Chapter 15 The Formation of Planetary Systems Chapter 15 opener. The formation of our solar system was a long-ago event, with much of the matter of our primordial galactic cloud eventually either comprising the Sun and planets or ejected back into deep space. Now, some 4.5 billion years later, it is not easy to reconstruct what exactly did happen here. Astronomers therefore observe other young star systems, hoping to gain some insight about the origins of our own solar system. Here, the Spitzer Space Telescope has taken this infrared image of W5, with its towering pillars of cool gas and dust illuminated at their tips with light from warm embryonic stars. (SSC/JPL)

Units of Chapter 15 15.1 Modeling Planet Formation 15.2 Formation of the Solar System 15.3 Terrestrial and Jovian Planets 15.4 Interplanetary Debris 15.5 Solar System Regularities and Irregularities The Angular Momentum Problem 15.6 Planets Beyond the Solar System 15.7 Is Our Solar System Unusual?

15.1 Modeling Planet Formation Any model must explain: Planets are relatively isolated in space Planetary orbits are nearly circular Planetary orbits all lie in (nearly) the same plane Direction of orbital motion is the same as direction of Sun’s rotation Direction of most planets’ rotation is also the same as the Sun’s

15.1 Modeling Planet Formation (cont.) 6. Most moons’ orbits are also in the same sense 7. Solar system is highly differentiated 8. Asteroids are very old, and not like either inner or outer planets 9. Kuiper belt, asteroid-sized icy bodies beyond the orbit of Neptune 10. Oort cloud is similar to Kuiper belt in composition, but farther out and with random orbits

15.1 Modeling Planet Formation Solar system is evidently not a random assemblage, but has a single origin. Planetary condensation theory, first discussed in Chapter 6, seems to work well. Lots of room for variation; there are also irregularities (Uranus’s axial tilt, Venus’s retrograde rotation, etc.) that must be allowed by the model.

15.2 Formation of the Solar System Review of condensation theory: Large interstellar cloud of gas and dust starts to contract, heating as it does so Sun forms in center; dust provides condensation nuclei, around which planets form As planets grow, they sweep up smaller debris near them Figure 15-1. Solar System Formation The condensation theory of planet formation. (a) An infalling interstellar cloud, actually very much larger than the resulting planetary system. (b) The solar nebula after it has contracted and flattened to form a spinning disk. The temperature is greatest in the center, near the red proto-Sun, and coolest at the edges. (c) Dust grains act as condensation nuclei, forming clumps of matter that collide, stick together, and grow into moon-sized (and larger) planetesimals. The composition of the grains and thus of any planetesimal depends on location within the nebula. (d) After a few million years, strong winds from the still-forming Sun begin expelling the nebular gas, and some massive planetesimals in the outer solar system have already accreted gas from the nebula. (e) With the gas ejected, planetesimals continue to collide and grow; the gas giant planets are already formed and the Sun has become a genuine star. (f) Over the course of a hundred million years or so, planetesimals are accreted or ejected, leaving a few large planets that travel in roughly circular orbits.

15.2 Formation of the Solar System This dust cloud is believed to be a site of star formation: Figure 15-2. Dark Cloud Interstellar gas and dark dust lanes mark this region of star formation. The dark cloud known as Barnard 86 (left) flanks a cluster of young blue stars called NGC 6520 (right). Barnard 86 may be part of a larger interstellar cloud that gave rise to these stars. (D. Malin/Anglo-Australian Telescope)

15.2 Formation of the Solar System These accretion disks surrounding stars in the process of forming are believed to represent the early stages of planetary formation: Figure 15-3. Newborn Solar Systems? (a) This infrared image, taken by the Spitzer Space Telescope, of the bright star Fomalhaut, some 25 light-years from Earth, shows a circumstellar disk in which the process of accretion is underway. The star itself is well inside the yellowish blob at center. The outer disk, which is falsely colored orange to match the cooler dust emission, is about three times the diameter of our solar system. (b) This higher-resolution Hubble Space Telescope image blocks out the central (circled) parts of another such disk around a more distant star HR4796A, but shows the edges more clearly. (NASA)

15.2 Formation of the Solar System The farther away one gets from the newborn Sun, the lower the temperature. This caused different materials to predominate in different regions—rocky planets close to the Sun, then the gas giants farther away. Figure 15-4. Temperature in the Early Solar Nebula (a) Theoretically computed variation of temperature across the primitive solar nebula illustrated in (b), which shows half of the disk in Figure 15.1(c). In the hot central regions, only metals could condense out of the gaseous state to form grains. At greater distances from the central proto-Sun, the temperature was lower, so rocky and icy grains could also form. The labels indicate the minimum radii at which grains of various types could condense out of the nebula.

