Chapter 6

Chapter 6 The Solar System Asteroids sometimes collide with Earth, so it is very much in our own interest to keep an eye on them! This image shows a close-up of the asteroid Itokawa, which is only 0.5 km long—about five soccer fields across. It was photographed as the Japanese spacecraft, Hayabusa, having launched from Earth in 2003, slowly approached the asteroid in 2005. The craft then soft-landed, scooped up some rocky debris, and took off for Earth, landing back home in 2010. A remarkable engineering achievement, this mission also scientifically proved that asteroids like this one are the source of most meteorites—the oldest matter in the solar system. (JAXA)

Units of Chapter 6 6.1 An Inventory of the Solar System 6.2 Measuring the Planets 6.3 The Overall Layout of the Solar System 6.4 Terrestrial and Jovian Planets Discovery 6-1 Gravitational “Slingshots” 6.5 Interplanetary Matter

Units of Chapter 6 (cont.) 6.6 How Did the Solar System Form? Discovery 6-2 Spacecraft Exploration of the Solar System More Precisely 6-1 Angular Momentum 6.7 Jovian Planets and Planetary Debris

6.1 An Inventory of the Solar System Early astronomers knew Moon, stars, Mercury, Venus, Mars, Jupiter, Saturn, comets, and meteors Figure 6-1. Early Telescope The refracting telescope with which Galileo made his first observations was simple, but its influence on astronomy was immeasurable. (Museo della Scienza; Scala/Art Resource, NY)

6.1 An Inventory of the Solar System Now known: The solar system has 169 moons, one star, eight planets (added Uranus and Neptune), eight asteroids, more than 100 Kuiper belt objects more than 300 km in diameter, and many smaller asteroids, comets, and meteoroids. Figure 6-4. Spirit on Mars The Mars rover Spirit took hundreds of images to create this true-color, 360° panorama of the Martian horizon from within Gusev crater. The robot, whose tracks into the shallow basin can be seen at right center, then measured the chemistry and mineralogy of soils and rocky outcrops. (JPL)

6.1 An Inventory of the Solar System More than 800 extrasolar planets have been found Understanding planetary formation in our own solar system helps understand its formation as well as formation of other systems

6.2 Measuring the Planets Distance from Sun known by Kepler’s laws Orbital period can be observed Radius known from angular size Masses from Newton’s laws Rotation period from observations Density can be calculated knowing radius and mass

6.2 Measuring the Planets

6.3 The Overall Layout of the Solar System All orbits but Mercury’s are close to the same plane Figure 6-5. Solar System Major bodies of the solar system include the Sun, planets, and asteroids. Except for Mercury, the orbits of the planets are almost circular (a) and lie nearly in the same plane (b). The distance between adjacent orbits increases farther from the Sun. The entire solar system spans nearly 100 AU—roughly the diameter of the Kuiper belt—and is very flat.

6.3 The Overall Layout of the Solar System Because the planet’s orbits are close to being in a plane, it is possible for them to appear in a straight line as viewed from Earth. This photograph was taken in April 2002. Figure 6-6. Planetary Alignment This image shows six planets—Mercury, Venus, Mars, Jupiter, Saturn, and Earth—during a planetary alignment in April 2002. The Sun and Moon are just below the horizon. As usual, the popular press contained many sensationalized predictions of catastrophes that would occur during this rare astronomical event. Also as usual, none came true. (J. Lodriguss)

6.4 Terrestrial and Jovian Planets In this picture of the eight planets and the Sun, the differences between the four terrestrial and four jovian planets are clear. Figure 6-7. Sun and Planets Relative sizes of the planets and our Sun, drawn to scale. Notice how much larger the jovian planets are than Earth and the other terrestrial planets, and how much larger still is the Sun. Explaining this planetary dichotomy is an important goal of comparative planetology, although by no means the only one.

