Chapter 11 Answers

Imagine that each of the five stars is orbited by a terrestrial planet at a distance of 1 AU (Earth's distance from the Sun). Rank the stars based on the planet's expected surface temperature (not including any greenhouse effect), from lowest to highest.

(Lowest Temperature) Barnard's Star (M4) 61 Cygnia A (K5) Alpha Centauri A (G2) Sirius (A1) Spica (B1) (Highest Temperature) (For a planet at any particular distance, such as 1 AU, a more luminous star means more energy (per unit area) reaching the planet's surface—and more energy will tend to make the planet hotter. For example, if the Sun were more luminous, Earth would be hotter. That is why the rankings again go in order of increasing luminosity, which for main-sequence stars means increasing mass.)

Listed below are the names, spectral types (in parentheses), and approximate masses of several nearby main-sequence stars. Rank the stars based on the distances to their habitable zones (from the central star), from shortest to longest.

(Smallest Distance) Barnard's Star (M4) 61 Cygnia A (K5) Alpha Centauri A (G2) Sirius (A1) Spica (B1) (Largest Distance) (Notice that the ranking is in order of mass, because lower mass main-sequence stars are less luminous than higher mass stars. Therefore a planet would have to be located closer to a lower mass star than to a higher mass star in order to have a temperature warm enough for liquid water to exist.)

Consider again the same set of five stars. This time, rank the stars based on the size (width) of their habitable zones, from smallest to largest.

(Smallest) Barnard's Star (M4) 61 Cygnia A (K5) Alpha Centauri A (G2) Sirius (A1) Spica (B1) (Largest) (Notice that the ranking is the same as that from Part A, because lower mass main-sequence stars not only have their habitable zones located closer to them but they also have the narrowest habitable zones. Higher mass stars have wider habitable zones because they have a greater range of distances in which a planet could potentially have liquid water on its surface.)

Astrometric Method

-Measures precise changes in a star's position in the sky, in fractions of arcseconds. (could detect a planet in an orbit face-on to the earth)

Transit Method

-Planet-detection strategy of NASA's Kepler Mission -Can Potentially detect planets in only a few percent of all planetary systems -Allows for the extrasolar planet's radius to be determined -this method was first to identify earth-sized extrasolar planets -looks for very slight, periodic dimming of a star. 1%

Doppler Method

-Used for most of the first 200 extrasolar planet detections -Currently best-suited to find Jupiter-sized extrasolar planets orbiting close to their stars.

What do we mean by a "hot Jupiter"? A. A massive planet that orbits very close to its star B. A planet so large that it actually puts out heat from fusion in its core C. A brown dwarf D. A planet similar to Jupiter that orbits an unusually massive star

A. A massive planet that orbits very close to its star (The planet is hot because it is so close to the star.)

Why does the Doppler method generally give a planet's "minimum mass" rather than an exact mass? A. The size of the Doppler shift that we detect depends on the tilt of a planet's orbit. B. The size of the Doppler shift that we detect depends on knowing the star's mass, which can be uncertain. C. Extrasolar planets are always increasing in mass. D. Doppler measurements are difficult, producing noisy data that often cause astronomers to underestimate a planet's mass.

A. The size of the Doppler shift that we detect depends on the tilt of a planet's orbit. (We get a precise mass from the Doppler method only if the planet's orbit is edge-on to us; we'll know if it is edge on because in that case we will see transits by the planet.)

What is the common property that distinguishes stars on the main sequence of the H-R diagram from other stars on the diagram? A. They are fusing hydrogen in their central cores. B. They are all quite young. C. They are all made mostly of hydrogen. D. They all have a mass approximately the same as that of our Sun.

A. They are fusing hydrogen in their central cores.

A giant or supergiant star is ________. A. a star that is near the end of its life B. a young star that is still forming C. an unusually massive star D. a star that has exhausted all possible forms of fuel for fusion

A. a star that is near the end of its life (Stars become giants or supergiants only after they have exhausted all their central core fuel for hydrogen fusion.)

