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--- courses:ast100:4 [2024/12/06 08:59] – [3. Earth] asad
+++ courses:ast100:4 [2024/12/14 09:41] (current) – [4. Detecting Planets] asad
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 **Socrates:** In about an hour and a half, we’ve witnessed the first 800 million years of the solar system. Now, Hermes, can you give us an overview of this world?
 ===== - Solar System =====
-{{:un:helios.webp?direct}}
+{{:bn:courses:ast100:helios.webp?nolink}}
-{{:bn:courses:ast100:planets.webp?nolink|}}
+**Hermes:** Since not all objects are equally bright, you cannot see everything in the solar system with the naked eye. I have shown the current state of the solar system in this image using three successive scales. On the largest scale, the solar system is actually a vast cloud made up of billions of icy fragments. Due to the gravitational influence of the four giant planets that formed near the frost line, most of the carbonaceous ice fragments between the frost line and the CO snow line were ejected from the solar system. However, they couldn't escape the Sun's gravity entirely. These fragments formed the **Oort Cloud**. Compared to the Oort Cloud, the solar system up to the Kuiper Belt (50 AU from the Sun) is just a speck, as the Oort Cloud begins at a distance of 2000 AU and ends around 100,000 AU. This makes the size of the solar system roughly **1 light-year**.
+**Socrates:** Zooming in, I see two images below. On the left, there are the four outer planets, the Kuiper Belt, and the heliosphere. On the right, there's a closer view showing the asteroid belt and the four inner planets within it. What is the heliosphere?
-===== - Earth =====
+**Hermes:** From the Sun, a continuous stream of charged particles like electrons and protons flows out across the solar system. This flow is called the **solar wind**, literally "the Sun's wind." Beyond the Kuiper Belt, this wind can no longer travel far because it slows down due to interaction with the interstellar wind from other stars and eventually stops. The boundary where the solar wind stops is called the **heliopause**, and the entire region filled with the solar wind and its magnetic field inside the heliopause is called the **heliosphere**. In the image on the left, the heliopause is represented by a blue shell, though, of course, we cannot see it with the naked eye.
-**Hermes:** Understanding how auroras are formed on Earth can help explain those on Saturn. The Solar System is now approximately 1 billion years old. If we travel from Saturn to Earth, we can see how auroras formed at Earth's poles even 3.6 billion years before the emergence of humans.
-<color purple>[Eight people approach Earth and observe the northern polar region from above.]</color>
+**Socrates:** In your image, I see a shock front in the direction the Sun is moving toward the galactic center, and in the opposite direction, there's a heliotail. What does this mean?
+**Hermes:** Just as a ship moving quickly in the sea creates a bow-shaped wave in front of it called a **bow shock**, the Sun creates a similar bow shock as it moves through the interstellar medium. The interaction between the solar wind and the interstellar wind creates this phenomenon. The solar wind spreads equally in all directions from the Sun, but near the heliopause, the interstellar wind pulls many of the solar wind particles behind the Sun, forming the **heliotail**. Human-made spacecraft have spread across the solar system, with Voyager 1 and 2 even crossing the heliosphere. Just as airplanes move through Earth's atmosphere, spacecraft travel through the Sun's atmosphere, the heliosphere.
+{{:bn:courses:ast100:ss-planets-motion.webp?nolink|}}
+**Socrates:** I can see that all the planets orbit the Sun in the same direction, and most also rotate in the same direction on their axes, except for Venus and Uranus. Is this because they were all formed from the same solar nebula? As the nebula contracted, it was rotating in a specific direction, so everything still rotates in that direction. But why are Venus and Uranus exceptions?
+**Hermes:** As shown in the image above, Venus rotates in the opposite direction, and Uranus rotates while tilted. The cause is undoubtedly some **catastrophic event** in the solar system's history, some **violent collisions**. During their formation or during the Late Heavy Bombardment, a large asteroid may have struck these two planets, leaving them in their current state. However, we are not yet certain about the exact reasons.
+**Socrates:** Earth's day is 24 hours, and its year is about 365 days. How do the days and years of the other seven planets compare?
