Excerpt
Introduction: The Birth of a Planet
Stars shine, comets streak across the sky, and asteroids tumble through space. But planets? Planets are different. They don’t glow on their own like stars, and they don’t zip around wildly like comets. They have a special way of existing—one that sets them apart in the vastness of space.
At first glance, a planet seems simple. It’s a big, round thing that moves around a star. Earth is one, Mars is one, Jupiter is one. Easy, right? Not exactly. Scientists spent centuries trying to define what makes a planet a planet. Even today, the definition is still debated. It wasn’t always clear what counted and what didn’t, and for a long time, astronomers weren’t sure how to tell the difference between planets and other space objects.
Long ago, people looked up and noticed that certain lights in the sky didn’t stay in the same place like the stars did. They moved. The ancient Greeks called them “wanderers” because they didn’t follow the usual starry patterns. These wanderers became what we now call planets. Back then, only a few were known—Mercury, Venus, Mars, Jupiter, and Saturn. That was it. No telescopes, no space probes, just bright dots in the sky that seemed to drift in their own mysterious way.
Everything changed when telescopes came along. Astronomers discovered Uranus and Neptune, and later, Pluto. They realized there were countless other things floating in space—asteroids, moons, and even distant icy worlds. The question became more complicated. Were all of these planets too? If not, where was the line?
To be a planet, an object has to follow three important rules. First, it must orbit a star. That’s why the Moon isn’t a planet—it circles Earth, not the Sun. Second, it has to be big enough for gravity to shape it into a ball. If something is too small, it might stay lumpy and irregular, like an asteroid. Planets need enough mass to pull themselves into a round shape. Third, and this is the tricky one, a planet has to clear its neighborhood. That means it must be the biggest thing in its orbit, sweeping up or pushing away smaller objects around it.
This last rule is what changed everything for Pluto. Pluto orbits the Sun, and it’s round, but it doesn’t dominate its part of space. Instead, it shares its orbit with other icy bodies in the Kuiper Belt, a region full of frozen leftovers from the early solar system. In 2006, astronomers decided that Pluto didn’t meet all three rules. It was reclassified as a “dwarf planet,” a name given to objects that are planet-like but don’t quite fit the definition. This upset a lot of people. Pluto had been a planet for 76 years, and suddenly, it wasn’t. But space doesn’t change just because we call something by a different name. Pluto is still there, still orbiting the Sun, still covered in ice and mystery.
If planets have to clear their neighborhood, does that mean Earth is still doing that today? Actually, yes. Earth has pulled in or pushed away most objects near its path, which is why there aren’t tons of asteroids zooming around at the same distance as our planet. Jupiter, the biggest planet in the solar system, is especially good at this—it uses its massive gravity to fling away space debris.
Not all planets are alike. Some are small and rocky, like Earth and Mars. Others are enormous and made mostly of gas, like Jupiter and Saturn. Even though they look completely different, they follow the same basic rules. But what about planets outside our solar system?
Astronomers have found thousands of planets orbiting other stars, called exoplanets. Some are much bigger than Jupiter, some are smaller than Mercury, and a few are even Earth-like, meaning they have solid surfaces and might have water. Since we can’t visit them (at least not yet), scientists use telescopes to study their size, temperature, and atmosphere. They’ve found some strange worlds—planets made of diamond, planets where it rains glass, and planets that orbit two stars instead of one. The universe is full of surprises.
How do planets form?
Space isn’t empty. It might look that way when you glance up at the night sky, but beyond the stars and planets, there’s a swirling sea of dust and gas stretching across the universe. This dust and gas might not seem like much—tiny bits of rock, ice, and elements floating aimlessly—but given enough time, these scattered particles can come together to form something as huge and complex as a planet.
The process begins inside a nebula, a massive cloud of gas and dust left behind by dying stars. Nebulas drift through space for millions of years, sometimes growing, sometimes shrinking, but mostly staying quiet. Then something changes. Maybe a nearby star explodes in a powerful blast called a supernova. Maybe two galaxies crash into each other, sending shockwaves rippling through space. Whatever the trigger, the peaceful cloud starts to stir.
