2. Chapter 15 The Formation of Planetary Systems
Astronomers routinely observe other young star systems, hoping to gain insight into the origins of our own solar system. The Spitzer Space Telescope recently took this infrared image of W5 about 6500 light-years distant, with its towering pillars of cool gas illuminated at their tips with light from warm embryonic stars—and probably planets. Several generations of stars can be identified in this one image; younger (bluer) ones are at the center. The widespread nebulosity is due to dust. (SSC/JPL)

3. Units of Chapter 15 15.1 Modeling Planet Formation
15.2 Terrestrial and Jovian Planets
15.3 Interplanetary Debris
15.4 Solar System Regularities and Irregularities
15.5 Searching for Extrasolar Planets
15.6 Properties of Exoplanets
15.7 Is Our Solar System Unusual?

4. 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

5. 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

6. 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 for by the model.

7. As planets grow, they sweep up smaller debris near them
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 Solar System Formation The condensation theory of planet formation is artistically illustrated by these half-dozen changes, from infalling interstellar cloud at the top to newly emerged planetary system at the bottom. Compare to Figure 6.17, and consult the text opposite for descriptions of each of the frames of this figure.

8. 15.2 Terrestrial and Jovian Planets
Terrestrial (rocky) planets formed near Sun, due to high temperature—nothing else could condense there.
Figure 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.

9. 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 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.

10. Jovian planets:
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 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 15.2(b). (b) Only a few thousand years later, four gas giants have already formed, preceding and circumventing the accretion process sketched in Figure With the nebula gone (c), the giant planets have taken their place in the outer solar system. (See Figure 15.2e.)

11. 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.

12. Any theory of the origin of the solar system must explain all of these EXCEPT
the orbits of the planets are nearly circular, and in the same plane.
the direction that planets orbit the Sun is opposite to the Sun’s spin.
the terrestrial planets have higher density and lower mass.
comets do not necessarily orbit in the plane of the solar system.

13. What is the name of the theory that is currently used to describe the formation of the solar system?
perturbation theory
condensation theory
differentiation theory
close-encounter theory

14. The condensation sequence theory explains why
our planet Earth has water and rain.
stars are more likely to form large planets orbiting very near.
terrestrial planets are different from jovian planets.
the Moon formed near the Earth.
Pluto has such a circular orbit.

15. The ________ theory would have the jovians growing quickly into giants after forming directly from the outer solar nebula.
large impact
core-accretion
gravitational instability
Perturbation
Oort cloud

16. Why did a major planet not form out in the Kuiper belt?
One did, and its name is Pluto.
Some may have, but they have since migrated to orbits nearer the Sun.
Sedna disrupted objects closer to the Sun.
Comets bombarded these bodies violently, breaking them apart.
It was not in the ecliptic like the planets; there was no material out there.

17. Compared to the formation of the terrestrial planets, Jupiter may have
formed at a much higher temperature.
condensed gravitationally rather than by accretion.
condensed with mostly metallic materials.
required a much longer time to form.
not followed the condensation sequence at all.

18. 15.3 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

19. General timeline of solar system formation
Figure 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.

20. 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 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.

21. 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.”

22. 15.4 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.

23. 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.
8. Binary Kuiper belt objects (including the Pluto-Charon system) could have formed through collisions before ejection by interactions with the jovian planets.

24. 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.

25. 15.5 Searching for Extrasolar Planets
Most extrasolar planets have been discovered indirectly, through their gravitational or optical effects, and they cannot be seen directly due to the glare of their star. This is one exception; the planet orbits about 100 AU from its star, Fomalhaut.
Figure Extrasolar Planet The Fomalhaut star system, about 25 light-years away, is seen here in this false-colored visible image taken by the Hubble telescope (see also Figure 6.18a for an infrared image taken by the Spitzer telescope). The parent star itself is the small white dot at center surrounded by a vast disk of dusty material that is likely condensing into planets. (The darkened areas near the star are instrumental artifacts.) The newly discovered planet, called Fomalhaut b, was found within the small boxed area, which is expanded at lower right to show the planet at two positions along its orbital track about the star. The Jupiter-type planet orbits about 100 AU from the star and has a period of nearly a thousand years. (NASA)

26. 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 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.)

27. 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 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).

28. 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 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 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.

29. 15.6 Properties of Exoplanets
More than 450 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

30. This plot shows the mass, semimajor axis, and eccentricity for 400 extrasolar planets, with the mass ranges corresponding roughly to Jupiter, Neptune, and Earth included for comparison
Figure Extrasolar Orbital Parameters (a) Masses and orbital semimajor axes of approximately 400 known extrasolar planets. Each point represents one planetary orbit. The corresponding points for Earth, Jupiter, and Neptune in our solar system are also shown. Planets are classified by familiar solar system names, depending on mass, and as hot or cold, depending on distance from their parent star. (b) Orbital semimajor axes and eccentricities of the same planets. The known extrasolar planets generally move on smaller, much more eccentric orbits than do the planets circling the Sun.

31. Orbits of 60 of the known extrasolar planets
Orbits of 60 of the known extrasolar planets. Note that some of them are very close to their star:
Figure 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!

32. Planets orbiting within 0
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.

33. 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.

34. The blue line is the same curve for Jupiter.
Recently, more Jupiter-like planets have been found; this one has almost the mass of Jupiter and an orbital period of 9.1 years.
The blue line is the same curve for Jupiter.
Figure Jupiter-like Planet? Velocity “wobbles” in the star HD 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 0.95-Jupiter-mass planet orbits at a distance of 4.2 AU with an orbital eccentricity of As in Figure 15.10, the blue line marks the corresponding plot for Jupiter itself.

35. Current theories include the possibility that Jupiter-like planets could migrate inward, through friction with the solar nebula
Figure 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.

36. 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.

37. This figure shows the size of the habitable zone – where there is a possibility of liquid water being present – as a function of the mass of the parent star.
Figure Habitable Zones Every star is surrounded by a habitable zone, within which the surface temperature on a terrestrial planet would lie in the range 273–373 K (0–100° C)—that is, the planet could have liquid water on its surface. The habitable zone is small for cool, faint stars and much larger for hotter, bright ones. Earth-like planets orbiting within the habitable zone of their parent stars are thought to be the best candidates for finding life as we know it beyond the solar system.

38. Astronomers have detected most extrasolar planets by observing
the “wobble” of their parent stars using spectroscopy.
starlight reflected by their surfaces.
eclipses when the planets block the light of their parent stars.
the planets’ changing phases as they orbit their stars.

39. Beyond our own solar system, the planets found to date have tended to be
Kuiper belt objects, far from the glare of their suns.
large jovians far from stars like our Sun.
large jovians with orbits more like terrestrial planets.
terrestrials very close to their star, and transiting its disk.
imaginary, with no present proof that they really exist.

40. Extrasolar planets the size of Earth have NOT been seen yet with current techniques because
small planets probably don’t exist.
the large planets nearby have swept them up.
Earth-like planets take time to form.
large planets orbiting near to their stars are more easily detected.
small planets can only be seen if they cross in front of their star.

41. 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.
Jovian planets may have formed directly from instabilities in the cloud.
Asteroids never condensed into a larger object.

42. 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 450 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.