The Hertzsprung-Russell (H-R) diagram is a way of ________. A. classifying stars according to their surface temperatures and luminosities B. classifying planets according to their sizes and the distances at which they orbit their stars C. classifying stars according to their masses and ages D. classifying planets according to their orbital periods and the distances at which they orbit their stars

A. classifying stars according to their surface temperatures and luminosities (The horizontal axis is surface temperature and the vertical axis is luminosity.)

Consider a star that is less massive than the Sun. For a planet to receive the same intensity of light from this star as Earth receives from the Sun, the planet would have to orbit ________. A. closer to the star than Earth is to the Sun B. faster than Earth orbits the Suns C. lower than Earth orbits the Sun D. farther from the star than Earth is to the Sun

A. closer to the star than Earth is to the Sun (Less massive stars are less luminous, so a planet would have to be closer to the star to get the same intensity of light.)

Recall that stars are classified with types from the spectra sequence OBAFGKM. In general, an ordinary (hydrogen-fusing, or main-sequence) star of spectral type A is ________ than a star of spectral type K. A. hotter, more luminous, and more massive B. cooler, less luminous, and less massive C. cooler, more luminous, and more massive D. hotter, more luminous, and less massive

A. hotter, more luminous, and more massive (As shown in the table in your textbook, surface temperature, luminosity, and mass all tend to decrease for ordinary (hydrogen-fusing) stars along the spectral sequence from O to M.)

In essence, the Kepler mission searched for extrasolar planets by ____________. A. monitoring stars for slight dimming that might occur as unseen planets pass in front of them B. obtaining high-resolution photographs of other star systems C. looking for slight back and forth shifts in a star's position in our sky D. observing Doppler shifts in a star's spectrum caused by an unseen planet

A. monitoring stars for slight dimming that might occur as unseen planets pass in front of them (This transit method can reveal planets as small as Earth (or smaller), but only if they happen to orbit their star in the plane of our line-of-sight.)

According to the "rare Earth hypothesis," ________. A. planets with intelligent life should be extremely rare B. Sun-like stars are extremely rare, and for that reason Earth-like planets will also be rare C. Earth-like planets are usually ejected from their star systems into interstellar space D. planets the size of Earth should be extremely rare

A. planets with intelligent life should be extremely rare (The basic idea is that a variety of special circumstances have combined to make our existence here a very low-probability event.)

In the movie Star Wars, the planet Tatooine is shown with two suns that rise and set together in its sky. According to present understanding, this is ________. A. possible, if the planet orbits around the two stars of a close binary B. not possible C. possible, if the planet orbits halfway between two stars with nearly equal masses. D. possible, if the planet orbits in a binary system in which one star is many times more massive than the Sun and the other is much less massive than the Sun

A. possible, if the planet orbits around the two stars of a close binary (Stable orbits are possible for planets around a close binary star system.)

The first confirmed detections of extrasolar planets occurred in ____________. A. the 1990s B. the mid-17th century C. the mid-20th century D. 2009

A. the 1990s (The first detection around a Sun-like star was a planet orbiting the star 51 Pegasi in 1995.)

This graph plots planetary mass on the horizontal axis and planetary radius on the vertical axis. Notice the four locations marked by the bold numbers 1 through 4. Which location represents a planet with the greatest density? A. 1 B. 2 C. 3 D. 4

B. 2

Combined, all stars more massive than the Sun (meaning spectral types O, B, A, and F) represent about ________ of all stars. A. 97% B. 3% C. 50% D. 20%

B. 3% (Therefore, even though the short lifetimes of these stars make them less likely candidates for life (and especially for complex life), they hardly affect the overall prospects of finding life among the stars.)

What do we mean by a "super Earth"? A. A planet with an Earth-like orbit that has more abundant life than Earth. B. A planet made of metal and rock that is larger in mass than Earth. C. A planet that is more massive than Earth and also has surface oceans. D. A planet similar in size to Earth but orbiting close to its star.

B. A planet made of metal and rock that is larger in mass than Earth. (In other words, a "super Earth" is "super" only in being more massive than Earth while sharing a similar rock and metal composition.)

Suppose you are using the Doppler method to look for planets around another star. What must you do? A. Carefully examine a single spectrum of the star. B. Compare many spectra of the star taken over a period of many months or years. C. Carefully examine a single spectrum of an orbiting planet. D. Compare the brightness of the star over a period of many months or years. E. Compare many spectra of an orbiting planet taken over a period of many months or years.