+**Hermes:** Just as we measure distance in AUs, we measure the days and years of other planets relative to Earth's. Jupiter has the shortest day, just **10 hours**, while Venus has an unusually long day, equivalent to **243 Earth days**. Mercury's day is about **59 Earth days**, while the rest can be measured in hours. The years are straightforward: the farther a planet is from the Sun, the longer its orbital path and the slower its speed. **Kepler** became famous for discovering the relationship between a planet's year and its distance from the Sun. Mercury's year lasts **88 Earth days**, while Neptune's year is **165 Earth years**.
+{{:bn:courses:ast100:ss-planets-structure.webp?nolink|}}
+**Socrates:** From your image, it seems the planets in the solar system fall into three categories: the four inner planets are similar, but the outer planets are of two types—Jupiter and Saturn are one type, while Uranus and Neptune are another.
+**Hermes:** Yes. The inner planets are called **terrestrial planets** or **rocky planets**. Jupiter and Saturn are **gas giants**, and Uranus and Neptune are **ice giants**. All four terrestrial planets have cores made of iron and nickel, surrounded by silicate, meaning a thick layer of rock. The gas giants have cores of iron and rock, surrounded first by water, then by **liquid metallic hydrogen**, and finally by layers of hydrogen gas. The hydrogen layer is much larger than the core; these two planets are about **90% hydrogen**. The ice giants also have iron and rocky cores, but outside their water layers, there is only a layer of hydrogen gas; they lack liquid hydrogen. Ice giants are about **80% hydrogen**. You could say that inside each outer giant planet is a rocky inner planet. The water outside the rocky-iron cores exists in a state that is neither fully solid nor liquid, known as a **supercritical fluid**. However, venturing there would not be wise.
+**Socrates:** Since we don’t have time to discuss all the planets, tell me only about Saturn. I’ve heard that all four giant planets have rings. Why are Saturn’s rings so bright that they alone inspire wonder?
+**Hermes:** Let’s zoom in and travel closer to Saturn.
+**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.
+<color purple>[The eight approach Earth and observe the northern polar region from above.]</color>
 {{:bn:courses:ast100:atmosphere-magnetosphere.webp?nolink|}}
-**Hermes:** In this image, we see Earth's **atmosphere** (the word comes from the Greek 'atmos,' meaning 'vapor') and the **magnetosphere**. The atmosphere is shown extending roughly 100 km above the surface and is divided into layers: first, the troposphere, the region of clouds, followed by the ozone-layer-rich stratosphere, and then, successively, the mesosphere and ionosphere. The ionosphere spans roughly 80 to 1000 km in height, where gases become ionized due to the Sun's strong radiation. Beyond the ionosphere lies the magnetosphere, Earth's magnetic field region, mapped by dipolar (two-pole) magnetic field lines. This enormous magnetic sphere, about 1 million kilometers in size, makes Earth and its atmosphere seem minuscule. Charged particles from the solar wind get trapped in Earth's magnetic field lines and oscillate along these lines in a spiraling path, moving between the northern and southern poles. The field lines near the poles are known as the Van Allen radiation belts, where particle acceleration generates significant radiation. As these particles reach the northern pole, their density increases, and the magnetic field kicks them back toward the south. It takes these particles only 1 second to oscillate between the poles. This oscillation causes particles near the poles to accelerate so much that they emit various colors of radiation, known as **auroras**. The northern aurora is called Aurora Borealis, named by Galileo after the north wind god. The southern one is called Aurora Australis.
