrial planets– Relatively small (with diameters of 5000 to 13,000 km)– High average densities (4000 to 5500 kg/m3)– Composed primarily of rocky materials27Jovian Planets are the outer planets (except for Pluto)• Jupiter, Saturn, Uranus and Neptune are Jovian planets– Large diameters (50,000 to 143,000 km)– Low average densities (700 to 1700 kg/m3)– Composed primarily of hydrogen and helium.The Jovian Planets8Pluto (dwarf planet) -Not terrestrial nor Jovian•Pluto is a special case– Smaller than any of the terrestrial planets– Intermediate average density of about 1900 kg/m3– Density suggests it is composed of a mixture of ice and rock9• Some (3) comparable in size to the planet Mercury (2 are larger)• The remaining moons of the solar system are much smaller than theseSeven largest moons are almost as big as the terrestrial planets10Spectroscopy reveals the chemical compositionof the planets• The spectrum of a planet or satellite with an atmosphere reveals the atmosphere’s composition• If there is no atmosphere, the spectrum indicates the composition of the surface.• The substances that make up the planets can be classified as gases, ices, or rock, depending on the temperatures and pressures at which they solidify• The terrestrial planets are composed primarily of rocky materials, whereas the Jovian planets are composed largely of gas11Phases and Phase Diagram(Not in text but important)12Spectroscopy of Titan (moon of Saturn)313Spectroscopy of Europa (moon of Jupiter)14Hydrogen and helium are abundant on the Jovianplanets, whereas the terrestrial planets arecomposed mostly of heavier elementsJupiterMars15Asteroids (rocky) and comets (icy)also orbit the Sun• Asteroids are small, rocky objects• Comets and Kuiper Belt Objects are made of “dirty ice”• All are remnants left over from the formation of the planets• The Kuiper belt extends far beyond the orbit of Pluto• Pluto (aka dwarf planet) can be thought of as a large member of the Kuiper Belt16Cratering on Planets and Satellites• Result of impacts from interplanetary debris– when an asteroid, comet, or meteoroid collides with the surface of a terrestrial planet or satellite, the result is an impact crater• Geologic activity renews the surface and erases craters– extensive cratering means an old surface and little or no geologic activity– geologic activity is powered by internal heat, and smaller worlds lose heat more rapidly, thus, as a general rule, smaller terrestrial worlds are more extensively cratered17A planet with a magnetic field indicates an interior in motion• Planetary magnetic fields are produced by the motion of electrically conducting substances inside the planet• This mechanism is called a dynamo•If a planet has no magnetic field this would be evidence that there is little such material in the planet’s interior or that the substance is not in a state of motion18• The magnetic fields of terrestrial planets are produced by metals such as iron in the liquid state• The magnetic fields of the Jovian planets are generated by metallic hydrogen419• The planets, satellites, comets, asteroids, and the Sun itself formed from the same cloud of interstellar gas and dust• The composition of this cloud was shaped by cosmic processes, including nuclear reactions that took place within stars that died long before our solar system was formed• Different planets formed in different environments depending on their distance from the Sun and these environmental variations gave rise to the planets and satellites of our present-day solar system Comparing Terrestrial and Jovian Planets20Any model of solar system origins must explainthe present-day Sun and planets1. The terrestrial planets, which are composed primarily of rocky substances, are relatively small, while the Jovian planets, which are composed primarily of hydrogen and helium, are relatively large2. All of the planets orbit the Sun in the same direction, and all of their orbits are in nearly the same plane3. The terrestrial planets orbit close to the Sun, while the Jovian planets orbit far from the Sun21The abundances of the chemical elements arethe result of cosmic processes• The vast majority of the atoms in the universe are hydrogen and helium atoms produced in the Big Bang22All heavy elements (>Li) were manufactured by stars after the origin of the universe itself, either by fusion deep in stellar interiors or by stellar explosions.23•The interstellar medium is a tenuous collection of gas and dust that pervades the spaces between the stars• A nebula is any gas cloud