B. Compare many spectra of the star taken over a period of many months or years. (We look for Doppler shifts in the star's spectrum that, over time, shift back and forth to indicate the influence of an orbiting planet.)

What role has Jupiter played in Earth's habitability? A. Jupiter's extended magnetosphere has protected Earth from radiation from distant supernovae. B. Jupiter's gravitational effects helped clear the inner solar system of objects that could cause impacts. C. Jupiter helps stabilize Earth's orbit. D. Jupiter helps stabilize Earth's axis.

B. Jupiter's gravitational effects helped clear the inner solar system of objects that could cause impacts. (We therefore expect that the impact rate would be higher without a Jupiter-like planet, though the implications of this fact to life are still debated)

In Part A you found that the terrestrial planet 10 AU from a 0.5MSun0.5�Sun star is unlikely to be habitable. Could this planet be habitable if it were in a different orbit around its star? A. Yes, if it were 1 AU from its star. B. Yes, but it would have to be less than 0.5 AU from its star. C. Yes, if it had an eccentric orbit that sometimes brought it within 0.01 AU of its star. D. No, because its star is too small to have a habitable planet.

B. Yes, but it would have to be less than 0.5 AU from its star. (Remember that a star half the mass of the Sun is much less than half as luminous as the Sun, so the star's habitable zone would be located much less than 1 AU from the star.)

Which of the following might once have been orbited by an Earth-like planet? A. A supernova B. A black hole C. A white dwarf D. A neutron star

C. A white dwarf (Stars like the Sun end up as white dwarfs after they die, and hence could have had Earth-like planets.)

Which of the following does not vary much among ordinary, hydrogen-fusing (main-sequence) stars? (That is, which does not change as you go along the spectral sequence OBAFGKM?) A. Surface temperature B. Luminosity C. Chemical composition D. Mass

C. Chemical composition (All stars are born with the same basic composition: roughly three-quarters hydrogen and one-quarter helium, with no more than about 2% of other elements.)

Which of the following will allow you to learn something about a transiting planet's atmospheric composition? A. Look for slight variations in the time between transits. B. Use the Doppler method to study the planet throughout a cycle from one transit to the next. C. Compare spectra obtained before and during an eclipse. D. Calculate the planet's size, and then use size to infer what its atmospheric composition must be.

C. Compare spectra obtained before and during an eclipse. (The spectrum before the eclipse is that of the star and planet combined. The spectrum during eclipse, when the planet is behind its star, is that of the star alone. The difference between the two spectra therefore represents the planet's spectrum, from which we learn about atmospheric composition.)

From the viewpoint of an alien astronomer, how does Jupiter affect observations of our Sun? A. It causes the Sun to move in a small ellipse in the sky, with the same ellipse repeated every night. B. It makes the Sun appear dimmer when viewed with infrared light. C. It causes the Sun to move in a small ellipse with an orbital period of about 12 years. D. It makes the Sun periodically get somewhat brighter.

C. It causes the Sun to move in a small ellipse with an orbital period of about 12 years. (Jupiter exerts a gravitational tug on the Sun, causing the Sun to move around their mutual center of mass with the same orbital period as Jupiter.)

Based on discoveries to date, which of the following conclusions is justified? A. Planets are common, but planets as small as Earth are extremely rare. B. Although planetary systems are common, few resemble ours with terrestrial planets near the Sun and jovian planets far from the Sun. C. Planetary systems are common and planets similar in size to Earth are also common. D. Most stars have one or more terrestrial planets orbiting within their habitable zones.

C. Planetary systems are common and planets similar in size to Earth are also common. (The statistics of extrasolar planets known to date strongly support these conclusions.)

Overall, what do current data suggest about planetary types in other planetary systems? A. All planets fall into the same terrestrial and jovian categories as the planets in our solar system. B. Jovian planets are common, whereas terrestrial planets are rare. C. Planets come in a wider range of types than the planets in our solar system. D. Planets that fall into either the terrestrial or jovian categories are extremely rare outside of our own solar system.

