422 PART 4^ | THE SOLAR SYSTEM
- Why do astronomers think the solar system formed about 4.6 billion
years ago? - If you visited another planetary system, would you be surprised to
fi nd planets older than Earth? Why or why not? - Why is almost every solid surface in our solar system scarred by
craters? - What is the difference between condensation and accretion?
- Why don’t Terrestrial planets have rings like the Jovian planets?
- How does the solar nebula theory help you understand the location of
asteroids? - How does the solar nebula theory explain the dramatic density
difference between the Terrestrial and Jovian planets? - What does the term differentiated mean when applied to a planet?
Would you expect to fi nd that planets are usually differentiated? Why? - What processes cleared the nebula away and ended planet building?
- What is the difference between the dense hot disks seen around some
stars and the low-density cold disks seen around some other stars? - What evidence can you cite that planets orbit other stars?
- How Do We Know? The evidence is overwhelming in support of the
idea that the Grand Canyon was dug over a span of millions of years
by the erosive power of the Colorado River and its tributary streams.
Is that a catastrophic theory or an evolutionary theory? - How Do We Know? Why must scientists be skeptical of new
discoveries even when they think that the discoveries are correct?
Discussion Questions
- If you visited some other planetary system in the act of building
planets, would you expect to see the condensation sequence at work,
or was it probably unique to our solar system? How do the properties
of the extrasolar planets discovered so far affect your answer? - In your opinion, do most planetary systems have asteroid belts?
Would all planetary systems show evidence of an age of heavy
bombardment? - If the solar nebula hypothesis is correct, then there are probably more
planets in the universe than stars. Do you agree? Why or why not? - The human race has intelligence and consequently has both the
ability and the responsibility to wonder about its origins. Do you
agree?
Problems
- If you observed the solar system from the nearest star (distance
1.3 parsecs), what would the maximum angular separation be between
Earth and the sun? (Hints: Use the small-angle formula, Chapter 3,
and see the defi nition of parallax on page 169.) - The brightest planet in our sky is Venus, which is sometimes as bright
as apparent magnitude –4 when it is at a distance of about 1 AU.
How many times fainter would it look from a distance of 1 parsec
(2.06 105 AU)? What would its apparent magnitude be? (Hints:
Remember the inverse square law from Chapter 9; also, review the
defi nition of magnitudes in Chapter 2.) - What is the smallest-diameter crater you can identify in the photo
of Mercury on page 402? (Hint: See Appendix A, “Properties of the
Planets,” to fi nd the diameter of Mercury in kilometers.) - A sample of a meteorite has been analyzed, and the result shows that
out of every 1000 nuclei of potassium-40 originally in the meteorite,
only 250 have not decayed. How old is the meteorite? (Hint: See
Figure 19-6.) - In Table 19-2, which object’s observed density differs least from its
uncompressed density? Why?
▶ (^) According to the condensation sequence (p. 408), the inner part of
the solar nebula was so hot that only metals and rocky materials could
form solid grains. The dense Terrestrial planets grew from those solid
particles and did not include many ices or low-density gases.
▶ (^) The outer solar nebula, beyond the ice line (p. 408), was cold
enough for ices as well as metals and rocky minerals to form solid
particles. The Jovian planets grew rapidly and incorporated large
amounts of low-density ices and gases.
▶ (^) Evidence that the condensation sequence was important in the
solar nebula can be found in the high densities of the Terrestrial
planets relative to the Jovian planets. Comparing the uncompressed
densities (p. 408) of the Terrestrial planets shows that the innermost
Terrestrial planets have the highest densities.
▶ (^) The Terrestrial planets may have formed slowly from the accretion of
planetesimals of similar composition. Later, radioactive decay plus
heat of formation (p. 410) melted each planet’s interior to cause
differentiation (p. 410) into layers of differing density. In that scenario,
Earth’s early atmosphere was probably supplied by a combination of
outgassing (p. 411) from Earth’s interior and planetesimal impacts.
▶ (^) Disks of gas and dust around protostars may not last long enough to
form Jovian planets by accretion and then by gravitational collapse.
Some models suggest the Jovian planets could have formed more
rapidly by direct collapse (p. 412), skipping the condensation and
accretion steps.
▶ (^) In addition to intense light from hot nearby stars and the gravita-
tional infl uence of passing stars, the solar nebula was eventually
cleared away by radiation pressure, the solar wind, and the sweeping
up or ejection of debris by the planets.
▶ (^) All of the old surfaces in the solar system were heavily cratered dur-
ing the heavy bombardment (p. 413) by debris that fi lled the solar
system when it was young.
▶ (^) Hot disks of gas and dust have been detected in early stages of star
formation and are believed to be the kind of disk in which planets
could form.
▶ (^) Cold dust disks, also known as debris disks (p. 415), appear to be
composed of dust released by collisions among comets, asteroids, and
Kuiper belt objects. Such disks may be signs that planets have already
formed in those systems.
▶ (^) Planets orbiting other stars, called extrasolar planets (p. 417), have
been detected by the way they tug their stars about, creating small
Doppler shifts in the stars’ spectra. Planets have also been detected in
transits (p. 418) as they cross in front of their star and partly block
the star’s light. A few planets have been detected when they orbited
behind their star and their infrared radiation was cut off. A few more
have been detected by gravitational microlensing (p. 418).
▶ (^) Nearly all extrasolar planets found so far are massive, Jovian worlds
orbiting close to their parent stars, called hot Jupiters (p. 419).
Lower-mass Terrestrial planets are harder to detect but may be
common.
Review Questions
- What produced the helium now present in the sun’s atmosphere? In
Jupiter’s atmosphere? In the sun’s core? - What produced the iron and heavier elements like gold and silver in
Earth’s core and crust? - What evidence can you cite that disks of gas and dust are common
around young stars? - According to the solar nebula theory, why is the sun’s equator nearly
in the plane of the planets’ orbits? - Why does the solar nebula theory predict that planetary systems are
common?