Overview of the Solar System

1. Overview of the Solar System • The solar system consists of the Sun, eight known planets (+ Pluto!), many satellites of the planets, and a large number of interplanetary bodies (comets, asteroids, meteoroids, interplanetary dust particles) • Much information has been gathered by telescopes and space probes • Images reveal surface features (geology) and atmospheric conditions • Spectroscopic information (analysis of light from excited atoms) provide insight on atmospheric composition • Radio and infrared measurements collect data on planetary surface temperatures • NASA’s Cassini probe currently in orbit around Saturn • Pluto fly-by mission (NASA’s New Horizons) launched in 2006 (will have closest approach to Pluto in about 7.5 yrs)

2. Overview of the Solar System • The solar system has an overall disk-like structure • The orbital planes of most planets are closely aligned with each other • The orbits of most planets are ellipses with small eccentricities (nearly circular) • All planetary orbits, and virtually all satellite orbits, revolve in the same direction • Counterclockwise as seen from above the north pole • The direction of rotation for nearly all planets and satellites is in the same sense as the orbital motions • Exceptions are Venus, Uranus, and Pluto

3. Planetary Orbits (inner planets) www.nineplanets.org (outer planets)

4. Overview of the Solar System • Terrestrial planets: Mercury, Venus, Earth, Mars • 4 innermost planets • Relatively small and dense with rocky surfaces • Average densities range from 3.5 – 5.5 g/cm3 • Small abundance of light gases such as H and He • High abundance of metals and rocky materials • Few or no satellites • Giant planets/gas giants/Jovian planets: Jupiter, Saturn, Uranus, Neptune • Large outer planets • Low density, no solid surface • Average densities range from 0.7 – 1.7 g/cm3 • Light gases (like H and He) are prolific (similar to Sun and stars) • Many satellites and ring systems

5. The Planets Pluto • Overview of the planets and Sun (not to scale): Sun Mars Neptune Earth Mercury Venus Uranus Jupiter Saturn

6. Relative Sizes of the Planets • Picture of the Sun and the planets, made to scale: (courtesy of Calvin J. Hamilton)

7. Scale Model of the Solar System • Distance scales are used frequently on maps • i.e. 1 inch = 10 miles • Consider a scale model of the solar system where 1 inch = 1 million miles • Diameter of Sun = 860,000 miles ≈ 1 million miles = 1 inch on this scale (size of golf ball or ping pong ball) • See next slide for table of average distance of each planet from Sun, including distances using this scale model • The planets out to Mars are much more closely spaced than are Jupiter and the planets beyond it • Solar system is almost entirely empty space • Nearest star (Proxima Centauri) would be about 400 miles away on this scale • Approximately the distance from Columbus to Philadelphia

8. Scale Model of the Solar System (1 inch = 1 million miles)

9. Scale Model of the Solar System • Other distances on the scale of 1 inch = 1 million miles: • Earth – Moon orbital diameter: about ½ inch • Diameter of Jupiter: about 1/10 of an inch (2.5 mm) • Diameter of Earth: 1/100 of an inch (0.25 mm) • Not realistic to represent planet sizes with this scaling • Another alternative scale: 1 foot = 1 million miles • Sun = 1 foot across (basketball size) • Jupiter and Saturn = 1 inch across (golf ball size) • Uranus and Neptune = ¼ inch across (large peas) • Earth and Venus = 1/10 inch across (mustard seed) • Mercury = 1/3 size of Earth, Mars = ½ size of Earth • Nearest star would be 4000 miles away

10. Theory of Solar System Formation • Catastrophic theories • Solar system formed as the result of a singular cataclysmic event caused by external forces • In this view, planetary systems are rare • Evolutionary theories • Solar system was the result of natural internal processes accompanying the formation of the Sun • In this view, planetary systems orbiting other stars would be common • Themes from both models are used today • Overall process of planet formation is thought to be evolutionary, and planetary systems are predicted to be common • Many details of formation are thought to be the result of singular, catastrophic events

11. Theory of Solar System Formation • Solar system is believed to have originated from a rotating, condensing cloud of interstellar gas and dust • The cloud then collapsed under its own gravity • Localized regions of high density formed (?) • Blast wave from exploding star caused compression (?) • As cloud collapsed, it flattened into a disk because of its rotation (“solar nebula”) • As the cloud collapsed, the compression was most rapid at the center • Central concentration became dense enough to form molecular hydrogen, and heating occurred (cooling slowed by molecular hydrogen) • Further gravitational collapse slowed by gas pressure • Temperature increased enough for nuclear fusion to occur • Process took about 100 million years (beginning of Sun)

