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--- courses:ast100:4 [2024/12/13 23:24] – [3. Earth] asad
+++ courses:ast100:4 [2024/12/14 09:41] (current) – [4. Detecting Planets] asad
@@ Line 105: / Line 105: @@
 **Socrates:** Let’s go, everyone.
+==== Saturn's Rings ====
+{{https://upload.wikimedia.org/wikipedia/commons/thumb/2/2d/Saturn_diagram.svg/1280px-Saturn_diagram.svg.png?nolink}}
+**Hermes:** Galileo first observed these rings with a telescope, but it was Huygens from the Netherlands who first understood that they were indeed rings. From here, it’s clear that these rings are not continuous disks but rather a collection of countless ice particles ranging from a few centimeters to several meters in size. Besides water, many of these particles also contain various carbon compounds. These rings extend from about 30,000 km above Saturn’s surface to nearly 150,000 km. Despite a diameter of 300,000 km, the thickness of these massive rings ranges from about 10 meters to a maximum of a few hundred meters. The A, B, C, D, and F rings can be seen here, and the gaps between the rings are named after different scientists. For instance, the gap between the A and B rings is called the Cassini Division and the Huygens Gap; between the B and C rings is the Coulomb Gap, and between the C and D rings is the Maxwell Gap. Maxwell, the founder of electromagnetic theory, was the first to understand that Saturn’s rings are not a single disk but rather a collection of countless small objects.
+**Socrates:** No need for further descriptions. Just tell me how these rings formed.
+**Hermes:** If a moon or asteroid comes too close to a planet, the planet’s gravitational force pulls the near side of the object more strongly than the far side, as gravity decreases with distance. This results in the object being stretched and eventually torn apart into fragments due to the gravitational pull. These fragments then form a ring around the planet. Saturn’s rings were formed in this way. Moreover, if you observe closely, you will see small rocky fragments, about 10–20 km in size, scattered in certain places within the rings. These are called moonlets. Due to these moonlets, spiral structures, similar to spiral galaxies, form within Saturn’s rings. Saturn’s rings and its 62 moons can be compared on one hand to an entire solar system and on the other hand to a spiral galaxy like the Milky Way, with the spiral patterns of the rings resembling those of a galaxy.
+**Socrates:** Hold on, hold on. During the stellar age, we should have understood how spiral arms are formed in galaxies. That wasn’t possible then. Now, do you want to explain spiral arms in galaxies using Saturn’s rings?
+**Hermes:** Why not? Because of the gravity of these moonlets in Saturn’s rings, small waves sometimes form in the ocean of the rings. Waves spread evenly outward from a moonlet. However, since the inner rings of Saturn rotate faster than the outer rings, the inward-moving waves outpace the outward-moving waves. The perfectly circular wave (technically called a __density wave__) becomes spiral due to this uneven velocity, just as a circular ring can be twisted into a spiral pattern. In the case of galaxies, replace the moonlets with nebulas and stars, and replace the ocean of rings with the gases and stars of a galaxy. Since the velocity of stars and gas in galaxies also decreases with distance, the density waves created within galaxies similarly give them their spiral shape.
+**Socrates:** Fascinating. It’s through such comparisons of one era with another, or one object with another, that we can progress. How are the pinkish-purple auroras seen around Saturn’s north pole formed?
 ===== - Earth =====
 **Hermes:** If you understand how auroras form on Earth, you’ll understand Saturn’s as well. The solar system is now about 1 billion years old. If we travel from Saturn to Earth, we can see how auroras formed near Earth’s poles even 3.6 billion years before the emergence of humans.
@@ Line 140: / Line 156: @@
 **Socrates:** Even if it doesn’t exist in our solar system, surely there are many planets around other stars in the Milky Way with tectonic movement. Isn’t that right?
 ===== - Detecting Planets =====
-{{youtube>IiCRZmrgB9g?large}}
+**Hermes:** Planets gravitationally bound to stars other than the Sun or free-floating in interstellar space are called exoplanets. It’s still unknown whether their crusts exhibit tectonic activity, though more than seven thousand exoplanets have already been discovered.
-\\
+**Socrates:** I don’t see the necessity of terms like exoplanet or any other such designations. A planet is a planet, whether it orbits the Sun or Sirius. Could you instead explain how these "planets" around other stars are actually "discovered"?