6.4 Terrestrial and Jovian Planets Terrestrial planets: Mercury, Venus, Earth, Mars Jovian planets: Jupiter, Saturn, Uranus, Neptune Terrestrial planets are small and rocky, close to the Sun, rotate slowly, have weak magnetic fields, few moons, and no rings Jovian planets are large and gaseous, far from the Sun, rotate quickly, have strong magnetic fields, many moons, and rings

6.4 Terrestrial and Jovian Planets Differences among the terrestrial planets: All have atmospheres, but they are very different; surface conditions vary as well Only Earth has oxygen in its atmosphere and liquid water on its surface Earth and Mars spin at about the same rate; Mercury is much slower, Venus is slow and retrograde Only Earth and Mars have moons Only Earth and Mercury have magnetic fields

Discovery 6-1: Gravitational “Slingshots” Gravitational “slingshots” can change direction of spacecraft, and also accelerate it

6.5 Interplanetary Matter Asteroids and meteoroids have rocky composition; asteroids are bigger Asteroid Vesta is 500 km across Figure 6-8a. Asteroid and Comet (a) Asteroids, like meteoroids, are generally composed of rocky material. This asteroid, Vesta, is nearly 500 km across and orbits between Mars and Jupiter. It was photographed by the Dawn spacecraft in 2011. (JPL)

6.5 Interplanetary Matter Comets are icy, with some rocky parts Comet McNaught Figure 6-8b. Asteroid and Comet (b) Most comets are composed largely of ice and so tend to be relatively fragile. This is comet McNaught, seen over the Pacific Ocean in 2007, with its vaporized tail extending away from the Sun for nearly a quarter of the way across the sky. (S. Deiries/ESO)

6.5 Interplanetary Matter Pluto, once classified as one of the major planets, is the closest large Kuiper belt object to the Sun Figure 6-8c. Asteroid and Comet (c) Pluto—seen here at center with three of its moons—is one of the largest members of the Kuiper belt. Formerly classified as a major planet, it was demoted during a heated debate among astronomers in 2006. (NASA)

Discovery 6-2 Spacecraft Exploration of the Solar System Soviet Venera probes landed on Venus from 1970 to 1978 Discovery 6-2 Figure (Sovfoto/Eastfoto)

Discovery 6-2 Spacecraft Exploration of the Solar System The most recent Venus expedition from the United States was the Magellan orbiter, 1990–1994 Discovery 6-2 Figure (NASA)

Discovery 6-2 Spacecraft Exploration of the Solar System Viking landers arrived at Mars in 1976 Discovery 6-2 Figure (NASA)

Discovery 6-2 Spacecraft Exploration of the Solar System Spirit took this image on Mars in 2005 Discovery 6-2 Figure (NASA)

Discovery 6-2 Spacecraft Exploration of the Solar System Pioneer and Voyager flew through outer solar system. This is Voyager. Discovery 6-2 Figure (NASA)

Discovery 6-2 Spacecraft Exploration of the Solar System Cassini mission arrived at Saturn in 2004, has returned many spectacular images Discovery 6-2 Figure (NASA)

6.6 How Did the Solar System Form? Nebular contraction: Cloud of gas and dust contracts due to gravity; conservation of angular momentum means it spins faster and faster as it contracts Figure 6-9. Nebular Contraction (a) Conservation of angular momentum demands that a contracting, rotating cloud must spin faster as its size decreases. (b) Eventually, a small part of it destined to become the solar system came to resemble a gigantic pancake. The large blob at the center ultimately became the Sun. (c) The planets that formed from the nebula inherited its rotation and flattened shape.

More Precisely 6-1: Angular Momentum Conservation of angular momentum says that product of radius and rotation rate must be constant

6.6 How Did the Solar System Form? Nebular contraction is followed by condensation around dust grains, known to exist in interstellar clouds such as the one shown here. Accretion then leads to larger and larger clumps; finally gravitational attraction takes over and planets form. Figure 6-11. Dark Cloud Interstellar gas and dark dust lanes mark this region of star formation. The dark cloud known as Barnard 86 (dark, empty space at left) flanks a cluster of young blue stars called NG C 6520 (right). Barnard 86 may be part of a larger interstellar cloud that gave rise to these stars. (D. Malin/Anglo-AustralianTelescope)

6.7 Jovian Planets and Planetary Debris Terrestrial (rocky) planets formed near Sun, due to high temperature—nothing else could condense there. Figure 6.13 Accreting Planets Initially in the inner solar system, many moon-sized planetesimals orbited the Sun. Gradually, they collided and coalesced, forming a few large planets in roughly circular orbits.

6.7 Jovian Planets and Planetary Debris T Tauri stars are in a highly active phase of their evolution and have strong solar winds. These winds sweep away the gas disk, leaving the planetesimals and gas giants. Figure 6.16 T Tauri Star (a) Strong stellar winds from the newborn Sun sweep away the gas disk of the solar nebula, (b) leaving only giant planets and planetesimals behind. This stage of stellar evolution occurs only a few million years after the formation of the nebula.

