Excerpt
Chapter 1: Welcome to Planet Earth
Earth is where we live, and it’s part of something much bigger: the solar system. That’s the group of objects, including planets, that move around the Sun. Our planet is one of eight in that group. But what even makes something a planet?
A planet isn’t just a big rock floating in space. To be a planet, an object has to meet three main rules. First, it has to orbit a star. In Earth’s case, that star is the Sun. Second, it has to be big enough that gravity pulls it into a round shape. And third, it has to have cleared other objects out of its path as it moves around the star. That last part means a planet uses its gravity to either pull in or push away smaller things near its orbit.
That’s why Earth is a planet—and why things like Pluto aren’t called planets anymore. Pluto orbits the Sun and is round, but it hasn’t cleared its orbit. There are still many icy objects moving near Pluto in the Kuiper Belt, a distant part of our solar system. That’s what makes Pluto a “dwarf planet” instead of a full planet like Earth.
Planets come in different types. Some, like Earth, are rocky. These are called terrestrial planets. They have solid surfaces, mountains, valleys, and sometimes even volcanoes. Other planets, like Jupiter and Saturn, are gas giants. They don’t have a solid surface, and they’re mostly made of gases like hydrogen and helium. There are also ice giants, like Uranus and Neptune, which are made of a mix of ice, gas, and rock.
Earth is the third planet from the Sun. That spot in the lineup is really important. It’s not too hot, like Mercury and Venus, and not too cold, like the outer planets. This “just right” zone is called the habitable zone, and it’s part of the reason Earth can support life.
When scientists study planets, they look at a few key things. One is the size. Earth isn’t the biggest, but it’s also not the smallest. It’s about 7,900 miles (12,700 kilometers) wide. That’s enough space to hold billions of people, animals, trees, mountains, oceans, and more. Another thing scientists look at is what the planet is made of. Earth is made mostly of rock and metal, with a thin outer layer called the crust and a hot center called the core.
Planets also have something called gravity. Gravity is a force that pulls things toward the planet’s center. Earth’s gravity is what keeps your feet on the ground. It’s also what keeps the Moon orbiting Earth and what holds our atmosphere in place. If Earth didn’t have gravity, everything—including air and water—would float off into space.
Not all planets have moons, but Earth does. The Moon is Earth’s only natural satellite. Some planets, like Jupiter, have dozens of moons. Mercury and Venus don’t have any. Earth’s Moon is special because it helps control the ocean tides. Its gravity pulls on Earth’s oceans, creating high and low tides each day. That movement of water helps shape coastlines and affects life in the ocean.
Planets can also have different kinds of surfaces. Earth’s surface is full of features like mountains, rivers, forests, and deserts. Other planets have their own features. Mars has the tallest volcano in the solar system. Venus has thick clouds of acid. Jupiter doesn’t have a solid surface at all—it’s mostly swirling gas. Even though Earth isn’t the biggest or the flashiest, it has something no other known planet has: life.
That’s the thing that makes Earth stand out more than anything else. So far, scientists haven’t found any other planet where life can survive the way it does here. Earth has the right temperature, water in liquid form, an atmosphere with oxygen, and a magnetic field that protects it from harmful space radiation. All those things work together to make Earth a home.
Our planet also moves in two important ways. It rotates, or spins, once every 24 hours. That’s what gives us day and night. It also revolves around the Sun, which takes about 365 days. That’s what gives us a year. The way Earth moves and the angle it’s tilted at also create the seasons.
Earth’s orbit isn’t a perfect circle. It’s a shape called an ellipse, which means it’s a bit stretched out. Even though Earth gets a little closer or farther from the Sun during its orbit, the seasons are actually caused more by the tilt of the planet than by the distance. When the Northern Hemisphere is tilted toward the Sun, it gets more sunlight and has summer. When it’s tilted away, it has winter.
Being a planet also means being part of a system. Earth doesn’t move through space alone. It’s part of the solar system, which includes the Sun, the eight planets, moons, asteroids, comets, and other objects. All of these things are held together by the Sun’s gravity. Even though the planets are millions of miles apart, they’re still connected by the way they move and affect one another.
Earth has a lot in common with other planets, but it also has many things that make it unique. Its breathable air, liquid water, and surface temperatures all help support a wide variety of life. From the smallest bacteria to the largest whales, every living thing depends on Earth’s systems working together.
