Our Solar System is a vast neighborhood in space, centered around our Sun, filled with planets, moons, and countless smaller objects, all held together by gravity.

Key idea: The Sun is the massive center of our Solar System, generating energy through nuclear fusion and creating a heliosphere that influences everything around it.

The Sun is the heart of our Solar System. It is incredibly huge, making up almost all the mass in our entire system. To be exact, the Sun holds 99.86% of the Solar System's total mass. Inside the Sun's core, tiny particles of hydrogen combine to form helium in a process called nuclear fusion. This process releases a huge amount of energy, which we see and feel as sunlight and heat. This energy creates a giant bubble of influence around the Sun called the heliosphere, and it also causes temperatures to get colder and colder the farther you get from the Sun.

Key idea: The Solar System has eight main planets: four rocky inner planets (Mercury, Venus, Earth, Mars) and four giant outer planets (Jupiter, Saturn, Uranus, Neptune).

After the Sun, the next most important objects are the eight planets. These planets are so big that their gravity has cleared out their orbital paths, meaning they are the dominant objects in their neighborhoods. The planets closest to the Sun are called the inner planets: Mercury, Venus, Earth, and Mars. These are also known as terrestrial planets because they have solid, rocky surfaces. Earth and Mars are special because they are in the Sun's habitable zone, where it is just the right temperature for liquid water to exist on their surfaces. Farther out, beyond a point called the frost line, are the outer planets: Jupiter, Saturn, Uranus, and Neptune. Jupiter and Saturn are called gas giants because they are mostly made of gas. Uranus and Neptune are called ice giants because they contain a lot of icy materials. Jupiter and Saturn together hold almost 90% of all the mass in the Solar System that isn't the Sun.

“Earth and Mars are special because they are in the Sun's habitable zone, where it is just the right temperature for liquid water to exist on their surfaces.”

Key idea: Beyond the main planets, the Solar System contains dwarf planets, asteroids in the asteroid belt, icy objects in the Kuiper belt, and moons orbiting planets.

Besides the main planets, there are many other fascinating objects. Dwarf planets are like smaller planets; they orbit the Sun and are round, but they haven't cleared their orbital paths of other debris. Examples include Ceres, Pluto, and Eris. Even smaller are countless small Solar System bodies, such as asteroids, comets, and meteoroids. Many asteroids, which are rocky chunks, are found in the asteroid belt between Mars and Jupiter. Other dwarf planets and icy objects are in the Kuiper belt, which is beyond Neptune. Some objects do not orbit the Sun directly. Instead, they orbit planets; these are called natural satellites or moons. For example, Earth has one moon. Some moons, like Jupiter's Ganymede and Saturn's Titan, are even larger than the planet Mercury!

Key idea: The Solar System extends far beyond the planets, with the heliopause marking the edge of the Sun's direct influence and the Oort cloud representing its outermost gravitational boundary.

The space between all these objects is not empty. It is filled with a thin mix of charged particles from the Sun (called the solar wind), dust, gas, and cosmic rays. This is known as the interplanetary medium. The solar wind creates the heliosphere, a giant bubble around our Solar System. This bubble extends far beyond the planets. At about 70 to 90 times the distance from the Earth to the Sun, the solar wind slows down as it hits the gas and dust between stars. This boundary is called the heliopause, and it marks the edge of the Sun's direct influence. Even farther out, much beyond the heliopause, scientists believe there is a huge, spherical cloud of icy objects called the Oort cloud. This cloud is thought to be the source of many comets and marks the true outer edge of our Solar System, where the Sun's gravity is no longer the strongest force.

Key idea: Our understanding of the Solar System evolved from Earth-centered views to the Sun-centered model, greatly advanced by telescopes and space probes.

Our understanding of the Solar System has grown a lot over time. For a long time, people thought Earth was the center of everything. This idea is called geocentrism. Then, scientists like Nicolaus Copernicus and Johannes Kepler showed that the planets actually orbit the Sun. This is called heliocentrism. Galileo used telescopes to see moons orbiting Jupiter, which further supported the idea that not everything orbits Earth. Isaac Newton later explained the laws of gravity that govern all these movements. In more recent times, space probes have visited all the main planets and even some dwarf planets and comets, giving us amazing close up views and data. For example, the Voyager probes have traveled so far that they have crossed the heliopause and are now in interstellar space. These missions continue to teach us new things about our cosmic home.

“Isaac Newton later explained the laws of gravity that govern all these movements.”