Astonomy Exam 3

Q3.1 From your textbook's chapter 21.3, explain why it is easier to search for planetary systems when the protoplanetary disk contains a lot of dust and gas before it gathers into planets instead of when planets have formed from all of the dust and gas in the disk.

It's easier to search because we can detect radiation from all the spread-out individual dust particles. Spread-out particles reflect lots of light.

Q3.2 From your textbook's chapter 21.4, describe two reasons why the majority of exoplanets found via the Doppler wobble technique are "hot Jupiters".

They are big and close to their star, so they pull on the star strongly and complete orbits quickly, causing faster and more noticeable Doppler shifts. (ai)

Q3.3 From your textbook's chapter 21.4, explain how we deduce the approximate size (or radius) of transiting exoplanets. Also, how do we deduce the density of these exoplanets in order to determine whether they are rocky or gaseous?

We find the size by checking the depth of the light curve. If the planet blocks more light, it's larger. By looking at Doppler wobbles we can see mass—if the Sun wobbles more, it's bigger.

Q3.4 From your textbook chapter 21.4, explain why it is easier to use the infrared part of the spectrum (instead of the visible part) in order to directly image planets orbiting around other stars.

Infrared light shows planets better because the star gives off less infrared compared to visible light, and planets emit more infrared from heat, making them stand out. (ai)

Q3.5 From your textbook chapter 21.5, figure 21.23 is a bar chart showing the percentage of each category of planet size detected by the Kepler mission. Figure 21.24 is a bar chart showing the ACTUAL percentage of each category of planet size in the galaxy. Explain why these two bar charts are different (in other words, why are there more small-sized planets out there compared to what Kepler found).

There are more small planets because we have to correct for the fact that smaller planets are harder to detect. Kepler is able to find 90% of planets 2-3x larger, but only 20-30% of Earth-sized planets. There are 3-4x more than what Kepler found.

Q3.6 From your textbook chapter 21.5, in figure 21.25, there is a curve that implies that as the mass (horizontal axis) of a mostly Hydrogen planet increases, the size (vertical axis) also increases. But that relationship changes once the planet's mass is about 1000 times Earth's mass. At that point, the more mass you add to the planet, the smaller it gets. Explain why this happens.

As mass increases, gravity compresses the gas more tightly. After a certain point, adding more mass squeezes the planet's material, making it smaller. (ai)

Q3.7 From your textbook chapter 21.5, we find that gas giant planet sizes for hot Jupiters (planets that are mostly Hydrogen) are much larger than predicted for our most basic assumptions about mass and planet size. We expect as we begin to find more "cold Jupiters" in more distant orbits, their sizes will be more "normal." Describe the two effects that cause "hot Jupiters" to be much larger than expected.

One effect is that they have another source of energy. The star raises tides that circularize the orbits. They stretch and heat up. They are close to the star and receive a lot of radiated energy.

Q3.8 From your textbook chapter 21.6, explain how astronomers think hot Jupiters form differently from the way we think our planet formed.

They likely formed far from their star where gas could condense, then moved inward due to gravity or interactions in the disk. (ai)

Q3.9 From your textbook chapter 21.6, why was it surprising to find a number of rocky worlds in orbit around the star Kepler-444?

Kepler 444 is 11 billion years old. We didn't think there were heavy elements in the universe at that time to form rocky planets.

Q3.10 From your textbook chapter 30.1, explain what is the Copernican Principle, and provide at least two historical examples that have motivated scientists to believe this is a valid idea.

The principle is the idea that nothing is special about our place in the universe. An example is that the Earth is not the center of our system, but the Sun is. Another is that we aren't the only solar system.

Q3.11 From your textbook's Chapter 30.2, describe what was discovered in the Miller-Urey experiments.

In the Miller-Urey experiment, they found that maybe life formed on Earth through natural conditions. Many building blocks for life occur naturally. They simulated early Earth's atmosphere, and amino acids formed.

Q3.12 From your textbook's Chapter 30.2, explain what is the definition of an extremophile organism, and explain why their existence on Earth makes us more optimistic that life in some form may exist elsewhere in the Universe.

