Earth is part of a group of planets, moons, and smaller objects, such as asteroids. All these objects are grouped together because they all orbit around a star, which we call the Sun. The Sun is so big that its gravity pulls all these large, distant objects into its orbit. We call this group of objects in space the Solar System.

Our Solar System contains:

• One star (the Sun)
• Eight planets
• Five dwarf planets
• 293 moons
• Millions of asteroids, comets, and other objects

Light year: The distance light travels in one year—about 5.88 trillion miles

"Our Solar System was fully formed 4.568 billion years ago."

Our Solar System is big. Earth is 93 million miles from the Sun. The furthest planet (Neptune) is 2.8 billion miles from the Sun. The outer edge of the Oort cloud—a shell of trillions of icy objects that scientists believe orbit our Sun—is about 1.5 light-years from the Sun. That means that our Solar System is 3 light-years—18.6 trillion miles—across. Traveling as fast as we currently can, it would take human spacecraft tens of thousands of years to reach the edge of our Solar System.

Our Sun began its life about 5 billion years ago as a giant cloud of gas called a nebula. Thanks to gravity, all the gas in the nebula began to pull together, collapsing in on itself in a way that created immense heat and pressure. Eventually, this generated enough energy to begin the process of nuclear fusion, and a star was born—our Sun.

The Sun is the largest object in the Solar System. The next largest are the eight planets that orbit the Sun. There are two types of planets. The four inner planets, those closest to the Sun, are Mercury, Venus, Earth, and Mars. They are smaller and mostly made of rocks and metals. The four outer planets—Jupiter, Saturn, Uranus, and Neptune—are larger and made mostly of gases.

What is a planet?

According to the International Astronomical Union, to qualify as a planet, an object in space needs to do three things:

• Orbit a star
• Be big enough that its gravity has shaped it into a sphereBe big enough that it cleared away other objects in its orbit around its star

All the planets in our Solar System started out as clouds of gas and dust orbiting a new star—the Sun. Over time, all that gas and dust began to stick together and slowly built up into balls. In astrophysics, this process is called accretion. As all these clumps of matter began orbiting the Sun, they banged into each other and sometimes stuck together, creating bigger and bigger clumps. In our Solar System, these collisions continued until we had eight large planets that each revolved around the Sun in its own path. In 2007, researchers determined that our Solar System was fully formed about 4.568 billion years ago.

Why are there two types of planets in our Solar System?

The young Sun sent lots of energy and particles blasting across the early Solar System. These are called solar winds. These winds were so strong that they blew off the gases of the four planets closest to the Sun, leaving them smaller, with only their rocks and metals intact. That’s why we have four rocky planets close to the Sun. The four outer planets were so far from the Sun that the solar winds didn’t blow off their ice and gases. They stayed big and gaseous, with small, rocky cores.

Conditions on Earth

Earth is one of the four inner rocky planets, but it’s not quite the same as the other three (Mercury, Venus, and Mars). When the rocky planets first formed, they were mostly molten (melted) rock. Over hundreds of millions of years, they slowly cooled. Their surfaces solidified, but their cores stayed molten.

Three of the inner rocky planets—Earth, Venus, and Mars—developed a layer of gases around them. This layer of gases, such as oxygen, is called an atmosphere. Mercury, the smallest inner planet, is closest to the Sun, and solar winds have ripped away any gases that could form an atmosphere. Mars, on the other hand, is far enough away from the Sun to have an atmosphere. However, because Mars is much smaller than Earth, its gravity is weaker, and so its atmosphere is very, very thin. On Venus, a very thick atmosphere developed that makes it the hottest planet in our Solar System. Earth, however, was just the right distance from the Sun and developed an atmosphere that was just right: not too thick and not too thin. These special conditions created the Goldilocks Conditions necessary for life to flourish here on Earth. Earth was just right: neither too hot nor too cold; not too big nor too little; neither too near nor too far from the Sun.

Earth’s Moon

The object that’s closest to Earth is the Moon. The Moon orbits Earth, not the Sun, so it’s not a planet. The Moon is about one fourth the width of Earth. The Earth has about 80 times more mass than the Moon. The exact origin of the Moon remains a mystery, but we do know more about it than we once did: In 1969, astronauts walked on the Moon and brought back rock and soil samples, which gave scientists some important clues.

The most likely explanation for the origin of the Moon is this: Early in Earth’s development, when it was still banging into other bodies, accreting matter, and forming a sphere, another body—about the size of Mars—collided with it. Some materials broke away in the collision and bounced off. But they didn’t get far. Instead, these materials were caught in the Earth’s gravity, clumped together, and began to orbit the Earth. Over time, gravity pulled them together, and they merged into a sphere—the Moon.

The Moon affects conditions on Earth in important ways. The collision that created the Moon also tilted Earth on its axis. This tilt causes Earth’s variations in temperature over the year, since the side tilted toward the Sun (for half the year) receives more direct sunlight. The Moon’s gravity also causes the tides of Earth’s oceans, reduces the Earth’s wobble (which helps stabilize climate), and slows Earth’s spin.

The origins of our Solar System and Earth are well understood today. Still, each year, scientists uncover new evidence that helps us add detail to our picture of how our piece of the Universe came to be.