Gravity takes over. This invisible force pulls everything toward the densest parts of the cloud, making it clump together. As the gas and dust collapse inward, they spin faster and flatten into a huge, spinning disk. Right in the center of that disk, something incredible is happening—a star is forming. The heat and pressure grow so intense that hydrogen atoms smash together, creating nuclear fusion. This is how a star is born.
But a star isn’t the only thing forming inside that disk. Away from the burning heat of the center, smaller clumps of dust and rock begin to stick together. It starts with tiny grains, no bigger than specks of sand. These grains bump into each other, sometimes breaking apart, sometimes sticking. When they stick, they form bigger pieces, growing from pebbles to boulders to massive chunks of rock called planetesimals.
Planetesimals are the building blocks of planets. They crash into each other constantly, some shattering into dust, others merging to form even larger bodies. The biggest ones pull in more and more material with their growing gravity, becoming something even larger—protoplanets. These are planets in the making, still growing, still fighting for space.
Not every protoplanet gets to become a full-fledged planet. Some collide violently, breaking apart into smaller pieces. Others are pulled too close to their star and burn up. Some never gather enough material to grow big enough, staying as asteroids or moons. But a few manage to hold on, growing larger, shaping themselves into spheres, and clearing out their orbits. These survivors become planets.
What kind of planet they become depends on where they form. Close to the star, where it’s hot, lighter materials like gas and ice get blasted away by powerful solar winds. This is where rocky planets like Earth, Mars, Venus, and Mercury form. They’re made mostly of metal and rock, materials that can withstand the intense heat.
Farther out, where it’s colder, gas and ice don’t get blown away as easily. Here, protoplanets can grow much larger because they’re able to pull in huge amounts of hydrogen and helium. This is how the gas giants—Jupiter and Saturn—and the ice giants—Uranus and Neptune—form. Instead of solid surfaces, they have thick atmospheres made of swirling clouds and storms.
Some planets don’t form alone. Moons can take shape in the same way, forming from leftover dust and debris orbiting a planet. Sometimes, a planet’s gravity pulls in passing asteroids, trapping them as moons. Other times, a giant collision can create a moon from the debris, like how Earth’s moon was likely formed when a massive object slammed into our planet long ago.
Stars shine, comets streak across the sky, and asteroids tumble through space. But planets? Planets are different. They don’t glow on their own like stars, and they don’t zip around wildly like comets. They have a special way of existing—one that sets them apart in the vastness of space.
At first glance, a planet seems simple. It’s a big, round thing that moves around a star. Earth is one, Mars is one, Jupiter is one. Easy, right? Not exactly. Scientists spent centuries trying to define what makes a planet a planet. Even today, the definition is still debated. It wasn’t always clear what counted and what didn’t, and for a long time, astronomers weren’t sure how to tell the difference between planets and other space objects.
Long ago, people looked up and noticed that certain lights in the sky didn’t stay in the same place like the stars did. They moved. The ancient Greeks called them “wanderers” because they didn’t follow the usual starry patterns. These wanderers became what we now call planets. Back then, only a few were known—Mercury, Venus, Mars, Jupiter, and Saturn. That was it. No telescopes, no space probes, just bright dots in the sky that seemed to drift in their own mysterious way.
Everything changed when telescopes came along. Astronomers discovered Uranus and Neptune, and later, Pluto. They realized there were countless other things floating in space—asteroids, moons, and even distant icy worlds. The question became more complicated. Were all of these planets too? If not, where was the line?
To be a planet, an object has to follow three important rules. First, it must orbit a star. That’s why the Moon isn’t a planet—it circles Earth, not the Sun. Second, it has to be big enough for gravity to shape it into a ball. If something is too small, it might stay lumpy and irregular, like an asteroid. Planets need enough mass to pull themselves into a round shape. Third, and this is the tricky one, a planet has to clear its neighborhood. That means it must be the biggest thing in its orbit, sweeping up or pushing away smaller objects around it.
This last rule is what changed everything for Pluto. Pluto orbits the Sun, and it’s round, but it doesn’t dominate its part of space. Instead, it shares its orbit with other icy bodies in the Kuiper Belt, a region full of frozen leftovers from the early solar system. In 2006, astronomers decided that Pluto didn’t meet all three rules. It was reclassified as a “dwarf planet,” a name given to objects that are planet-like but don’t quite fit the definition. This upset a lot of people. Pluto had been a planet for 76 years, and suddenly, it wasn’t. But space doesn’t change just because we call something by a different name. Pluto is still there, still orbiting the Sun, still covered in ice and mystery.