+**Hermes:** In this image, you can see Earth’s atmosphere (the word originates from the Greek ‘atmos,’ meaning ‘vapor’) and __magnetosphere__. The atmosphere extends roughly 100 km above the ground and is divided into several layers: first comes the troposphere, the region of clouds; then the stratosphere, home to the ozone layer; above that are the mesosphere and ionosphere, in that order. The ionosphere spans approximately 80 km to 1,000 km in height, where gases become ionized due to the Sun’s intense radiation. Beyond the ionosphere lies the magnetosphere, the region governed by Earth’s magnetic field, mapped by the dipolar magnetic field lines. Compared to Earth and its atmosphere, this massive magnetic field, about 1 million km in size, makes Earth appear tiny. Charged particles from the solar wind are trapped by Earth’s magnetic field lines, spiraling around the lines and oscillating between the North and South Poles. These field lines are known as the Van Allen radiation belts because the acceleration of charged particles in these regions generates significant radiation. When particles reach the northern pole, their density increases greatly, and the magnetic field kicks them back toward the south. It takes only 1 second for these particles to oscillate between the poles. Due to this oscillation, the particles’ acceleration near the poles is so high that they emit radiation of various colors, creating what we call __auroras__. The northern aurora is called aurora borealis, named by Galileo after the god of the northern wind. The southern aurora is called aurora australis.
-**Socrates:** But how is this enormous magnetic field generated? It seems like Earth is a massive bar magnet with its north magnetic pole slightly offset from its geographic north pole. Your diagram shows a tilt of over ten degrees between the magnetic axis and the rotation axis.
+**Socrates:** But how is this vast magnetic field created? It looks like Earth is a giant bar magnet with its magnetic north pole slightly offset from the geographic North Pole. In your diagram, the magnetic axis appears to be over ten degrees off from the rotational axis.
 {{:bn:courses:ast100:earth.webp?nolink|}}
-**Hermes:** Yes, the actual tilt between the two axes is 11 degrees. And Earth indeed acts as a giant bar magnet. To understand its source, we must look inside Earth. Earth's body is divided into three parts: the core, the mantle, and the crust. Within Earth's radius of approximately 6500 km, the crust is only 50 km thick, acting like Earth's skin. Below the crust lies the mantle, around 3000 km thick, and beneath that, the core, which is about 3500 km thick and divided into the outer and inner core. The inner core, roughly 1300 km in size, is solid iron and nickel, while the outer core, about 2000 km thick, consists of liquid iron and nickel. If we compare the inner core to a furnace, the outer core can be likened to a boiling pot of water. As water bubbles rise due to heat and cool down to sink back, a circular flow called convection occurs. In the outer core, convection currents arise due to heat from the inner core. Since the flowing material is electrically conductive metal, this generates a convection current. Ampère, a 19th-century French scientist, showed that a loop of electric current generates a magnetic field perpendicular to the loop's area. The convection currents in Earth's outer core act similarly, producing the magnetic field.
+**Hermes:** Yes, the distance between the two axes is actually 11 degrees. Earth is indeed a giant bar magnet, and to understand its source, we must examine Earth’s interior. Earth’s structure can be divided into three parts: the core, mantle, and crust. With a radius of about 6,500 km, the crust is only 50 km thick, akin to Earth’s skin. Beneath the crust lies the mantle, about 3,000 km thick, and below that, the core, about 3,500 km thick, is divided into the outer and inner core. The inner core, roughly 1,300 km in size, is composed of solid iron and nickel, while the outer core, about 2,000 km thick, contains the same elements but in liquid form. If we compare the inner core to a furnace, the outer core can be likened to a pot of water on that furnace. As water boils, hot water rises as bubbles, cools, becomes heavy, and sinks again, creating a circular flow called convection. Similarly, the outer core exhibits convection due to the inner core’s heat. This flow of conductive metallic liquid generates convection currents. In the 19th century, French physicist Ampère showed that when electric current flows through a wire loop, a magnetic field is generated perpendicular to the loop’s area. The convection currents in Earth’s outer core act like such loops, and these electric currents are the source of Earth’s magnetic field.
-**Socrates:** So, the magnetic field lines you showed earlier emerge from Earth's interior. And around each of these lines in the outer core, there are electric current loops caused by the convection of liquid iron-nickel. Is that correct?
+**Socrates:** So, the magnetic field lines you showed earlier emerge from Earth’s core. Around each of these lines in the outer core, we’ll find loops of electric current generated by the convection of liquid iron and nickel. Correct?