15.3 Terrestrial and Jovian Planets Terrestrial (rocky) planets formed near Sun, due to high temperature—nothing else could condense there. Figure 15-5. Making the Inner Planets Accretion in the inner solar system: Initially, many moon-sized planetesimals orbited the Sun. Over the course of about 100 million years, they gradually collided and coalesced, forming a few large planets in roughly circular orbits.

15.3 Terrestrial and Jovian Planets 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 15-6. 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.

15.3 Terrestrial and Jovian Planets Once they were large enough, may have captured gas from the contracting nebula Or may have formed from instabilities in the outer, cool regions of the nebula Figure 15-7. Jovian Condensation As an alternative to the growth of massive protoplanetary cores followed by the accretion of nebular gas, it is possible that some or all of the giant planets formed directly through instabilities in the cool gas of the outer solar nebula. Part (a) shows the same instant as Figure 15.1(b). (b) Only a few thousand years later, four gas giants have already formed, preceding and circumventing the accretion process sketched in Figure 15.1. With the nebula gone (c), the giant planets have taken their place in the outer solar system. (See Figure 15.1e.)

15.3 Terrestrial and Jovian Planets 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.

15.4 Interplanetary 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

15.4 Interplanetary Debris General timeline of solar system formation: Figure 15-8. 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.

15.4 Interplanetary 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 15-9. Planetesimal Ejection The ejection of icy planetesimals 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 interior to Neptune’s orbit had been ejected. As depicted here, Neptune was affected most and may have moved outward by as much as 10 AU.

15.4 Interplanetary Debris Kuiper-belt objects have been detected from Earth recently; 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.”

15.5 Solar System Regularities and Irregularities Condensation theory covers the 10 points mentioned at the beginning. What about the exceptions? 1. Mercury’s large metallic core may be the result of a collision between two planetesimals, where much of the mantle was lost. 2. Two large bodies may have merged to form Venus. 3. Earth–Moon system may have formed after a collision.

15.5 Solar System Regularities and Irregularities (cont.) 4. Late collision may have caused Mars’s north–south asymmetry and stripped most of its atmosphere. 5. Uranus’s tilted axis may be the result of a glancing collision. 6. Miranda may have been almost destroyed in a collision. 7. Interactions between jovian protoplanets and planetesimals could be responsible for irregular moons.

15.5 Solar System Regularities and Irregularities (cont.) Many of these explanations have one thing in common—a catastrophic, or near-catastrophic, collision at a critical time during formation. Normally, one does not like to explain things by calling on one-time events, but it is clear that the early solar system involved almost constant collisions. Some must have been exceptionally large.

Discovery 15-1: The Angular Momentum Problem As it collapsed, the nebula had to conserve its angular momentum. However, at the present day, the Sun has almost none of the solar system’s angular momentum: Jupiter alone accounts for 60% Four jovian planets account for more than 99%

Discovery 15-1: The Angular Momentum Problem Theory: The Sun transferred most of its angular momentum to outer planets through friction.

15.6 Planets Beyond the Solar System Most extrasolar planets have been discovered indirectly, through their gravitational or optical effects, and cannot be seen directly due to the glare of their star. This is one exception: The star is a brown dwarf, and the planet is clearly visible. Figure 15-10. Extrasolar Planet Most known extrasolar planets are too faint to be detectable against the glare of their parent stars. However, in this system, called 2M1207, the parent itself (centered) is very faint—a so-called brown dwarf (see Chapter 19)—allowing the planet (lower left) to be detected in the infrared. This planet has a mass about 5 times that of Jupiter and orbits 55 AU from the star, which is 230 light-years away. (ESO)

15.6 Planets Beyond the Solar System Planets around other stars can be detected if they are large enough to cause the star to “wobble” as the planet and star orbit around their common center of mass. Figure 15-11. Detecting Extrasolar Planets As a planet orbits its parent star, it causes the star to “wobble” back and forth. The greater the mass of the planet, the larger is the wobble. The center of mass of the planet–star system stays fixed. If the wobble happens to occur along our line of sight to the star, as shown by the yellow arrow, we can detect it by the Doppler effect. (In principle, side-to-side motion perpendicular to the line of sight is also measurable, although there are as yet no confirmed cases of planets being detected this way.)

