Objects in our Solar System

Structure of Our Solar System
Did you know that when you look up at the night sky, you are looking at more than just stars and the Moon? Some of the bright objects that you see are actually the planets in our solar system. The Sun and the objects that move around it make up the solar system. These celestial bodies are bound to the Sun by gravity. The planets and their moons, dwarf planets, meteors, asteroids, and comets are all members of our cosmic neighborhood, the solar system.

Distances in Space
Have you ever wondered how the distances between objects in space are measured? The units of measurement that we use here on Earth such as meters (m) become overwhelming when measuring vast distances in space. Objects in the solar system are measured in a larger measurement unit called an astronomical unit. An astronomical unit (AU) is the average distance from Earth to the Sun—about 150 million km. For example, Neptune's distance from the Sun is 30.05 AU. Neptune is 30.05 times farther from the Sun than Earth.

Solar System Objects
Our solar system formed 4.5 billion years ago when gravity caused a dense cloud of interstellar gas, ice, and dust called a nebula to collapse. The rotating cloud flattened into a disk. Gravity pulled gas and dust toward the center of the disk, forming the Sun. Next, the planets began to form as gravity continued to pull particles together. As they collided, they stuck to each other and formed larger objects that were shaped into spheres by gravity. The leftover particles formed into moons, asteroids, meteors, dwarf planets, and comets.

The Sun
The Sun, a star, is the largest object in the solar system. The Sun is the star in the center of our solar system around which the planets revolve and receive light. Its diameter is about 1.4 million km—about 109 times the diameter of Earth. The Sun is made up of about 71% hydrogen and 27% helium, with other elements in much smaller quantities. It is not solid but is made up of gases in the form of plasma. Its mass is about 99 percent of the entire solar system's mass. The hottest part of the Sun is its core where temperatures reach up to 15 million degrees Celsius.

Unlike the other objects in the solar system that reflect light, the Sun produces its own light. The Sun's core has very high temperatures and high pressure. Inside the core, nuclear fusion produces an enormous amount of energy. Fusion occurs when two atoms combine to make a heavier element, which causes the release of large amounts of energy. The Sun emits some of this energy as light, which is reflected off of objects in the solar system.

‘You may think of the Sun as a fixed central object of our solar system, but the Sun is constantly moving through space. The Sun orbits the center of the Milky Way galaxy, just as the planets orbit the Sun. It takes the Sun 230 million years to complete its orbit around the Milky Way. The Sun also rotates on its axis. Different parts of the Sun rotate at different rates because the Sun is gaseous. The gravitational pull from the planets causes the Sun to wobble a bit.

Planets and Moons
Astronomers classify the eight largest objects that orbit the Sun as planets. A planet is an object that orbits a star, is large enough to be nearly spherical in shape, and has no other large object in its orbital path. Additionally, the mass of a planet must be much larger than the total mass of all other objects whose orbits are close by, such as natural satellites. All of the planets move around the Sun in the same direction, but the. lengths of their journeys vary depending on how far the planet is from the Sun. Each planet rotates on an axis as it revolves around the Sun.

Inner Planets
Imagine you are hiking outdoors. How would you describe the ground? You might say that itis dusty, grassy, or wet. Beneath the ground is a layer of solid rock. The inner planets—Mercury, Venus, Earth, and Mars—are called terrestrial planets.

The terrestrial planets are the planets closest to the Sun that are made of rock and metallic minerals and have solid outer layers. The inner planets are called “terrestrial” because they are made of rock and metal and have solid outer layers They have different sizes, atmospheres, and surfaces. The inner planets are much closer together than the outer planets are to each other. They are also smaller and denser than the outer planets. The asteroid belt separates the inner planets from the outer planets.