-**Hermes:** Correct. It's also worth mentioning that the core's heat doesn't just create convection currents in the outer core; they also form impressively in another layer. The topmost part of the mantle, called the asthenosphere, is made of semi-solid rock and soil, not fully liquid but semi-solid. The crust lies above this layer. However, like the skin of a body, Earth's crust is not continuous but broken into pieces like a jigsaw puzzle. These pieces are called tectonic plates, which float on the semi-solid asthenosphere. Convection currents in the asthenosphere cause these plates to move, roughly 2 cm per year. When two plates collide head-on, mountains form at their boundary. For example, the Indian plate collided with the Eurasian plate about 50 million years ago, forming the Himalayas. This collision is so recent that the Himalayas are still rising, about 5 mm per year.
+**Hermes:** Correct. It’s also important to note that convection currents are not confined to the outer core due to its heat; they are also prominent in another layer. The uppermost layer of the mantle, called the asthenosphere, is made of semi-solid rock and soil—not fully liquid but semi-solid. Above it lies the crust. However, like human skin, Earth’s crust isn’t a continuous layer but is divided into several pieces like a jigsaw puzzle, called tectonic plates. These plates float on the semi-solid asthenosphere. Convection currents within the asthenosphere cause these plates to move, roughly 2 cm per year. When two plates collide head-on, mountains form along their boundary. For instance, the Himalayas arose about 50 million years ago when the Indian plate collided with the Eurasian plate. This collision is so recent that the Himalayas are still rising, about 5 mm annually.
 {{https://upload.wikimedia.org/wikipedia/commons/f/f2/Tectonic_plate_model_1Ga.webm?nolink&800}}
-**Socrates:** Does this video show the movement of tectonic plates?
+**Socrates:** Can’t we see Earth’s last billion years as shown in this video? Speed up time to show us how the plates in Earth’s crust have moved.
+**Ishtar:** I think the movement of plates is better observed from space than on a globe. For now, we should return to the Brahmaputra River by boat. The peace and vastness of the Siang River descending into the Assam valley will give us a sense of tranquility akin to the calm after an epoch of chaos.
+<color purple>[The eight board a boat at Dibrugarh in Assam and travel along the Brahmaputra toward Dhubri.]</color>
+**Hermes:** Meanwhile, watch this video showing how Earth’s major plates have moved over the last 1 billion years. This simulation, published in the journal *Earth-Science Reviews* in 2021, reveals that plates sometimes move slowly, sometimes quickly, sometimes approach each other, and sometimes drift apart. About 500 million years ago, most plates shifted southward, and as they moved northward, nearly all plates merged into a single supercontinent around 300 million years ago, called Pangaea. The combined oceans at that time were collectively called Panthalassa. Over the last 300 million years, the plates have separated again, forming the current seven continents. India’s northward journey toward Asia is particularly clear in the video.
+**Socrates:** Do the other three planets with solid crusts in our solar system have moving plates like this?
+**Hermes:** No. Mercury, Venus, and Mars don’t exhibit tectonic activity. However, Saturn’s moon Enceladus might have it.
+**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 =====
+**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.
-**Hermes:** Yes, it's a simulation showing how Earth's major crustal plates have moved over the past billion years. It was published in 2021 in the journal *Earth-Science Reviews*. It shows that the plates sometimes move slowly, sometimes rapidly, sometimes approach each other, and sometimes drift apart. About 500 million years ago, most plates shifted southward, and by 300 million years ago, they had coalesced into a single supercontinent called Pangaea, surrounded by the global ocean Panthalassa. Over the past 300 million years, the plates have broken apart to form today's seven continents. India's sprint toward Asia is particularly clear in the video.
+**Socrates:** I see that each bubble represents a planet, but what is the "equilibrium temperature" indicated by the bubble color?
-**Socrates:** Do the solid-crust planets in our Solar System experience such tectonic activity?
+**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.
-**Hermes:** No. Mercury, Venus, and Mars have no tectonic activity on their surfaces. However, Saturn's moon Enceladus might have it.
+**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?
-**Socrates:** Well, even if our Solar System doesn't have more, surely many planets orbiting other stars in the Milky Way must have tectonic movement. Am I right?
+**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.
-===== - Detecting Planets =====
-{{youtube>IiCRZmrgB9g?large}}
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+**Socrates:** What are super-Earths?
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-===== - 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.