Extremophiles are organisms that live in extreme conditions like high heat, cold, or acid. Their existence shows that life can survive in environments once thought impossible, increasing the chances of life elsewhere. (ai)

Q3.13 From your textbook's Chapter 30.3, explain how we know (thanks to observations made by NASA's Cassini mission) that Saturn's tiny moon Enceladus likely has a sub-surface ocean like Europa.

The evidence is the river beds which indicate liquid water.

Q3.14 From your textbook's Chapter 30.3, suppose we find traces of Oxygen in the atmosphere of a planet orbiting a dim, red M-dwarf star. Would it be correct to conclude that life likely exists there? Explain why or why not.

It would be incorrect to assume that, because there are many non-biological ways to have oxygen.

Q3.15 From your textbook's Chapter 30.4, explain why Astronomers believe that if aliens do exist, our first contact is more likely to come through some sort of distant communication as opposed to an in-person visit by a UFO.

The distance between stars is so vast that traveling physically would take far too long, so communication through radio or light signals is much more likely. (ai)

Q3.16 From the Scientific American article "A Planet is Born," explain why some stars appear brighter in the infrared part of the spectrum than we expected.

Dust absorbs visible light but radiates heat as infrared, making the star appear brighter in that part of the spectrum. (ai)

Q3.17 From the Scientific American article "A Planet is Born," we learn that there are, broadly speaking, two different sizes of dust particles: tiny (like smoke particles) and large (like sand grains). Why do we think it is more productive to look for the larger sandlike grains in order to determine whether planets are present in orbit around another star?

Larger grains clump together faster, indicating that planet formation is happening. They also emit more detectable infrared radiation. (ai)

Q3.18 From the Scientific American article "A Planet is Born," what is the main weakness (or bias) in the two most popular exoplanet detection techniques (transit and radial velocity), and how do the observations of the ALMA telescopes enable us to spot solar systems more like our own?

The main weakness is they can only detect massive and large planets. There's also bias toward planets close to their star since it's easier to notice changes in faster orbits. ALMA enables us, by being good at seeing dust, to see holes in disks and evidence of planets. It can estimate planet size by the size of the disk.

Q3.19 From the short Scientific American article "The Earth Next Door," (a) What are the properties of Proxima Centauri's suspected planet, Proxima b (estimated mass and orbital period), and (b) what is Proxima Centauri and how does it relate to Alpha Centauri?

Proxima b is 1/3 heavier than Earth and has a 12-day orbit, so it could have life.

Q3.20 From the short Scientific American article "The Earth Next Door," name and briefly explain three potential problems Proxima b might have due to its close orbit that may prevent the planet from providing a stable environment in which to host life.

Red dwarfs have violent, unstable paths and could be really hot or cold. Extreme tides could make Proxima b rotate only once per orbit, baking one side while keeping the other in darkness. Tidal heating can also heat it up from friction. Star flares, which are much more violent there, are another issue because of its proximity.

Q3.21 From the Scientific American article "The Galactic Archipelago," describe three of the proposed "solutions" to Fermi's question that the author discusses to explain why we have seen no evidence of extraterrestrial life.

The cost of resources to rapidly travel the galaxy is too high even for advanced species. Population growth is not a strong motivation for space travel. The "Great Filter" is the third option.

Q3.22 From the Scientific American article "The Galactic Archipelago," explain why it would be difficult for us to determine whether or not an advanced civilization lived on our planet millions of years ago. What is an example of evidence we might plausibly find that would indicate the existence of a past advanced civilization on Earth?

Look for odd, non-natural isotopes.

Q3.23 From the Scientific American article "Alone in the Milky Way," the author says that even if life is common in the galaxy, the planet Earth is perhaps one of the first planets to be inhabited by life. Explain why (as part of your answer, explain the concept of metallicity).

Our sun formed 5 billion years ago, which is when stars started to have enough metals for life. The sun was one of the first stars to have enough metal.

Q3.24 From the Scientific American article "Alone in the Milky Way," the author argues that we live in a galactic version of the habitable zone. That is, we are not too close to the center of the Milky Way galaxy and not too far away. Explain why life is less likely to thrive on planets orbiting stars that are closer to the center of the galaxy (two reasons).

The center of the galaxy is more packed and there are lots of supernovas which shoot out radiation. The super black hole and gamma-ray bursts affect the center more often with deadly radiation.

Q3.25 From the Scientific American article "Alone in the Milky Way," explain two reasons why life is less likely to thrive on planets orbiting stars that are further from the center of the galaxy.

