Socrates: Hermes, thank you for taking us back 4.6 billion years to the Milky Way. There aren’t many stars here in the Orion Spur, but in the distance, I can see a dark cloud. You’ll have to show us how, over the next 150 million years, our solar system will form from this molecular cloud. Don’t worry, just as you can travel through space, Ishtar can travel through time. With Ishtar’s help, you can speed up or slow down time as needed.

Hermes: But weren’t we supposed to sit in a boat on the Brahmaputra, after starting from the Siang River, to discuss the Planetary Era?

Ishtar: We will, we will. First, let’s spend 100 million years in this void, witness the birth of our solar system, and then head to the Brahmaputra to discuss the planets forming around other stars in the Milky Way.

Hermes: Fine. Then Ishtar, speed up time so that one minute equals ten million years, and we can directly witness the first 150 million years of the solar system’s formation in just 15 minutes.

Ishtar: Starting now. In 15 minutes, the solar system will be born. Let’s see if you can narrate its creation within that time.

Hermes: During the Stellar Era, we observed how a molecular cloud spanning several light-years collapsed to form a disk only 100 AU in size, with most of the gas accumulating at the center to form a protostar. Here, AU refers to an Astronomical Unit, the distance between the Earth and the Sun, approximately 150 million km. In the Planetary Era, we’ll use AU instead of light-years, as the scale of distances has now become much smaller. The gas and dust disk we see before us is called the solar nebula, with the protosun at the center. But instead of focusing on the protosun, let’s turn our attention to the disk. Our task is to observe how this disk, divided into several rings, gives rise to eight planets, two belts, and the vast Oort Cloud.

Socrates: My student Plato’s successor, the great European philosopher Immanuel Kant, is said to have been the first to propose that a rotating cloud could give birth to a planetary system.

Hermes: Kant didn’t explain the birth of planets but was the first to theorize stellar formation from a nebula, in 1755. Later, in 1796, Laplace attempted to extend the theory to explain the formation of planets. According to Laplace, a battle occurs between rotation and gravity, where gravity dominates closer to the center, and rotational force dominates further out. Gas and dust at the very outer edge of the disk become detached due to rotational forces but can’t drift too far because of gravity, forming a ring orbiting the center. The outermost ring forms first, and as more material detaches from the disk, additional rings form between the disk and the first ring. Eventually, the disk transforms into several rings, and gravity within each ring consolidates the material into individual planets.

Socrates: That’s quite an elegant theory.

Hermes: Elegant but incorrect. In the 18th century, electromagnetic forces hadn’t been discovered, so everything was explained using gravity, which is not sufficient. Modern computer simulations show that planets cannot form purely due to gravity in such rings. Electromagnetic forces played a crucial role initially, even more so than gravity.

Socrates: But we can almost see rings forming now in the solar nebula. Look! The disk is dividing into three rings, with the middle one being the largest, though the gaps between them seem nearly equal.

Hermes: Yes, within just 100,000 years, the solar nebula has divided into three rings, but not into eight, as Laplace predicted. Electromagnetic forces are heavily involved in the formation of these rings.

Socrates: What kind of influence do they have?

Hermes: In the top panel of this image, three sublimation lines are shown. Sublimation refers to the process by which material transitions from gas to solid. The closer you get to the Sun, the higher the temperature. At 1.5 AU, the temperature drops to 1,100°C, allowing silicates (the material that makes up rocks) to solidify, but water (H$_2$O) or carbon monoxide (CO) cannot. At 8 AU, where the temperature is -100°C, water can exist as ice, and this boundary is called the water snow line. At 45 AU, where the temperature is -240°C, carbon monoxide also solidifies, forming the CO snow line. As the distance from the Sun increases, the temperature gradually decreases, but these three specific temperatures are critical because the solar nebula contains a significant abundance of these associated materials. As a result, solid material begins to accumulate more near these three distances from the Sun, giving rise to the faint rings you see now, which will become more pronounced over time.

Socrates: One minute has passed. The material near the three lines is condensing further.

Hermes: Yes. Now, if you glance back at my diagram, the middle panel shows the solar system 1 million years after its formation began. Near the silicate line, I’ve marked NC-type planetesimals in red.

Socrates: Wait, wait! Are you going to drown us in jargon?

Hermes: If you give me a moment, I’ll explain what NC and planetesimals are. NC stands for non-carbonaceous chondrites, which are chondrites that lack carbon compounds. Chondrites are meteorites that have remained largely unchanged since the solar system’s formation. During meteor showers, many such chondrites fall to Earth, and their analysis helps scientists understand the solar system’s early conditions. So, CC stands for carbonaceous chondrites, which contain carbon compounds. These form near the water snow line, depicted in white, while green represents icy fragments near the CO snow line, which formed the early Kuiper Belt.