+{{youtube>TbNGEkAuAjU?large}}
+\\
+**Hermes:** Most planets have been discovered through the transit method. Think of transit as a smaller sibling of an eclipse, as shown in this video. From our perspective, a planet passing in front of its star is called a transit. As the planet crosses the star’s face, it partially blocks the starlight, causing a slight decrease in brightness. This phenomenon is observed through a light curve, depicted in the bottom-left corner of the video. A light curve represents changes in the star’s brightness over time. While the planet is in front of the star, the light intensity drops; once it moves away, the light returns to its original level. This creates a dip in the light curve.
+**Socrates:** Is the depth of this dip related to the size of the planet?
+**Hermes:** Yes, the larger the planet, the deeper the dip in the light curve. If a star has multiple planets orbiting it, the light curve becomes even more interesting. The video illustrates a system with three planets undergoing transit. Each planet creates a separate dip in the curve. However, since the second and third planets transit simultaneously, a smaller dip is embedded within a larger one. Analyzing such complex patterns in multi-planet systems reveals not only the planets’ sizes but also their orbital periods. Each dip occurs at intervals corresponding to the time a planet takes to complete an orbit around its star.
+**Socrates:** You mentioned that a planet can be observed crossing its star—does this mean the planet can literally be "seen"?
+**Hermes:** No, planets aren’t directly visible. They don’t emit visible light, and the infrared light they do emit is negligible compared to their stars. This is why stars dominate daytime skies on Earth, and similarly, planets around distant stars cannot be directly seen. Only the star is visible, and the planet’s transit is detected through the slight dimming of the star’s light.
+**Socrates:** But planets reflect starlight; that’s why the planets in our solar system look somewhat like stars in the night sky. Don’t planets around other stars reflect their starlight similarly?
+**Hermes:** They do. However, due to the immense distance, it’s currently impossible to distinguish reflected light from the planet itself. Compared to the distance between us and a star, the distance between a star and its planets is incredibly small. This makes it challenging to resolve planets separately using existing telescopic resolution. Images of nearby planetary systems have been captured using "direct imaging," where special masks block the star’s light in the telescope’s focal plane. Much like the Moon blocking the Sun during an eclipse, these artificial masks reveal the planets around a star. In this image, taken over seven years at Hawaii’s Keck Observatory, you can see such a planetary system in motion.
+{{https://upload.wikimedia.org/wikipedia/commons/4/48/Hr8799_orbit_hd.gif?nolink&500}}
+**Socrates:** Ah, so the "★" symbol at the center indicates where the star would have been. Here’s another question about the transit method: If the total amount of light includes both starlight and the planet’s reflected light, wouldn’t the total light decrease slightly when the planet moves behind the star?
+{{:bn:courses:ast100:transits.webp?nolink&650|}}
+**Hermes:** Absolutely. This event is called a secondary transit, as illustrated in this diagram. A primary transit occurs when a planet passes in front of its star, while a secondary transit happens when it moves behind the star. When the planet is to the side, no transit occurs, but phases similar to those of the Moon are observed, with different parts of the planet illuminated by the star. The diagram’s lower panel shows a complete light curve from one secondary transit to the next, along with images of the various phases. The dip from the secondary transit is much shallower compared to the primary transit, so only its upper part is visible here.
+**Socrates:** Understood. Back in the Galactic Age, Shashi explained the Doppler effect to us: when something approaches, its light’s wavelength shortens, and when it moves away, the wavelength lengthens. So, as a planet moves from secondary to primary transit, it comes closer, and its reflected light should appear shorter (bluer). As it moves from primary to secondary transit, it moves farther away, and its light should appear longer (redder). Can this be measured?
+{{youtube>IiCRZmrgB9g?large}}
+\\
+**Hermes:** No, it’s nearly impossible to measure the reflected light of a planet separately. However, an interesting fact is that if a star has orbiting planets, not only the planet but also the star orbits a common center of mass. People assume planets orbit stars, which is only partially true; both planets and stars orbit the center of mass of their planetary system. In this video, the small circle represents the star’s orbit, the larger one represents the planet’s orbit, and their shared center is the center of mass of the system. You can see that the star itself moves, and when it moves toward us, its light becomes bluer (shorter wavelengths), and when it moves away, its light becomes redder (longer wavelengths).
+**Socrates:** Oh yes, I recall Shashi explaining center of mass during the Galactic Age using a seesaw. Since stars are much heavier than planets, they stay much closer to the center of mass, resulting in the smallest orbit.