The farther from the center of the galaxy, stars get less common. This means fewer chances for life. Also, further from the center, stars have a lower metal content. This means it's less likely for planets to form.

Q3.26 From the Scientific American article "Alone in the Milky Way," explain how the origin of Earth's Moon likely led Earth to have a very thin crust (compared to Venus) that is capable of life-sustaining plate tectonics.

Venus's thick crust keeps heat in; its surface melted due to it. We have a thin, mobile crust. A big collision formed the moon, which took out a big portion of our crust, making it thinner.

Q3.27 From the Scientific American article "Alone in the Milky Way," although the appearance of life on Earth only a billion years into the planet's history would make us optimistic about discovering life elsewhere, the development of more complex life (like us) seems to be extremely unlikely because of the history of life's development since that first single-celled organism appeared. Explain why.

About 4 billion years ago the first single-celled organisms formed. If it was common, complex life would have formed faster, but it took 3.6 billion years after single-celled life for it to form.

Q3.28 From the "Crash Course: Exoplanets" video: (a) Around what kind of star were the first two planets discovered? (b) Why was this discovery not very "satisfying" to Astronomers who were hoping to gain more insight into our own solar system?

(a) They were discovered around a pulsar—a dead neutron star. (b) It wasn't satisfying because pulsars are so different from our Sun that it didn't help explain how planets like ours form. (ai)

Q3.29 From the "Crash Course: Exoplanets" video: The first planet discovered around an "ordinary" star was 51 Peg b. The fact that it is very massive (more massive than Jupiter) and also orbits very close to its parent star (8 million km compared to the 55 million km radius of Mercury's orbit, with an orbital period just over 4 days) was surprising, because our model of planetary formation predicted that no giant planets could form that close to their parent star. So (a) how do we think 51 Peg b got there, exactly, and (b) why didn't a similar process happen with our own Jupiter (according to the idea presented in the video)?

51 Peg had drag from gas and dust which spiraled it in close to the star. Saturn could have prevented it from being pulled in.

Q3.30 From the "Crash Course: Exoplanets" video: (a) Explain how the HD 209458B system was different from other exoplanet systems discovered previously, and (b) what was so significant about the discovery of the exoplanet HD 209458b? Explain.

It wobbled and also had a transiting effect (planet orbiting the star). Since it's transiting, the system is edge-on and not tilted, so the mass using the Doppler method is accurate.

Q3.31 From the video "Life Beyond Earth, Part 1," we search the cosmos for the three things terrestrial life required. What are these three things, and for each ingredient, what is an example of a place where it may be found outside of our solar system?

Water, energy, and organic molecules. (Clouds → stars → cloud of gas and dust.)

Q3.32 From the video "Life Beyond Earth, Part 1," on the five km highway of life, the first life forms are found about 3.75 km away from where we are today (3.75 billion years ago). What were the first life forms that were common on Earth for 3 billion years after this time, and what were they like? Briefly describe them.

They were stromatolites—single-celled bacteria and algae.

Q3.33 From the video "Life Beyond Earth, Part 1," critics of Darwinian evolution say it is impossible to produce complex life forms just from random mutations (just like it is impossible to randomly organize letters of the alphabet to produce "King Lear"). How do biologists respond to this argument?

Natural selection filters out unsuccessful random changes, allowing complexity to build gradually over time. (ai)

Q3.34 From the video "Life Beyond Earth, Part 1," (a) explain what is the habitable zone in our solar system. Also, (b) explain two reasons why, about the time we were starting to venture into space, astronomers felt that life may exist on Mars or Venus.

(a) The habitable zone is the distance from a star where liquid water can exist. (b) Mars has polar ice caps, and Venus was once thought to have clouds like Earth's atmosphere. (ai)

Q3.35 From the video "Life Beyond Earth, Part 1," describe what the Magellan spacecraft discovered about the past history of the planet Venus, starting about 500 million years ago.

It discovered that Venus, 500 million years ago, turned itself inside out through volcanic activity, which released a lot of greenhouse gases and increased the temperature drastically. It shows that recent changes can happen to planets.

Q3.36 From the video "Life Beyond Earth, Part 1," what evidence discovered by the Viking orbiters on Mars indicated that Mars must have once had a thicker, warmer atmosphere than it does today?