Socrates: Hold on, hold on. Why are the three lines shown closer to the Sun in your middle panel compared to the first one?

Hermes: Because over the 1 million years shown here, the Sun’s temperature has decreased, allowing sublimation to occur closer to it. Consequently, the three lines have shifted inward. From now on, the Sun’s temperature won’t drop significantly, so the sublimation lines won’t move much further. Silicate lines stabilize around 1 AU, the water snow line around 4 AU, and the CO snow line around 20 AU. The silicate line is often called the soot line, while the water snow line is referred to as the frost line. Over the next 14 minutes (equivalent to 140 million years), you’ll see how the soot line spawns four rocky planets and the frost line gives rise to four giant planets.

Mars: I can already see how electromagnetic forces are causing micrometer-sized dust grains to clump together into millimeter-sized pebbles. Look, many pebbles are joining together to form kilometer-sized planetesimals. Electromagnetic forces work hard to create kilometer-sized planetesimals, and then gravity takes over to bind planetesimals together into planets. Depending on which materials dominate in a region, planetesimals are forming from those specific materials. And as planets grow larger, the number of pebbles around them decreases, meaning the rings are becoming clearer. From the soot (silicate) line’s ring, four rocky planets are forming: Mercury, Venus, Earth, and Mars. There are no planetesimals left between Mercury and the Sun. But something catastrophic seems to be happening in the frost line’s ring. What’s going on, Hermes?

Hermes: Among the three rings, the frost line’s ring contains the most material. Look at the mass distribution in the first panel of my diagram. The frost line’s ring holds material equivalent to about 85 Earth masses. Here, planetesimals have grown so large that they’ve begun attracting hydrogen gas. The addition of rock, ice, and gas has caused Jupiter to collapse inward due to gravity. A similar collapse may occur for Saturn, though it’s difficult to see clearly due to nebular dust. For Uranus and Neptune, it’s unclear whether collapse is occurring or planetesimals are still clumping together. Jupiter and Saturn are so massive that they are forming many moons around them, almost like mini solar systems of their own.

Socrates: Three minutes have passed. The Sun suddenly seems much calmer now.

Hermes: That’s because, Socrates, nuclear fusion has started in the Sun’s core. The pressure from nuclear reactions and gas counteracts gravity, preventing further contraction. The Sun is now a main-sequence star—a fully grown, adult star.

Socrates: Five minutes have passed. What just happened? Did another planet collide with Earth?

Hermes: Have you ever wondered why the Moon is so large relative to Earth? No other satellite in the solar system is proportionally as large as the Moon compared to its host planet. This collision is why. See how a huge amount of material has been ejected from Earth into space, but Earth’s gravity prevents it from escaping far. Within a million years, the scattered debris consolidates into a beautiful round Moon.

Socrates: The asteroid belt lies between Mars and Jupiter. You mentioned NC-type planetesimals migrating there from the soot line earlier, but now I see rocks coming here from outside as well.

Hermes: Yes, that’s shown with arrows in the last panel of my diagram. Red arrows indicate how asteroids are being flung into the asteroid belt by the influence of the inner four planets, while gray arrows show how CC-type asteroids from the outer four giant planets are entering the belt. This process created the asteroid belt.

Socrates: Our 15 minutes are up, and 150 million years of the solar system’s history are complete. Like the solar system, let’s rest for a while.

[Half an hour passes for our eight characters, during which 300 million more years of the solar system’s history unfold. Suddenly, the entire solar system is thrown into turmoil.]

Juno: What’s happening? The entire solar system seems to be in a massive world war. Because we’re experiencing time so quickly, the intensity of the war feels overwhelming. Planetesimals are crashing onto nearly all the planets and moons. Are the fragments of rocks, soil, and ice that failed to form planets or merge with any planet seeking revenge on them?

Hermes: This is called the Late Heavy Bombardment. Those that couldn’t form planets near the soot and frost lines have ended up in the asteroid belt, while those near the CO snow line became part of the Kuiper Belt. Their bombardment on planets and moons will continue for another half hour in our time—that’s 300 million years.

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?

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?

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.

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.

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.

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.

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?

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.

[The eight approach Earth and observe the northern polar region from above.]

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 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.

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 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 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.

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.

[The eight board a boat at Dibrugarh in Assam and travel along the Brahmaputra toward Dhubri.]

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?

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”?

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.

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?

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?

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.

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.

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.

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.

Socrates: What are super-Earths?

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.

Socrates: After hearing about water on Kepler-22b, I’ve lost interest in studying. Let’s watch the rhinos bathing in Kaziranga's Brahmaputra now.