Earth is where we live, and it’s part of something much bigger: the solar system. That’s the group of objects, including planets, that move around the Sun. Our planet is one of eight in that group. But what even makes something a planet?
A planet isn’t just a big rock floating in space. To be a planet, an object has to meet three main rules. First, it has to orbit a star. In Earth’s case, that star is the Sun. Second, it has to be big enough that gravity pulls it into a round shape. And third, it has to have cleared other objects out of its path as it moves around the star. That last part means a planet uses its gravity to either pull in or push away smaller things near its orbit.
That’s why Earth is a planet—and why things like Pluto aren’t called planets anymore. Pluto orbits the Sun and is round, but it hasn’t cleared its orbit. There are still many icy objects moving near Pluto in the Kuiper Belt, a distant part of our solar system. That’s what makes Pluto a “dwarf planet” instead of a full planet like Earth.
Planets come in different types. Some, like Earth, are rocky. These are called terrestrial planets. They have solid surfaces, mountains, valleys, and sometimes even volcanoes. Other planets, like Jupiter and Saturn, are gas giants. They don’t have a solid surface, and they’re mostly made of gases like hydrogen and helium. There are also ice giants, like Uranus and Neptune, which are made of a mix of ice, gas, and rock.
Earth is the third planet from the Sun. That spot in the lineup is really important. It’s not too hot, like Mercury and Venus, and not too cold, like the outer planets. This “just right” zone is called the habitable zone, and it’s part of the reason Earth can support life.
When scientists study planets, they look at a few key things. One is the size. Earth isn’t the biggest, but it’s also not the smallest. It’s about 7,900 miles (12,700 kilometers) wide. That’s enough space to hold billions of people, animals, trees, mountains, oceans, and more. Another thing scientists look at is what the planet is made of. Earth is made mostly of rock and metal, with a thin outer layer called the crust and a hot center called the core.
Planets also have something called gravity. Gravity is a force that pulls things toward the planet’s center. Earth’s gravity is what keeps your feet on the ground. It’s also what keeps the Moon orbiting Earth and what holds our atmosphere in place. If Earth didn’t have gravity, everything—including air and water—would float off into space.
Not all planets have moons, but Earth does. The Moon is Earth’s only natural satellite. Some planets, like Jupiter, have dozens of moons. Mercury and Venus don’t have any. Earth’s Moon is special because it helps control the ocean tides. Its gravity pulls on Earth’s oceans, creating high and low tides each day. That movement of water helps shape coastlines and affects life in the ocean.
Planets can also have different kinds of surfaces. Earth’s surface is full of features like mountains, rivers, forests, and deserts. Other planets have their own features. Mars has the tallest volcano in the solar system. Venus has thick clouds of acid. Jupiter doesn’t have a solid surface at all—it’s mostly swirling gas. Even though Earth isn’t the biggest or the flashiest, it has something no other known planet has: life.
That’s the thing that makes Earth stand out more than anything else. So far, scientists haven’t found any other planet where life can survive the way it does here. Earth has the right temperature, water in liquid form, an atmosphere with oxygen, and a magnetic field that protects it from harmful space radiation. All those things work together to make Earth a home.
Our planet also moves in two important ways. It rotates, or spins, once every 24 hours. That’s what gives us day and night. It also revolves around the Sun, which takes about 365 days. That’s what gives us a year. The way Earth moves and the angle it’s tilted at also create the seasons.
Earth’s orbit isn’t a perfect circle. It’s a shape called an ellipse, which means it’s a bit stretched out. Even though Earth gets a little closer or farther from the Sun during its orbit, the seasons are actually caused more by the tilt of the planet than by the distance. When the Northern Hemisphere is tilted toward the Sun, it gets more sunlight and has summer. When it’s tilted away, it has winter.
Being a planet also means being part of a system. Earth doesn’t move through space alone. It’s part of the solar system, which includes the Sun, the eight planets, moons, asteroids, comets, and other objects. All of these things are held together by the Sun’s gravity. Even though the planets are millions of miles apart, they’re still connected by the way they move and affect one another.
Earth has a lot in common with other planets, but it also has many things that make it unique. Its breathable air, liquid water, and surface temperatures all help support a wide variety of life. From the smallest bacteria to the largest whales, every living thing depends on Earth’s systems working together.