6.7 Jovian Planets and Planetary Debris Once they were large enough, may have captured gas from the contracting nebula Or may not have formed from accretion at all, but directly from instabilities in the outer, cool regions of the nebula Figure 6.17 Jovian Condensation As an alternative to the growth of massive protoplanetary cores followed by the accretion of nebular gas, some or all of the giant planets might have formed directly through instabilities in the cool gas of the outer solar nebula. Part (a) shows the same instant as Figure 6.15(a). (b) Only a few thousand years later, four gas giants have already formed (red blobs), circumventing the accretion process sketched in Figure 6.15(b) and (c). With the nebula gone (c), the giant planets have taken their place in the outer solar system. (See Figure 6.15d.)

6.7 Jovian Planets and Planetary Debris Detailed information about the cores of jovian planets should help us distinguish between the two possibilities. Also possible: The jovian planets may have formed farther from the Sun and “migrated” inward.

6.7 Jovian Planets and Planetary Debris Asteroid belt: Orbits mostly between Mars and Jupiter Jupiter’s gravity kept them from condensing into a planet, or accreting onto an existing one Fragments left over from the initial formation of the solar system

6.7 Jovian Planets and Planetary Debris General timeline of solar system formation Figure 6.18 Solar System Formation Schematic time line of some key events occurring during the first billion years of our solar system. The various tracks show the evolution of the Sun and the solar nebula, as well as that of the inner and outer solar system. Note that the tracks are intended to illustrate approximate relationships between events, not the precise times at which they occurred. Planetary scientists think that all of the processes represented here should have occurred in other planetary systems, too.

6.7 Jovian Planets and Planetary Debris Icy planetesimals far from the Sun were ejected into distant orbits by gravitational interaction with the jovian planets, into the Kuiper belt and the Oort cloud. Some were left with extremely eccentric orbits and appear in the inner solar system as comets. Figure 6.19 Planetesimal Ejection The ejection of icy planetesimals help to form the Oort cloud and Kuiper belt. (a) Initially, once the giant planets had formed, leftover planetesimals were found throughout the solar system. Interactions with Jupiter and Saturn apparently “kicked” planetesimals out to very large radii (the Oort cloud). Interactions with Uranus and especially Neptune tended to keep the Kuiper belt populated, but also deflected many planetesimals inward to interact with Jupiter and Saturn. (b) After hundreds of millions of years and as a result of the inward and outward “traffic,” the orbits of all four giant planets were significantly modified by the time the planetesimals inside Neptune’s orbit had been ejected. As depicted here, Neptune was affected most and may have moved outward by as much as 10 AU.

6.7 Jovian Planets and Planetary Debris Kuiper belt objects have been detected from Earth; a few are as large as, or larger than, Pluto, and their composition appears similar. About 1/3 of all Kuiper belt objects (including Pluto) have orbits that are in a 3:2 resonance with Neptune; such objects are called “plutinos.”

Summary of Chapter 6 Solar system consists of Sun and everything orbiting it Asteroids are rocky, and most orbit between orbits of Mars and Jupiter Comets are icy and are believed to have formed early in the solar system’s life Major planets orbit Sun in same sense, and all but Venus rotate in that sense as well Planetary orbits lie almost in the same plane

Summary of Chapter 6 (cont.) Four inner planets—terrestrial planets—are rocky, small, and dense Four outer planets—jovian planets—are gaseous and large Nebular theory of solar system formation: cloud of gas and dust gradually collapsed under its own gravity, spinning faster as it shrank Condensation theory says dust grains acted as condensation nuclei, beginning formation of larger objects

Summary of Chapter 6 (cont.) The solar system is orderly, not random; need formation theory that explains this Condensation theory is the current favorite—large cloud of interstellar gas and dust starts to collapse, the Sun forms at the center, and dust particles act as accretion nuclei to form the planets Rocky planets would form close to the Sun; outer planets contain materials that would vaporize or escape at higher temperatures Jovian planets may have formed directly from instabilities in the cloud Asteroids never condensed into a larger object

Summary of Chapter 6 (cont.) Leftover planetesimals were ejected from the main solar system and are now in the Kuiper belt and the Oort cloud. Some occasionally enter the inner solar system as comets