If planets have to clear their neighborhood, does that mean Earth is still doing that today? Actually, yes. Earth has pulled in or pushed away most objects near its path, which is why there aren’t tons of asteroids zooming around at the same distance as our planet. Jupiter, the biggest planet in the solar system, is especially good at this—it uses its massive gravity to fling away space debris.
Not all planets are alike. Some are small and rocky, like Earth and Mars. Others are enormous and made mostly of gas, like Jupiter and Saturn. Even though they look completely different, they follow the same basic rules. But what about planets outside our solar system?
Astronomers have found thousands of planets orbiting other stars, called exoplanets. Some are much bigger than Jupiter, some are smaller than Mercury, and a few are even Earth-like, meaning they have solid surfaces and might have water. Since we can’t visit them (at least not yet), scientists use telescopes to study their size, temperature, and atmosphere. They’ve found some strange worlds—planets made of diamond, planets where it rains glass, and planets that orbit two stars instead of one. The universe is full of surprises.
How do planets form?
Space isn’t empty. It might look that way when you glance up at the night sky, but beyond the stars and planets, there’s a swirling sea of dust and gas stretching across the universe. This dust and gas might not seem like much—tiny bits of rock, ice, and elements floating aimlessly—but given enough time, these scattered particles can come together to form something as huge and complex as a planet.
The process begins inside a nebula, a massive cloud of gas and dust left behind by dying stars. Nebulas drift through space for millions of years, sometimes growing, sometimes shrinking, but mostly staying quiet. Then something changes. Maybe a nearby star explodes in a powerful blast called a supernova. Maybe two galaxies crash into each other, sending shockwaves rippling through space. Whatever the trigger, the peaceful cloud starts to stir.
Gravity takes over. This invisible force pulls everything toward the densest parts of the cloud, making it clump together. As the gas and dust collapse inward, they spin faster and flatten into a huge, spinning disk. Right in the center of that disk, something incredible is happening—a star is forming. The heat and pressure grow so intense that hydrogen atoms smash together, creating nuclear fusion. This is how a star is born.
But a star isn’t the only thing forming inside that disk. Away from the burning heat of the center, smaller clumps of dust and rock begin to stick together. It starts with tiny grains, no bigger than specks of sand. These grains bump into each other, sometimes breaking apart, sometimes sticking. When they stick, they form bigger pieces, growing from pebbles to boulders to massive chunks of rock called planetesimals.
Planetesimals are the building blocks of planets. They crash into each other constantly, some shattering into dust, others merging to form even larger bodies. The biggest ones pull in more and more material with their growing gravity, becoming something even larger—protoplanets. These are planets in the making, still growing, still fighting for space.
Not every protoplanet gets to become a full-fledged planet. Some collide violently, breaking apart into smaller pieces. Others are pulled too close to their star and burn up. Some never gather enough material to grow big enough, staying as asteroids or moons. But a few manage to hold on, growing larger, shaping themselves into spheres, and clearing out their orbits. These survivors become planets.
What kind of planet they become depends on where they form. Close to the star, where it’s hot, lighter materials like gas and ice get blasted away by powerful solar winds. This is where rocky planets like Earth, Mars, Venus, and Mercury form. They’re made mostly of metal and rock, materials that can withstand the intense heat.
Farther out, where it’s colder, gas and ice don’t get blown away as easily. Here, protoplanets can grow much larger because they’re able to pull in huge amounts of hydrogen and helium. This is how the gas giants—Jupiter and Saturn—and the ice giants—Uranus and Neptune—form. Instead of solid surfaces, they have thick atmospheres made of swirling clouds and storms.
Some planets don’t form alone. Moons can take shape in the same way, forming from leftover dust and debris orbiting a planet. Sometimes, a planet’s gravity pulls in passing asteroids, trapping them as moons. Other times, a giant collision can create a moon from the debris, like how Earth’s moon was likely formed when a massive object slammed into our planet long ago.