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 ====== 4. Planetary Age ======
-**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.
+===== - Timeline =====
-**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?
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+    <title>Planetary Age Table</title>
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-**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.
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-**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.
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-**Ishtar:** Starting now. In 15 minutes, the solar system will be born. Let’s see if you can narrate its creation within that time.
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-===== - Birth of the Solar System =====
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-**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.
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-**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.
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-**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.
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-**Socrates:** That’s quite an elegant theory.
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-**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.
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-**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.
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-**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.
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-**Socrates:** What kind of influence do they have?
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-**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.
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-**Socrates:** One minute has passed. The material near the three lines is condensing further.
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-**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.
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-**Socrates:** Wait, wait! Are you going to drown us in jargon?
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-**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.
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+                <div class="col-time">9.1 Gy</div>
+                <div class="col-title">Accretion of Planetesimals</div>
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+                    Following the gravitational collapse of the solar nebula, the newly formed protoplanetary disk became a busy theater of cosmic construction. Within this swirling disk, microscopic dust grains began to collide and stick together through electrostatic forces, gradually growing into larger clumps. As these clumps accumulated more mass, their gravitational pull increased, allowing them to rapidly accrete surrounding material to form kilometer-sized bodies known as planetesimals. These planetesimals continuously collided and merged over millions of years, eventually coalescing into the distinct protoplanets that would become the inner terrestrial and outer Jovian worlds of our Solar System.
+                </div>
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-**Socrates:** Hold on, hold on. Why are the three lines shown closer to the Sun in your middle panel compared to the first one?
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+                <div class="col-time">9.1 Gy</div>
+                <div class="col-title">The T-Tauri Solar Wind</div>
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+                    During this critical period of planetary formation, the young Sun entered a highly erratic and energetic stage of its evolution known as the T-Tauri phase. Characterized by violent magnetic activity, the youthful star began to generate incredibly intense solar winds that radiated outward across the solar system. This relentless stream of charged particles effectively cleared the remaining primordial gas and dust from the protoplanetary disk. Consequently, this stellar clearing process halted the rapid gas accumulation of the massive Jovian planets and aggressively stripped away the delicate, primordial atmospheres of the inner terrestrial worlds.
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-**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.
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+                <div class="col-time">9.2 Gy</div>
+                <div class="col-title">Planetary Differentiation</div>
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+                    As the early Earth continued to accrete mass, the immense kinetic energy from constant impacts, combined with the heat released from the decay of short-lived radioactive isotopes, caused the entire young planet to melt. This molten state allowed for a profound global reorganization known as planetary differentiation. Governed by gravity, heavy metallic elements, primarily iron and nickel, sank toward the center to form a dense, churning core, which would eventually generate the planet’s protective magnetic field. Simultaneously, lighter silicate minerals floated outward to form a thick, insulating mantle and a primitive, cooling crust.
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-**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?
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+                <div class="col-title">Formation of the Moon</div>
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+                    Amidst the chaotic environment of the early Solar System, the newly differentiated Earth experienced a cataclysmic collision that would forever alter its developmental trajectory. A rogue, Mars-sized protoplanet, often referred to as Theia, smashed into the young Earth with unimaginable force. The monumental impact liquefied the planet's surface once again and ejected a colossal volume of vaporized crust and mantle into orbit. This superheated debris rapidly formed a dense ring around the Earth, which soon accreted under its own gravity to form the Moon, establishing a crucial gravitational relationship that would later stabilize Earth's axial tilt.
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-**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 their gravity to collapse inward, forming Jupiter. 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.
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+                    As the violently disrupted Earth gradually cooled, extensive volcanic activity dominated its recovering surface, initiating a massive outgassing of trapped interior volatiles. Endless eruptions expelled vast quantities of water vapor, carbon dioxide, and other gases, forming a dense secondary atmosphere to replace the one lost to early solar winds. As global temperatures continued to fall below the boiling point, this atmospheric water vapor finally condensed, unleashing torrential, planet-wide rains. Over millions of years, these deluges, augmented by the icy deliveries from continuous comet impacts, filled the low-lying impact basins to form the world's first primordial oceans.
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-**Socrates:** Three minutes have passed. The Sun suddenly seems much calmer now.
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+                <div class="col-time">9.6 Gy – 10 Gy</div>
+                <div class="col-title">Late Heavy Bombardment</div>
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+                    Despite the formation of early oceans, the inner Solar System remained an incredibly violent neighborhood. A prolonged period of catastrophic cosmic impacts, known as the Late Heavy Bombardment, subjected the terrestrial planets to a relentless barrage of leftover asteroids and comets. This intense storm of debris heavily scarred planetary surfaces, repeatedly pulverized Earth's fragile early crust, and routinely vaporized portions of the newly formed oceans. This chaotic "cleaning up" phase of solar system evolution ensured that the Earth's surface remained a hostile, largely molten battlefield for hundreds of millions of years.
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-**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.
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+                <div class="col-time">10 Gy – 11 Gy</div>
+                <div class="col-title">Stabilization of the Lithosphere</div>
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+                    Eventually, the frequency of massive impacts subsided, allowing the Planetary Age to draw to a close through the gradual stabilization of Earth's solid lithosphere. The crust thickened and cooled sufficiently to support the formation of the first permanent continental landmasses, such as the ancient supercontinent Vaalbara. The establishment of a stable rock cycle and the persistent presence of liquid water oceans fundamentally transformed the Earth from a violent, hellish sphere into a relatively tranquil world. This newfound geological stability provided the crucial, protected environments required to ignite the complex chemical evolution that would soon follow.
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-**Socrates:** Five minutes have passed. What just happened? Did another planet collide with Earth?
+    </div>
-**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.
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-**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.
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+</html>
-**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 collisions among 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.
+===== - Telescope =====
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-**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 war. Because we’re experiencing time so quickly, the intensity of the chaos 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 900 million years of the solar system. Now, Hermes, can you give us an overview of this world?
-Let me know if you'd like the continuation or any other adjustments!
-===== - Solar System =====
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-===== - Earth =====
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+The Kepler Space Telescope, the definitive instrument for the Planetary Age in the AST 100 curriculum, revolutionized our understanding of the cosmos by transitioning exoplanet study from theory to statistical reality. Launched in 2009, Kepler utilized a high-resolution photometer to monitor over 150,000 stars simultaneously in a fixed field of view within the Cygnus and Lyra constellations. By detecting the minute, periodic dimming of starlight caused by a planet crossing in front of its host star—a technique known as the transit method—Kepler proved that planets are ubiquitous throughout our galaxy.
-===== - Detecting Planets =====
+Technologically, Kepler’s mission was defined by its incredible precision, capable of detecting brightness changes as small as twenty parts per million. This sensitivity allowed it to identify "Earth-size" planets orbiting within the habitable zones of Sun-like stars, where liquid water could potentially exist on the surface. The data history of the mission includes the discovery of over 2,700 confirmed exoplanets and thousands of additional candidates, revealing a startling diversity of worlds, from "Super-Earths" to "Hot Jupiters," that challenged existing models of solar system formation and paved the way for future atmospheric studies.
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-===== - Classification of Planets =====
+In the broader context of cosmic history, Kepler’s legacy provides the essential bridge between the Stellar Age and the Chemical Age. By identifying the frequency of planetary systems, it shifted the scientific focus from how stars form to how frequently they produce environments capable of hosting complex chemistry. Although the primary mission ended in 2018, its vast archive of data continues to be mined by researchers and "citizens of the universe" alike. These discoveries ensure that the study of the Planetary Age remains a cornerstone of our efforts to map our place among the stars.
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