15.6 Planets Beyond the Solar System If the “wobble” is transverse to our line of sight, it can also be detected through the Doppler shift as the star's motion changes. Figure 15-12. Planets Revealed (a) Measurements of the Doppler shift of the star 51 Pegasi reveal a clear periodic signal indicating the presence of a planetary companion of mass at least half the mass of Jupiter. (b) Radial-velocity data for Upsilon Andromedae are much more complex, but are well fit (solid line) by a three-planet system orbiting the star. For reference in parts (a) and (b), the maximum possible signal produced by Jupiter orbiting the Sun (i.e., the wobble our Sun would display, as seen by a distant observer looking edge-on at our solar system) is shown in blue. (c) A sketch of the inferred orbits of three planets from the Upsilon Andromedae system (in orange), with the orbits of the terrestrial planets superimposed for comparison (in white).

15.6 Planets Beyond the Solar System More than 400 extrasolar planets have been discovered so far: Most have masses comparable to Jupiter’s Orbits are generally much smaller, and in some cases very much smaller, than the orbit of Jupiter Orbits have high eccentricity

15.6 Planets Beyond the Solar System An extrasolar planet may also be detected if its orbit lies in the plane of the line of sight to us. The planet will then eclipse the star, and if the planet is large enough, some decrease in luminosity may be observed. Figure 15-13. An Extrasolar Transit (a) If an extrasolar planet happens to pass between us and its parent star, the light from the star dims in a characteristic way. (b) Artist’s conception of the planet orbiting a Sun-like star known as HD 209458. The planet is 200,000 km across and transits every 3.5 days, blocking about 2 percent of the star’s light each time it does so.

15.6 Planets Beyond the Solar System This plot shows the semimajor axis and eccentricity for each of the known extrasolar planets, with Jupiter and Earth included for comparison: Figure 15-14. Extrasolar Orbital Parameters Orbital semimajor axes and eccentricities of nearly half of the approximately 200 known extrasolar planets. Each point represents one planetary orbit, and to plot them all would make a mess. The corresponding points for Earth and Jupiter in our solar system are also shown. The known extrasolar planets generally move on smaller, much more eccentric orbits than do the planets circling the Sun.

15.6 Planets Beyond the Solar System Orbits of 60 of the known extrasolar planets. Note that some of them are very close to their star: Figure 15-15. Extrasolar Orbits The orbits of many extrasolar planets residing more than 0.15 AU from their parent star, superimposed on a single plot, with Earth’s orbit shown for comparison. All these extrasolar planets are comparable in mass to Jupiter. A plot of all known extrasolar planets would be very cluttered, but the message would be much the same: These planetary systems don’t look much like ours!

15.6 Planets Beyond the Solar System Planets orbiting within 0.1 AU of their stars are called “hot Jupiters”; they are not included in the previous figure but are numerous. Stars with composition like our Sun are much more likely to have planets, showing that the “dusty disk” theory is plausible. Some of these “planets” may actually be brown dwarfs, but probably not many.

15.7 Is Our Solar System Unusual? The other planetary systems discovered so far appear to be very different from our own. Selection effect biases sample toward massive planets orbiting close to parent star; lower-mass planets cannot be detected this way.

15.7 Is Our Solar System Unusual? Recently, more Jupiter-like planets have been found; this one has twice the mass of Jupiter and an orbital period of 6 years: The blue line is the same curve for Jupiter. Figure 15-16. Jupiter-like Planet? Velocity “wobbles” in the star HD 70642 reveal the presence of the extrasolar planet with the most “Jupiter-like” orbit yet discovered. The parent star is almost identical to the Sun, and the 2-Jupiter-mass planet orbits at a distance of 3.3 AU with an orbital eccentricity of 0.1. Again, the blue line marks the corresponding plot for Jupiter itself.

15.7 Is Our Solar System Unusual? Current theories include the possibility that Jupiter-like planets could migrate inward, through friction with the solar nebula. Figure 15-17. Sinking Planet Friction between a giant planet and the nebular disk in which it formed tends to make the planet spiral inward. The process continues until the disk is dispersed by the wind from the central star, possibly leaving the planet in a “hot-Jupiter” orbit.

15.7 Is Our Solar System Unusual? About 5% of stars that have been measured have planets around them of the sort that can now be detected. A method of detecting Earth-like planets is much desired but will not be available for some time. The most promising detection method involves looking for changes in a star’s brightness as a planet transits across it. Until we can observe such planets, we will not be able to draw conclusions about the uniqueness of our own system.

Summary of Chapter 15 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. Asteroids never condensed into a larger object.

Summary of Chapter 15 (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. Collisions probably explain oddities of planets and moons. Over 200 extrasolar planets have been observed; most are massive and orbit very close to their star. This is probably the result of selection bias. Further conclusions cannot be drawn until it is possible to detect terrestrial planets.