12. Theory of Solar System Formation • Material from the solar nebula was still condensing slowly after Sun was formed • Gradually, the gas in the nebula began to condense into planetesimals through process called accretion • Temperature highest in the inner regions of the nebula (radiation from Sun) • Prevented light gases from condensing into solid form • Heavier elements still condensed into solids • Inner concentrations were smaller as a result • Temperature was much cooler in the outer regions • All elements condensed and incorporated • Outer concentrations were thus larger • Larger concentrations attracted more matter through gravity, including that which formed disks around them (giving rise to moons and rings) • Collisions between planetesimals formed larger bodies (giving rise to planets)

13. Theory of Solar System Formation • Other, later, collisions thought to cause some irregularities • Late collisions between planetesimals thought to cause unusual tilts of Venus, Uranus, and possibly Pluto • Formation of the Moon is thought to be the result of a collision between Earth and a very large planetesimal • Mercury may have lost much of its outer portion due to a collision • Many craters are visible on planets and satellites resulting from collisions with leftover debris in young solar system • Some planetesimals did not help form planets • Tidal forces exerted by young Jupiter formed asteroids between Jupiter and Mars • Pluto is thought of as a planetesimal that was never included into one of the larger planets • More minor planetary bodies are thought to be in Kuiper belt disk that exists from beyond Uranus to 50 AU from Sun (source of comets) • Oort cloud exists at 100,000 AU (another source of comets) • Solar system is thought to be about 5 billion years old

14. Planetology • Planetology is the comparative study of Earth and the other planets • Several general processes and principles apply to all planets • All planets began from same material, but various processes have defined and altered their individual characteristics • For example, relative amount of atmosphere on each planet (and the Moon) can be understood in terms of a common “kinetic theory” of gases • Another example: amount of volcanic activity • Understood in terms of common theory on heat transfer and cooling mechanisms inside planets • By comparing information from other planets to that from Earth, we learn more about Earth as well

15. Internal Structures of Planets • Differentiation causes relatively heavy elements to sink toward the center of each planet • Requires a fluid medium • Differentiation in terrestrial planets has helped to create a layered structure • Thin outer crust • Intermediate-density (semi-rigid) mantle • Dense nickel-iron core (which is partially fluid in some cases) • Differentiation in gas giants has created a small, solid core beneath fluid layers that make up most of the interior • Internal circulation, driven by planetary rotation, occurs in any planet that has fluid zones in its interior • Occurs in the core of the terrestrial planets (if at all) • Complex in interiors of the gas giants

16. Internal Structures of Planets • Magnetic fields and particle belts are manifestations of internal circulation • Internal fluid motions in a planet can create electrical currents • Electrical currents create a magnetic field • Fluid motions in Earth’s nickel-iron core have created a significant magnetic field • Mercury has weak magnetic field despite a slow rotation – thus it probably has a large, partially molten core • All of the gas giants have intense magnetic fields due to internal circulation • A planetary magnetic field can trap electrically charged particles in belts or zones surrounding the planet

17. Surface features of Terrestrial Planets • Slow-flowing motions in the mantle are responsible for tectonic activity (movement of crustal plates) • Associated with earthquakes and volcanic activity • Flows in the mantle may be caused by convection (overturning motion caused by heated bubbles rising in a gravitationally confined fluid) • On Earth, tectonic activity causes continental drift • On Venus, crustal plates are locked together (no drifting) • Tectonic activity is responsible for major structures of the crusts of terrestrial planets (mountains, continental distributions) • Several conditions influence surface geology • Composition of crust determined by original composition of the planet (after differentiation) and volcanism • Erosion by groundwater • Collisions with other bodies in space

18. Planetary Atmospheres • A balance between gains and losses of various constituents controls atmospheric composition • Gains • Accretion of gases during and after planetary formation • Venting of gases from the interior • Chemical and biological processes that occur on the surface • Losses • Escape of gases into space (determined by mass of atoms/molecules and temperature of the atmosphere) • Chemical and biological reactions at the surface • Similar factors in all planets control the circulation of an atmosphere • Convection creates high- and low-pressure zones and vertical flowing motions • Planetary rotation converts simple convective flows into rotary flow systems (creates both calm and stormy weather)