C. Planets come in a wider range of types than the planets in our solar system. (For example, there may be other planets that are Jupiter-like in size but much higher or lower in density, or planets that are Earth-like in size but made mostly of water.)

What do the astrometric, Doppler, and transit methods share in common? A. They all allow us to measure a planet's size (radius). B. They all require very high-resolution imaging. C. They all search for planets by measuring properties of a star rather than of the planets themselves. D. They all require high-resolution spectroscopy. E. All of the above.

C. They all search for planets by measuring properties of a star rather than of the planets themselves. (This is why we say they are indirect methods, as opposed to direct methods that seek to acquire images or spectra of planets themselves.)

In general, which type of planet would you expect to cause the largest Doppler shift in the spectrum of its star? A. a low-mass planet that is far from its star B. a low-mass planet that is close to its star C. a massive planet that is close to its star D. a massive planet that is far from its star

C. a massive planet that is close to its star (Doppler shifts measure velocity, and the star will move faster if it experiences a greater gravitational tug from its planet. A massive, close-in planet will cause the strongest gravitational tug.)

Based on current data, about what fraction of stars have one or more planets? A. approximately 20% B. approximately 1% C. at least about 70% D. no more than 30%.

C. at least about 70% (There is no longer any real doubt that planetary systems are common.)

We are not yet capable of detecting life on planets around other stars. But as our technology develops, our first real chance of detecting such life will probably come from _________. A. examining high-resolution images of the planets made by orbiting telescopes B. sending spacecraft to study the planets up close C. examining spectral lines from the atmospheres of distant planets D. determining the orbital properties of the planets

C. examining spectral lines from the atmospheres of distant planets (Certain spectral signatures may lead us to conclude that life is present.)

Suppose a planet is discovered by the Doppler method and is then discovered to have transits. In that case, we can determine all the following about the planet except ______________. A. its physical size (radius) B. its density C. its rotation period D. its precise mass E. its orbital period

C. its rotation period (We do not get any direct information about rotation period. (However, if the planet is close to its star, we can sometimes conclude that it has synchronous rotation, in which case the rotation period and orbital period are the same.))

The astrometric method looks for planets with careful measurements of a star's _________. A. Brightness B. velocity toward or away from us C. position in the sky D. all of the above

C. position in the sky

Stars of spectral type A and F are considered ________. A. reasonably likely to have Earth-like planets with complex plant- and animal-like life B. unlikely to have planets of any kind C. reasonably likely to have habitable planets but much less likely to have planets with complex plant- or animal-like life D. unlikely to have habitable planets

C. reasonably likely to have habitable planets but much less likely to have planets with complex plant- or animal-like life (These stars live only 1 to 2 billion years, which is plenty long enough for habitable planets to form and life to take hold, but based on the example of Earth, they are much less likely to be long enough for complex plant and animal life to evolve.)

Consider a star orbiting the center of mass of its star system. We can tell when the star is on the side of its orbit in which it is coming toward us, because during that time ________. A. the star is slightly brighter than its average for the entire orbit B. the star's spectral lines will be shifted to slightly longer wavelengths than their average wavelengths for the entire orbit C. the star's spectral lines are shifted to slightly shorter wavelengths than their average wavelengths for the entire orbit D. the star is slightly dimmer than its average for the entire orbit

C. the star's spectral lines are shifted to slightly shorter wavelengths than their average wavelengths for the entire orbit (This is an example of the Doppler shift; objects coming toward us have blue shifts (shorter wavelengths), whereas objects moving away from us have red shifts (longer wavelengths).)

In the context of astrobiology, what is an "orphan planet"? A. A planet with no moons, such as Mercury or Venus. B. A planet that has microbial life but no larger organisms. C. A planet orbiting a star that is not a member of a galaxy. D. A planet-size object that does not orbit a star.

D. A planet-size object that does not orbit a star. (It may either have been ejected from its star system or, perhaps, formed independently from the collapse of a gas cloud in interstellar space.)

Why is it so difficult to take pictures (direct images) of extrasolar planets? A. No telescope is powerful enough to detect the faint light from a distant planet.B. Telescopes are too busy with other projects. C. Extrasolar planets give off light at different wavelengths than planets in our solar system. D. The light of the planets is overwhelmed by the light from their star.

