Questions

Review Questions

  1. Describe three properties of the solar system that are thought to be a result of how the solar system formed.

  2. The graphite in your pencil is a form of carbon. Where were these carbon atoms formed?

  3. What is the interstellar medium? How does it become enriched over time with heavy elements?

  4. What is the evidence that other stars existed before our Sun was formed?

  5. Why are terrestrial planets smaller than Jovian planets?

  6. How do radioactive elements make it possible to determine the age of the solar system? What are the oldest objects that have been found in the solar system?

  7. Consider a 4.54 × 109 year old meteorite. Are the atoms in that meteorite—which were created in stars—4.54 × 109 years old? Explain your reasoning.

  8. What is the tidal hypothesis? What aspect of the solar system was it designed to explain? Why was this hypothesis rejected?

  9. The half-life for uranium-238 in Box 8-1 is 4.5 billion years. There is plenty of uranium-238 in Earth, and much of Earth’s heat comes from the radioactive decay of this isotope. Compared to today, about how much more uranium-238 was there when Earth formed? (Hint: See Box 8-1.)

  10. What is the nebular hypothesis? Why is this hypothesis accepted?

  11. What was the protosun? What caused it to shine? Into what did it evolve?

  12. Why is it thought that a disk appeared in the solar nebula?

  13. What are protoplanetary disks? What do they tell us about the plausibility of our model of the solar system’s origin?

  14. What is meant by a substance’s condensation temperature? What role did condensation temperatures play in the formation of the planets?

  15. At distances within the snow line, what is the state of water (solid, liquid, or gas)? How does this affect the formation of terrestrial planets?

  16. Why are terrestrial planets smaller than Jovian planets?

  17. What is a planetesimal? How did planetesimals give rise to the terrestrial planets?

  18. (a) What is meant by accretion? (b) Why are the terrestrial planets denser at their centers than at their surfaces?

  19. If hydrogen and helium account for 98% of the mass of all the atoms in the universe, why aren’t Earth and the Moon composed primarily of these two gases?

  20. Why did the terrestrial planets form close to the Sun while the Jovian planets formed far from the Sun?

  21. How did the Jovian planets form?

  22. Explain how our current understanding of the formation of the solar system can account for the following characteristics of the solar system: (a) All planetary orbits lie in nearly the same plane. (b) All planetary orbits are nearly circular. (c) The planets orbit the Sun in the same direction in which the Sun itself rotates.

  23. In the Grand Tack model, why is Mars much smaller than Earth or Venus?

  24. How does the Grand tack model help us understand the composition of the asteroid belt?

  25. Why do we think that Neptune must have formed closer to the Sun, and later migrated outward to its present position?

  26. In the Nice model of the Jovian planets, which planet migrates more, Jupiter or Neptune?

  27. In the Nice model, how does the Oort cloud and the Kuiper belt form?

  28. What is the Late Heavy Bombardment? How does the Nice model explain this event?

  29. Explain why most of the satellites of Jupiter orbit that planet in the same direction that Jupiter rotates.

  30. What is the radial velocity method used to detect planets orbiting other stars? Why is it difficult to use this method to detect planets like Earth?

  31. What type of planets and orbits are easiest to detect in the radial velocity method? What are “hot Jupiters”? Explain your answer.

  32. Summarize the differences between the planets of our solar system and those found orbiting other stars.

  33. Is there evidence that planets have fallen into their parent stars? Explain your answer.

  34. What does it mean for a planet to transit a star? What can we learn from such events?

  35. What combination of methods are required to determine the average density of an exoplanet? Explain your reasoning. (Hint: The average density is the planet’s mass divided by its volume.)

  36. What is so special about the habitable zone? Is every planet in this zone like Earth?

  37. What is microlensing? How does it enable astronomers to discover extrasolar planets?

  38. A 1999 news story about the discovery of three planets orbiting the star Upsilon Andromedae (“Ups And” is near the top of stars listed in Figure 8-18) stated that “the newly discovered galaxy, with three large planets orbiting a star known as Upsilon Andromedae, is 44 light-years away from Earth.” What is wrong with this statement?

Advanced Questions

Questions preceded by an asterisk (*) involve topics discussed in Box 4-4 or Box 8-1.

Problem-solving tips and tools

The volume of a disk of radius r and thickness t is πr2t. Box 1-1 explains the relationship between the angular size of an object and its actual size. An object moving at speed v for a time t travels a distance d = vt; Appendix 6 includes conversion factors between different units of length and time. To calculate the mass of 70 Virginis or the orbital period of the planet around 2M1207, review Box 4-4. Section 5-4 describes the properties of blackbody radiation.

  1. Figure 8-4 shows that carbon, nitrogen, and oxygen are among the most abundant elements (after hydrogen and helium). In our solar system, the atoms of these elements are found primarily in the molecules CH4 (methane), NH3 (ammonia), and H2O (water). Explain why you suppose this is.

  2. (a) If Earth had retained hydrogen and helium in the same proportion to the heavier elements that exist elsewhere in the universe, what would its mass be? Give your answer as a multiple of Earth’s actual mass. Explain your reasoning. (b) How does your answer to (a) compare with the mass of Jupiter, which is 318 Earth masses? (c) Based on your answer to (b), would you expect Jupiter’s rocky core to be larger, smaller, or the same size as Earth? Explain your reasoning.

