AST 105 HW #13 Solution
Week of November 16th and November 23rd, 2015
Note: All Problems are from The Cosmic Perspective (6ed)
Chapter 13
Review Problems
1. Why are extrasolar planets hard to detect directly?
 Extrasolar planets are difficult to detect directly because they are small when viewed from Earth,
many light-years away. In addition to their tiny sizes, the light from the stars near the planets is
much, much brighter than the planets themselves. As a result, the tiny, dim planets get lost in the
glare of the stars so that we have great difficulties seeing them.
2. What are the three major methods used to detect extrasolar planets indirectly?
 The three different techniques to detect extra solar planets are the astrometric techniques, the
Doppler technique, and the transient observation. The Doppler technique measures the shift in the
wavelength of light from stars. The presence of such shifts indicates orbital motion of stars which is
caused by the presence of extrasolar planets. Such motions can be also measured by astrometric
techniques. Transit technique is used to determine the changes in the brightness of the star when a
planet crosses the star. This change in the brightness is the outcome of gravitational lensing.
8. Have any extrasolar planets been detected directly? Explain
 Yes, in some cases, direct detection of extrasolar planets are possible. We have even seen few of
them in class. These are, however, rare, because the direct detection is hard due to the reasons laid
down in problem 1.
10. How do the orbits of known extrasolar planets differ from those of Jovian planets in our solar system?
Why are these orbits surprising?
 The orbits of many known extrasolar planets are much more eccentric and much nearer their stars
than the jovian planets of our solar system. This is surprising since our planet formation model
suggests that planets should have nearly circular orbits and that jovian planets, which require ice to
form, should form only farther out in the solar system.
15. Based on current data, does it seem likely that our solar system has a particularly unusual layout?
Explain
 With current technologies, we are only likely to detect massive planets in close-in orbits. Therefore,
with current data, our solar systems looks somewhat unique. However, star systems like ours might
be common.
Test Your Understanding
Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false).
Explain clearly; not all of these have definitive answers, so your explanation is more important than your
chosen answer.
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19. An extraterrestrial astronomy surveying our solar system with the Doppler technique could discover the
existence of Jupiter with just a few days of observation.
 This is unlikely. Unless her technology operates on fundamentally different principles, it's necessary
to observe one or more orbits before a planet's existence (and period) can be known.
20. The fact that we have not yet discovered an Earth-size extrasolar planet tells us that such planets must
be very rare.
 The statement is inconclusive. The reason why we have not discovered an Earth-size extra solar
planet might be that they are very difficult to discover. They are could be plenty of these planets out
there that we haven’t discovered yet.
22. Although hot Jupiters are unlikely places to find life, they could be orbited by moons that would have
pleasant, Earth-like temperatures.
 It is unlikely. First of all, hot Jupiters rarely have moons. Because these planets exist close to their
stars, there is a large gravitational pull from the sun. Therefore, potential moons are likely to get
pulled into the star, get ejected out of the star system, and crash to the planet.
 The tidal force from large planet is very strong. So even when we find a moon, it will be likely a
molten ball with a thin surface skin.
29. Astronomers announce that all the Doppler technique discoveries of extrasolar planets made to date are
actually more massive brown dwarfs, and we had thought they were less massive only because we didn’t
realize that they have nearly face-on orbits.
 This is unlikely. We can only deduce the minimum mass of a planet using the Doppler technique. The
minimum mass is equal to the true mass only and only if the planet have edge-on orbits. So all extra
solar planets we have discovered so far have face-on orbits, we can’t precisely determine their
masses.
Process of Science
43. Refining the Theory. Consider the following three hypothetical observations: (1) the discovery of a lone
planet that is small and dense like a terrestrial planet but has a Jupiter-like orbit; (2) the discovery of a
planetary system in which three terrestrial planets orbit the star beyond the orbital distance of two Jovian
planets; (3) the discovery that a majority of planetary systems have their Jovian planets located nearer to
their star than 1 AU and their terrestrial planets beyond 5 AU. Each of these observations would challenge
our current theory of solar system formation, but would any of them shake the very foundations of the
theory? Explain clearly for each of the three hypothetical observations.
 (1) It will not shake the very foundations of the nebular theory. It is possible that, in this star system,
the nebula was very hot even at the orbital distance of Jupiter. Given that assumption, the nebular
theory can reasonably explain the presence of such extra solar planet.
 (2) It is an interesting exception. But if it is just a single planetary system, the nebular theory can
reasonably explain its existence. One possible explanation goes like this. After the planets formed in
usual manner, due to the strong gravitational pull form the star, Jovian planets may move close to
the star. Terrestrial planets are less massive than Jovian planets, so they may experience a weaker
gravitational pull leaving them closer to where they initially formed. In this case, the order of Jovian
planets and terrestrial planets can get flipped.
 (3) If a majority of planetary systems have their Jovian planets located closer to their stars than
terrestrial planets, that would be a significant blow to the nebular theory. We must look for other
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theories than can explain the formation of our solar system and other planetary systems equally
well.
Investigate Further
48. Comparing Methods. What are the advantages and disadvantages of the Doppler and transit technique?
What kinds of planets are easiest to detect in each case? Are there certain planets that each method cannot
detect, even if the planets are very large? Explain. What advantages are gained if a planet can be detected
by both methods?
 The Doppler technique is best suited to detect massive planets that orbit relatively close to their
star. So, a relatively limited number of star systems can be studied by this technique. Also, to resolve
radial velocity, this technique often requires a large telescope. This technique is useful because,
beside indicating the existence of an extra solar planet, we can learn something about the mass of
the planet at the same time.
 The transit technique only works for the planetary systems whose orbits are oriented edge-on to
Earth. It also favor for the planets that are closer to the star and thus have a shorter period. It can
detect smaller planets than Doppler Effect and it work with both large and small telescopes. Lastly,
this technique can tell us about the size of the planet.
 If a planet can be discovered by both techniques, we can learn both the size and mass of the planet.
If we know both information, we can easily figure out the density of the planet, which is new
information.
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