+**Hermes:** Exactly. This motion of the star is called a "wobble." Since you mentioned mass, you can see that the extent of a star’s wobble depends on the planet’s mass. By measuring the star’s color change using this method called the "radial velocity" method, the planet’s mass can be determined.
+**Socrates:** Fascinating. So, if the transit method provides a planet’s size, and radial velocity reveals its mass, combining the two could determine the planet’s density.
+===== - Classification of Planets =====
+**Hermes:** Exactly. From size, we can derive volume, and dividing mass by volume gives density. This figure shows the radius of all discovered planets plotted against their mass. The x-axis represents planetary mass relative to Earth, while the y-axis represents planetary radius relative to Earth. Each bubble represents a planet, and the color of the bubble indicates the discovery method.
+{{:bn:courses:ast100:planets-r-m.webp?nolink|}}
+**Socrates:** And those two diagonal dashed lines represent constant density lines, correct?
+**Hermes:** Yes. A planet on the lower diagonal line has the same density as Earth, meaning its mass per cubic centimeter is 5 grams. Planets on the upper diagonal line have the same density as Saturn, about 0.7 grams per cubic centimeter. Planets between these lines have densities between Earth and Saturn. Planets below the lower line are denser than Earth, and those above the upper line are less dense than Saturn. The discovery methods are also shown here.
+**Socrates:** It’s clear that the transit method is the most successful. Direct imaging has only been possible for the largest and heaviest planets (the blue bubbles in the upper right corner). But what about the planets shown with yellow and red bubbles? You haven’t said much about their discovery methods.
+**Hermes:** Not all methods need to be discussed for now.
+**Socrates:** It seems you’re always looking for ways to skip explanations.
+**Hermes:** Life is short, and watching rhinos in the Kaziranga National Park along the banks of the Brahmaputra is much more enjoyable than endless study. Look, two rhinos bathing over there.
+**Socrates:** Explain this figure first, and then we’ll all enjoy the rhinos together.
+{{:bn:courses:ast100:planets-r-p.webp?nolink&800|}}
+**Hermes:** Since we’ve already discussed the classification of galaxies and stars, it’s only fair to talk about planetary classification too. In this scatter plot, the y-axis again shows planetary radius, but the x-axis now shows orbital period instead of mass, meaning the time a planet takes to orbit its star.
+**Socrates:** I see that each bubble represents a planet, but what is the "equilibrium temperature" indicated by the bubble color?
+**Hermes:** A planet’s equilibrium temperature is the temperature it would have if it emitted as much energy as it receives from its star. This may differ from its actual surface temperature. For example, Earth’s average surface temperature is 15°C, but its equilibrium temperature is -18°C. The actual temperature is higher because some reflected energy is retained in Earth’s atmosphere due to the greenhouse effect. In this plot, you can see that planets with shorter orbital periods are closer to their stars and thus have higher temperatures (more red-colored bubbles). Those with longer periods are farther from their stars and cooler (more blue-colored bubbles). Kepler discovered the relationship between period and distance 400 years ago, and we met him during the Stellar Age.
+**Socrates:** Got it. Now explain the classification. Planets at the bottom are Earth-like, those at the top (10 times larger than Earth) are like Jupiter and Saturn, and those in the middle (4 times larger) resemble ice giants like Uranus and Neptune. But how is the division along the x-axis determined, and what does the black diamond icon represent?
+**Hermes:** The black diamond icon represents Earth, with a period of 365 days. Planets with very short periods (1–10 days) are extremely hot, potentially lava worlds (if rocky like Earth), ocean worlds (covered in water), or hot Jupiters (gaseous like Jupiter). On the other hand, planets with very low temperatures are cold gas giants or ice giants.
-{{youtube>TbNGEkAuAjU?large}}
+**Socrates:** What are super-Earths?
-\\
-===== - Classification of Planets =====
+**Hermes:** Planets at least twice the size of Earth but smaller than Neptune are commonly called super-Earths. These range from molten-hot to ice-cold worlds. One well-known super-Earth, Kepler-22b, might be an ocean world, almost entirely covered by water. NASA’s Kepler Space Telescope has discovered nearly 2,500 planets, out of the 7,000 known so far. That’s why these planets are named after our dear friend Kepler.
-{{:bn:courses:ast100:exoplanets.webp?nolink|}}
+**Socrates:** After hearing about water on Kepler-22b, I’ve lost interest in studying. Let’s [[https://youtu.be/YCXOtcharM0|watch the rhinos bathing in Kaziranga's Brahmaputra]] now.