D. The light of the planets is overwhelmed by the light from their star. (A Sun-like star is about a billion times brighter than the light from a Jupiter-size planet orbiting it.)

Some scientists speculate that super-Earths and water worlds might be habitable at greater distances from their stars than Earth-size planets. Why? A. Their larger sizes mean they might have more of the ingredients needed for life. B. These planets may retain more internal heat than Earth, which might drive greater geological activity. C. For the water worlds, at least, they are certain to have oceans no matter how far they are from a star. D. Their larger sizes might allow them to retain hydrogen atmospheres.

D. Their larger sizes might allow them to retain hydrogen atmospheres. (Hydrogen can act as a greenhouse gas, and therefore might keep the planet's surface warm enough for life even at relatively large distances from a star.)

Which statement explains the observations that make it seem possible that Mars could have life underground? A. Mars is located within the Sun's habitable zone. B. We have found surface liquid water on Mars, so it should also have water underground. C. We have detected subsurface wells of liquid water in equatorial regions of Mars. D. We have detected water ice on Mars, and Mars still has some volcanic heat.

D. We have detected water ice on Mars, and Mars still has some volcanic heat. (The combination of water ice and volcanic heat suggests that there could be liquid water underground.)

The transit method allows us in principle to find planets around _________. A. only stars of about the same mass and size as our Sun B. only stars located within about 100 light-years of Earth C. all stars that have planets of any kind D. only a small fraction of stars that have planets

D. only a small fraction of stars that have planets (We can see transits only if the planetary orbits are nearly precisely edge-on as viewed from Earth, which means that most planetary systems cannot be detected through transits. The Kepler mission overcame this limitation by studying a large number of stars (about 150,000).)

More massive stars live ________. A. only near the center of the galaxy B. longer lives C. only near the outskirts of the galaxy D. shorter lives

D. shorter lives (Massive stars are so much more luminous than lower mass stars that they use up all their fuel for fusion in a much shorter time.)

Stars of spectral type O and B are unlikely to have planets with life because ________. A. they do not have enough rocky and metal material to make terrestrial planets B. their intense ultraviolet radiation would sterilize planets C. their high luminosities would prevent the formation of terrestrial planets D. they don't live long enough for planets to form

D. they don't live long enough for planets to form (Planets probably cannot form fully in less than a few tens of millions of years, and most O and B stars do not live this long.)

Discovering planets through the _______ requires obtaining and studying many spectra of the same star.

Doppler method

The _______ successfully discovered thousands of extrasolar planets with a spacecraft that searched for transits among some 100,000 stars.

Kepler mission

The items below describe worlds or selected localities on worlds. Based on our current scientific understanding, match these items to the appropriate category below.

Likely to be habitable- underground on Mars- subsurface ocean on Europa- moon with atmosphere orbiting jovian planet 1 AU from 1 Msun star Unlikely- surface of Mars- surface of terrestrial planet 10 AU from 0.5 Msun star- volcanoes on Io

The diagrams below each show the motion of a distant star relative to Earth (not to scale). The red arrows indicate the speed and direction of the star's motion: Longer arrows mean faster speed. Rank the stars based on the Doppler shift that we would detect on Earth, from largest blueshift, through no shift, to largest redshift.

Show image

The ________ was used to find a Jupiter-sized planet through careful measurements of the changing position of a star in the sky.

astrometric method

Observations indicating that other planetary systems often have jovian planets orbiting close to their stars are best explained by what we call _______.

migration

Each diagram below shows a pair of spectra with a set of spectral lines. The top spectrum always shows the lines as they appear in a spectrum created in a laboratory on Earth ("Lab") and the bottom spectrum shows the same set of lines from a distant star. The left (blue/violet) end of each spectrum corresponds to shorter wavelengths and the right (red) end to longer wavelengths. Rank the five stars based on the Doppler shifts of their spectra, from largest blueshift, through no shift, to largest redshift.

see image

An extrasolar planet that is rocky and larger than Earth is often called a ______.

super-Earth.

The _________ is used to find extrasolar planets by carefully monitoring changes in a star's brightness with time.

transit method

Compared to the planets of our solar system, the composition of a _______ most resembles the compositions of Uranus and Neptune.

water world