  3. *If you start with 0.80 kg of radioactive potassium (40K), how much will remain after 1.3 billion years? After 2.6 billion years? After 3.9 billion years? How long would you have to wait until there was no 40K remaining?

  4. *Three-quarters of the radioactive potassium (40K) originally contained in a certain volcanic rock has decayed into argon (40Ar). How long ago did this rock form?

  5. Suppose you were to use the Hubble Space Telescope to monitor one of the protoplanetary disks shown in Figure 8-8b. Over the course of 10 years, would you expect to see planets forming within the disk? Why or why not?

  6. The protoplanetary disk at the upper right of Figure 8-8b is seen edge-on. The diameter of the disk is about 700 AU. (a) Make measurements on this image to determine the thickness of the disk in AU. (b) Explain why the disk will continue to flatten as time goes by.

  7. The accompanying infrared image shows IRAS 04302+2247, a young star that is still surrounded by a disk of gas and dust. The scale bar at the lower left of the image shows that at the distance of IRAS 04302+2247, an angular size of 2 arcseconds corresponds to a linear size of 280 AU. Use this information to find the distance to IRAS 04302+2247.

    R I V U X G
    (Courtesy of D. Padgett and W. Brandner, IPAC/Caltech; K. Stapelfeldt, JPL; and NASA)

  8. The image accompanying Question 45 shows a dark, opaque disk of material surrounding the young star IRAS 04302+2247. The disk is edge-on to our line of sight, so it appears as a dark band running vertically across this image. The material to the left and right of this band is still falling onto the disk. (a) Make measurements on this image to determine the diameter of the disk in AU. Use the scale bar at the lower left of this image. (b) If the thickness of the disk is 50 AU, find its volume in cubic meters. (c) The total mass of the disk is perhaps 2 × 1028 kg (0.01 of the mass of the Sun). How many atoms are in the disk? Assume that the disk is all hydrogen. A single hydrogen atom has a mass of 1.673 × 10−27 kg. (d) Find the number of atoms per cubic meter in the disk. Is the disk material thick or thin compared to the air that you breathe, which contains about 5.4 × 1025 atoms per cubic meter?

  9. *The planet discovered orbiting the star 70 Virginis (70Vir is near the middle of the stars listed in Figure 8-18), 59 light-years from Earth, moves in an orbit with semimajor axis 0.48 AU and eccentricity 0.40. The period of the orbit is 116.7 days. Find the mass of 70 Virginis. Compare your answer with the mass of the Sun. (Hint: The planet has far less mass than the star.)

  10. *Because of the presence of Jupiter, the Sun moves in a small orbit of radius 742,000 km with a period of 11.86 years. (a) Calculate the Sun’s orbital speed in meters per second. (b) An astronomer on a hypothetical planet orbiting the star Vega, 25 light-years from the Sun, wants to use the astrometric method to search for planets orbiting the Sun. What would be the angular diameter of the Sun’s orbit as seen by this alien astronomer? Would the Sun’s motion be discernible if the alien astronomer could measure positions to an accuracy of 0.001 arcsec? (c) Repeat part (b), but now let the astronomer be located on a hypothetical planet in the Pleiades star cluster, 360 light-years from the Sun. Would the Sun’s motion be discernible to this astronomer?

  11. (a) Figure 8-19c shows how astronomers determine that the planet of HD 209458 has a surface temperature of 1130 K. Treating the planet as a blackbody, calculate the wavelength at which it emits most strongly. (b) The star HD 209458 itself has a surface temperature of 6030 K. Calculate its wavelength of maximum emission, assuming it to be a blackbody. (c) If a high-resolution telescope were to be used in an attempt to record an image of the planet orbiting HD 209458, would it be better for the telescope to use visible or infrared light? Explain your reasoning.

  12. *(a) The star 2M1207 shown in Figure 8-15b is 170 light-years from Earth. Find the angular distance between this star and its planet as seen from Earth. Express your answer in arcseconds. (b) The mass of 2M1207 is 0.025 that of the Sun; the mass of the planet is very much smaller. Calculate the orbital period of the planet, assuming that the distance between the star and planet shown in Figure 8-15b is the semimajor axis of the orbit. Is it possible that an astronomer could observe a complete orbit in one lifetime?

Discussion Questions

  1. Propose an explanation why the Jovian planets are orbited by terrestrial-like satellites.

  2. Suppose that a planetary system is now forming around some protostar in the sky. In what ways might this planetary system turn out to be similar to or different from our own solar system? Explain your reasoning.

  3. Suppose astronomers discovered a planetary system in which the planets orbit a star along randomly inclined orbits. How might a theory for the formation of that planetary system differ from that for our own?

Web/eBook Question

  1. Search the World Wide Web for information about recent observations of protoplanetary disks. What insights have astronomers gained from these observations? Is there any evidence that planets have formed within these disks?

  2. In 2000, extrasolar planets with masses comparable to that of Saturn were first detected around the stars HD 16141 (also called 79 Ceti) and HD 46375. Search the World Wide Web for information about these “lightweight” planets. Do these planets move around their stars in the same kind of orbit as Saturn follows around the Sun? Why do you suppose this is? How does the discovery of these planets reinforce the model of planet formation described in this chapter?

  3. In 2006 a planet called XO-1b was discovered using the transit method. Search the World Wide Web for information about this planet and how it was discovered. What unusual kind of telescope was used to make this discovery? Have other extrasolar planets been discovered using the same kind of telescope?