The Solar System was formed 4.6 billion years ago by the gravitational collapse of an interstellar molecular cloud. It contains the Sun and its eight planets and their moons, plus a belt of icy dwarf planets and many asteroids.
At the outer edge of our solar system, beyond the Kuiper belt, is the Oort cloud, which is a spherical shell containing thousands of small ice objects. It is estimated that it extends to a distance of around 100,000 astronomical units (AU).
The Sun
The Sun is one of the most well-known stars in the world, and it’s the center of our solar system. It formed around 4.5 billion years ago from the remnants of a cloud of gas that collapsed in the outer part of our galaxy.
Our sun is a glowing, spinning ball of very hot gases primarily hydrogen (92.1%) and helium (7.9%), plus trace amounts of oxygen, carbon, nitrogen, silicon, magnesium and iron. The heat causes these gases to become plasma – a soup of ionized, electrically charged particles. More matter exists as a plasma than as any other form, and it’s a key reason the universe is so bright and vibrant.
Next, the core extends outward from the surface of the Sun in a region called the radiative zone, which accounts for about 45 percent of its radius. The energy that comes from the core is carried outward by radiation, or photons, which travel about 1 micron (1 millionth of a meter) before being absorbed by a gas molecule and then re-emitted.
After the radiative zone, the convective zone begins, which takes up about 30 percent of the Sun’s radius. There, hot bubbles of plasma – a soup of ionized atoms – begin to “boil” outward and toward the photosphere, or the visible layer of the sun’s surface.
In the convective zone, temperature reaches about 3.5 million degF (2 million degC), a few hundred degrees below that of the radiative zone. There, atoms of hydrogen and helium become very hot, so they start to interact with each other, forming large bubbles of hot plasma that are then heated further by thermal columns until they reach the edge of the convective zone.
Planets
In order for a planet to orbit, it needs to have a strong gravitational attraction toward the Sun. In the case of our solar system, this attraction is provided by the interaction of the Sun’s magnetic field and the planets’ own gravity.
In addition, the Sun creates a stream of charged particles called the solar wind that flows outward from its surface. This flow also forms a giant electromagnetic shield, the heliosphere. The heliosphere protects the Solar System from harsh interstellar radiation and extends to about 2 light-years away.
These boundaries are defined by the heliopause and the Oort Cloud, which are remnants of a larger cloud of icy debris from the early days of our solar system. The heliopause is at about 3 AU (astrophysical unit, or half the distance between the Sun and the nearest star) from the Sun, while the Oort cloud reaches almost halfway to the closest star.
When the nebula collapsed in on itself 4.6 billion years ago, most of the dust and gas in the original cloud was pulled into the Sun, creating our Solar System. But some of the dust remained behind and began to clump together. These chunks of leftover material started to form planets.
Some of these chunks grew to be very large and formed into gas giants like Jupiter, Saturn and Uranus. Other pieces were smaller and became the terrestrial planets of Mercury, Venus, Earth and Mars.
Once the planets were formed, they started to revolve around the Sun. They do this in a circular path that is often seen as an ellipse.
The planets are made of different materials, including rocks, ice and gases. All of them are about 10 times the size of Earth. The smallest of the terrestrial planets are Mercury, Venus and Mars; the largest are Jupiter, Saturn, Uranus and Neptune.
Moons
A planet’s moon is a small object that orbits around it. There are a variety of moons in the solar system, and they vary in size and properties.
Some of them are rocky and some are ice-covered. The majority of the moons in our solar system move around their planets in the same direction as they orbit the Sun. Some of these moons are wracked by intense volcanism, while others are covered in frozen seas.
Many of the moons have a unique history and are the subject of scientific research. For example, Jupiter’s moon Io is constantly erupting matter into space. The resulting impact craters are evidence of past bombardments by meteoroids.
The Earth’s Moon is also constantly hit by asteroid impacts, creating a record of that history on its surface. Because it doesn’t have an atmosphere, the craters aren’t eroded away by weather, so they serve as a record of all the impacts that have happened to it over time.
Similarly, the outermost Jovian moon Callisto has been battered by a huge number of asteroid hits. It has been called the “cratered Moon” and the “dark Moon.”
Other unusual moons in our Solar System include Rhea, which was originally thought to be a small asteroid, and Triton, which is a captured icy body from the Kuiper Belt. Its retrograde orbit around Neptune makes it look a lot like a comet, and scientists think the material it contains is nitrogen ice.
Other interesting moons are Deimos, which is a dwarf planet and a close companion to Mars; Phobos, which is a tiny icy planet that’s only 22.7 km (14 mi) across; and Pluto, which has five small moons.
Asteroids
Asteroids are the rocky and dusty remnants of the material left over from the formation of the Sun and the planets around 4.6 billion years ago. The vast majority of asteroids are found in the main asteroid belt between Mars and Jupiter, but there are also groups of asteroids farther out in space called trojans.
The asteroids are made up of ice, dust, metals, and rocks. They orbit the sun in highly flattened, elliptical circles, often rotating erratically and tumbling and falling through space.
Occasionally, Earth is hit by a very large asteroid, which can cause catastrophic damage to the planet and its environment. One of the most devastating impacts occurred 6.5 million years ago, in Mexico’s Chicxulub region.
Smaller asteroids are much less likely to cause problems on Earth. Desk-sized asteroids, for example, usually hit the planet about once a century. When they do, they create bright fireballs that burn up in the atmosphere and are then released as meteorites.
These smaller asteroids are generally rocky and stony, but some are rich in iron and nickel alloys. These materials can expand when they enter the atmosphere, increasing their speed as they reach Earth.
Asteroids move through the solar system at a pace that is far too fast for them to remain on their own orbits. As they fall into the atmosphere, they absorb the Earth’s heat, which causes them to expand and then move faster.
They can also be affected by the YORP effect, which can cause them to change their orbits. The YORP effect is a powerful force in the crowded asteroid belt between Mars and Jupiter.
The YORP effect affects asteroids with high eccentricities, which means that they tend to shift to planet-crossing orbits as their perihelion approaches the Sun. This process is what leads to the elongation of the orbits of asteroids located in the Kirkwood gaps in the main asteroid belt.
Comets
Comets are icy balls of frozen gases, rocks and dust that form in the outer regions of the solar system. They have highly elliptical orbits that take hundreds of thousands of years to complete.
When a comet gets close to the sun, it begins to heat up and melts its solid ice into gas. This gas contains water vapor, carbon monoxide and other trace substances. Some of the ice is left behind, and the rest forms a thick cloud of gases and dust that surrounds the nucleus of the comet. This material is called the coma and can be millions of kilometers wide.
The coma is what makes a comet glow and shine brightly when it’s close to the sun. The coma is also what makes a comet’s tail. The tail is made up of a combination of plasma and dust particles.
There are many different ways that comets move through space, but the most common way is by using gravity. The sun’s gravity is what pulls the ice inside the comet to change into gas, and it’s also what causes the ejection of dust particles from the surface.
Comets can also travel in a circular path (like a hot dog) called an orbit. This is what Halley’s Comet did in 1986, and it takes some comets hundreds or even thousands of years to make their orbit around the sun once.
Most comets originate in the Oort cloud, a huge spherical cloud of icy bodies that extends several thousand times farther from the sun than Pluto. They are often disturbed by stars that pass nearby, and then comets can become visible in the night sky. They can also be flung out of the inner solar system into interstellar space by gravitational disturbances from other planets.
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