The evidence is the riverbeds, which indicate liquid water.

Q3.37 From the video "Life Beyond Earth, Part 1," Freeman Dyson explains that there are two possibilities regarding the origin of life. Either it came into being gradually through chemistry and steps we could hope to retrace (and could presumably be reproduced elsewhere) or life is some kind of extraordinary fluke. (a) If the answer to the question of the origin of life is the first possibility, what does that imply about life beyond Earth? (b) What if the answer is the latter possibility?

(a) It implies that life should exist everywhere. (b) The second option implies that the universe is probably pretty empty.

Q3.38 From the video "Life Beyond Earth, Part 1," explain (a) what discoveries on Earth have led us to make plans to search non-traditional places like Europa for life, and (b) what is meant by the term "gravitational habitable zone"?

There is life at the bottom of the ocean with hydrothermal vents and no sunlight. The gravitational habitable zone is there because Jupiter's gravity provides the energy.

Q3.39 From the TESS Planet Hunter discover video, NASA scientists have created two possible models to represent TOI 700 d, the Earth-sized planet orbiting in the habitable zone of its parent star. They predict that the mostly-rocky planet will have an extra feature in its spectrum that wouldn't appear in a mostly-water planet model. What is the extra spectral signature?

Methane or carbon dioxide would appear as extra spectral features for rocky planets. (ai)

Q3.40 From the TED talk about ALMA, describe four reasons why we do Astronomy (one complete sentence that describes each reason, not just a list of words).

We study astronomy to understand our origins, to learn how planets and stars form, to explore if we are alone, and to advance technology that helps us see the universe. (ai)

Q3.41 From the TED talk about ALMA, why do Astronomers move the individual radio telescopes in the array into different configurations? What is the purpose of a "large" vs a "small" configuration?

To get different resolutions. A large configuration is better for small, detailed things, and a small configuration is better for large-scale faint structures.

Q3.42 From the TED talk about ALMA, one of the ALMA observations discussed is an observation of Carbon Monoxide snow around the newly formed star TW Hydrae. How is this related to the possible origin of life on Earth?

Carbon Monoxide can freeze onto dust grains, forming icy coatings that can later create organic compounds. These compounds can lead to the building blocks of life.

Q3.43 From the film "Life Beyond Earth, Part 2," explain two reasons why radio wavelengths are the best way (we think) to send messages back and forth across the vast distances of interstellar space (as opposed to flaming triangles in Siberia or strings of firecrackers). Paul Horowitz (sitting on the edge of a radio telescope) will talk about this, too, a bit later.

Radio waves pass easily through dust and gas and travel long distances without being absorbed.

Q3.44 From the film "Life Beyond Earth, Part 2," explain why our radio signals from Earth are only detectable for the nearest 1000 or so stars nearest to our Sun.

Radio signals become weaker the farther they go. Cosmic background radiation is stronger, so past a certain point they can't be detected.

Q3.45 From the film "Life Beyond Earth, Part 2," explain what happens in the experiment that Timothy Ferris performs to test the validity Fermi's question to show that "absence of evidence is not evidence of absence." What was Ferris' logical mistake in the experiment, the same logical mistake we may be making when we try to explain why we haven't detected any aliens?

He dropped marbles in a field and didn't find any. He concluded there were none, but he just wasn't able to detect them. The same goes for aliens—we may not have the ability to detect them.

Q3.46 From the film "Life Beyond Earth, Part 2," explain the argument Stephen Jay Gould (a prolific author of many interesting short essays on the nature of science, biology and human evolution) makes to infer that if other life forms do exist out there is the cosmos, intelligence is very unlikely to be a common trait among them.

Intelligence is a random event. Even if life happens often, intelligence probably happens rarely.

Q3.47 From the film "Life Beyond Earth, Part 2," (a) explain what an "emergent property" is and give two examples, and (b) explain why this phenomenon may make intelligent life more probable to exist in the Universe.

An emergent property is something that happens when smaller parts combine to form something new. Example: water molecules form snowflakes, or neurons form consciousness. It could make intelligent life more common because complexity can appear naturally.

Q3.48 From the film "Life Beyond Earth, Part 2," describe the probability-based argument (also based on the Copernican Principle) made by J. Richard Gott to suggest that our human species is unlikely to last longer than a maximum of about 7.8 million years.