Outer Planets
Have you ever seen water drops on the outside of a glass with ice in it? They form because water vapor in the air changes to a liquid on the cold glass. Gases also change to liquids at high pressures. At even higher pressures, liquids become ices. These properties of gases affect the outer planets. The outer planets are Jupiter, Saturn, Uranus (YOOR uh nus), and Neptune, and are the farthest planets from the Sun in our solar system

The outer planets are much larger than Earth. Jupiter and Saturn are sometimes called gas giants. They get their distinction as gas giants because they are primarily made of hydrogen and helium, elements that are usually gases on Earth. Because their interiors are compositionally different from Jupiter and Saturn, Uranus and Neptune are often called ice giants.

Dwarf Planets
The definition of a planet has changed over time as we advance our knowledge of the universe. Some objects in the solar system are classified as dwarf planets. Our old friend, Pluto, was once classified as a planet but is now a dwarf planet. A dwarf planet is an object that orbits a star and is nearly spherical in shape but shares its orbital path with other objects of similar size but also isn't classified as a satellite.

Moons
A moon is a natural satellite of a planet. The inner planets have few or no moons orbiting them. Earth has one moon, named the Moon. In contrast, the outer planets have many moons that orbit them. Our solar system is home to over 200 Moons

Some dwarf planets and asteroids even have small moons that orbit them. The formation of most moons resulted from gas and dust moving around the planets in the early solar system. Small moons are usually solid large ones can have a large ocean under an ice-covered surface. Some even have atmospheres. Moons also rotate on an axis. Have you ever heard of the dark side of the Moon? Itis not actually dark~—it just appears dark to us. We always see the same side of the Moon because it has the same period of rotation as its period of revolution around Earth.

Meteors and Asteroids
Have you ever wished upon a shooting star? What you know as a shooting star is. actually not a star at all, but instead it is a small piece of material that burns up as it enters Earth's atmosphere. It is called a meteor. A meteor is a meteoroid that has entered Earth's atmosphere and produces a streak of light. Before they enter Earth's atmosphere, they are called meteoroids. A meteoroid is a small, rocky particle that moves through space made up of rock and metal. Meteoroids can be dust-sized grains up to a meter in diameter. When a meteor impacts the surface of a planet or a moon, it is called a meteorite and creates an impact crater.
Meteoroids orbit the Sun and can be found throughout the solar system.

Millions of small, rocky objects called asteroids orbit the Sun in the asteroid belt between the orbits of Mars and Jupiter. An asteroid is a small rocky object that orbits the Sun. Unlike planets and dwarf planets, asteroids usually are not spherical, but are more irregularly shaped due to their smaller sizes resulting in a weaker gravitational pull

Comets
A comet is a small, rocky, icy object that orbits the Sun. A comet is made of gas, dust, and ice and moves around the Sun in a highly elliptical orbit. The particles in a comet are loosely held together by the gravitational attractions among the particles. There might be 1 trillion comets orbiting the Sun. As a comet moves closer to the Sun, it absorbs thermal energy and can develop a bright tail. Heating changes the ice in the comet into a gas. Energy from the Sun pushes some of the gas and dust away from the nucleus and makes it glow. This produces the comet's bright tail and glowing nucleus, called a coma.

Kuiper Belt and Oort Cloud
Beyond Neptune, in the far reaches of our solar system are many icy objects that astronomers consider to be the leftovers from the formation of the solar system. Both the Kuiper Belt and the Oort Cloud are understood by astronomers to be where comets come from.

Kuiper Belt
The Kuiper Belt is a band of small celestial bodies beyond the orbit of Neptune. Shaped like a donut, the Kuiper Belt is home to millions of small objects made of rocks and frozen water, ammonia, methane, and other gases. Itis about 30 AU from the Sun, starting at Neptune's orbit. The outermost boundary of the Kuiper Belt expands to almost 1,000 AU. Objects in the Kuiper Beit are referred to as trans-Neptunian objects. Objects moving throughout the Kuiper Belt are not evenly distributed and differ in their shapes and sizes. The dwarf planet Pluto is part of the Kuiper Belt,

Oort Cloud
The Oort Cloud is a spherical shell of celestial bodies believed to surround the Sun far beyond the orbits of the outermost planets. Contrary to the flatter, donut shape of the Kuiper Belt, the Oort Cloud is a spherical shell that surrounds our solar system,