Solar System
The solar system is the Sun together with everything bound to it by gravity: eight planets, dwarf planets, moons, asteroids, comets, and dust.
Definition
The solar system is the Sun and everything that travels around it: eight planets and their moons, dwarf planets like Pluto, and countless asteroids and comets, with Earth as the third planet from the Sun. It includes at least five dwarf planets, over $200$ known moons, the asteroid belt, the Kuiper belt, and the distant Oort cloud, and it formed about $4.6$ billion years ago from a collapsing cloud of gas and dust called the solar nebula, with four rocky inner planets and four gas and ice giants beyond. The Sun holds $99.86\%$ of the mass and Jupiter most of the rest. In the original protoplanetary disk, the frost line at roughly $3$ to $5$ AU separated the inner region where only rock and metal condensed from the outer region where ices let giant cores grow and capture gas, and the heliosphere, the bubble of solar wind, extends to about $120$ AU where Voyager 1 crossed into interstellar space.
Example
In order from the Sun the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, often remembered with a sentence like "My Very Educated Mother Just Served Us Nachos." Neptune orbits at about $30$ AU, the Kuiper belt extends to about $50$ AU, and the Oort cloud may reach tens of thousands of AU. The planets follow $T^2 = a^3$ closely: Mars at $1.52$ AU has a $1.88$-year period and Neptune at $30.1$ AU a $165$-year period, and deviations from such predictions led to Neptune's discovery in 1846.
Key Insight
The Sun holds more than $99\%$ of the solar system's mass, so everything else, planets included, is a small leftover. The solar system is mostly empty space: if the Sun were a basketball, Earth would be a small bead about $26$ meters away and Neptune a marble almost $800$ meters away. The nearly circular, coplanar, same-direction orbits of the planets are fossils of the original spinning disk, which is why astronomers were confident of the nebular origin long before other stars' disks could be imaged.