He argues that if we are observing our species at a random point in its lifetime, it's more likely that we are near the middle rather than the beginning, so we probably don't have much longer left than we've already existed.

Q3.49 From the film "Life Beyond Earth, Part 2," explain why it will be easier to communicate (or find) other intelligent alien civilizations if the average lifetime of a civilization is extremely long (millions or tens of millions of years) instead of short (less than 100,000 years or so).

If civilizations last longer, there is a greater chance of overlap between their existence and ours, so we'd be more likely to detect each other.

Q3.50 From lecture, explain what are the three rules of Doppler shifting.

1. When something moves toward you, light waves are shorter (blue shift). 2. When something moves away, light waves are longer (red shift). 3. The faster the object moves, the greater the shift in wavelength.

Q3.51 From lecture, (a) explain the difference between radial velocity and transverse velocity. (b) If you have two stars with the same velocity but star A is moving directly away while star B is moving transverse to your line of sight, which will show a larger Doppler shift and why?

Radial velocity is motion toward or away from us, while transverse velocity is side to side. Star A shows a larger Doppler shift because only radial motion causes wavelength changes.

Q3.52 From lecture, in the Doppler wobble method of exoplanet detection, explain what we observe and how that turns into a graph of radial velocity vs time.

We look for the change in wavelength in the spectrum of the star. It's red-shifted when moving away and blue-shifted when moving toward. A graph of these shifts over time creates a wave pattern showing the planet's orbit.

Q3.53 From lecture, (a) once we have a graph of a star's radial velocity vs time, what two things on that graph do we measure? (b) For each of these two things, explain what property of the companion planet do we deduce (explain why there is a relation between what we measure and the property of the planet).

The two things we measure are amplitude and period. Amplitude gives us the mass (larger amplitude = heavier planet). Period tells us the distance (shorter period = closer planet).

Q3.54 From lecture, if the planetary system is face-on with respect to our line of sight instead of edge-on, we will not be able to detect any exoplanets with the Doppler wobble technique, even if the star is wobbling. Explain why not.

Because the motion is side to side instead of toward or away from us, so there's no wavelength shift to measure.

Q3.55 From lecture, explain why it is very difficult to use the astrometric (positional astronomy) technique, which is to measure the side-to-side (angular) motion of a star over time in response to its companion planet unless that star is one of the closest 100 or so stars to us.

Because the angular shift is so tiny that our instruments can only detect it if the star is extremely close. (ai)

Q3.56 From lecture, if the planet system is tilted with respect to our line of sight, our estimate of the companion planet's mass will be too small. First (a) draw a diagram to show why a "tilted" system will show a small Doppler shift compared to the same system seen edge-on, and (b) explain why our companion planet mass estimate will be too small for a tilted system.

If it's tilted, we don't see the full velocity, only a small part of it. Because the Doppler shift looks smaller, we underestimate the planet's true mass.

Q3.57 From lecture, explain why some Astronomers feel that if intelligent life currently exists or has ever existed in our galaxy, then we should be able to find evidence of its existence fairly easily, even on the Earth.

Because intelligent life leaves behind detectable artifacts, signals, or chemical traces that could persist for millions of years.

Q3.58 From lecture, explain what the purpose of the Drake Equation is and how it applies to SETI. I'm not asking for a definition of every part of it.

It estimates the number of intelligent civilizations we might contact by multiplying probabilities like how many stars have planets, how many develop life, and how many communicate. It gives SETI a framework for the search.

Q3.59 From lecture, if we were to encounter or detect evidence (by finding artificial signals through our searches of the sky) another intelligent, communicative civilization besides our own, it is highly likely that that other civilization will be much more advanced than our own. Explain why they would likely be more advanced than us (this is also covered in OpenStax Chapter 30.4).

Because the odds are higher of finding civilizations that have existed for millions of years. The longer they survive, the more likely they are to overlap with us.

Q3.60 From lecture, (a) explain the difference between "passive SETI" and "active SETI" and (b) describe an argument in favor of passive SETI (against active SETI), and (c) describe an argument in favor of active SETI.

(a) Passive SETI is listening for alien signals, active SETI is sending our own. (b) Passive is safer since we don't risk revealing ourselves to possibly dangerous civilizations. (c) Active could help us contact advanced life faster.