You live in the solar system, which consists of that big bright thing in the center called the Sun, and thousands if not millions of various other objects ranging from giant Jupiter all the way down to microscopic bits of dust. Our view of the solar system changed dramatically over four hundred years ago when we realized that all of these objects circle the Sun in elliptical orbits.We can state, in general terms, that there are four types of objects that orbit the Sun:•PlanetsHow many are there? Of course there is still some debate as to the exact definition of a planet.•Asteroids (Minor Planets)At last count, we have discovered over 700000 of these objects•CometsMade primarily of ice, these are visitors from the outer solar system•Kuiper ObjectsPluto was the first one discovered, and there are hundreds more•MoonsAlthough they do not orbit the Sun, they are still members of the solar system
PlanetsPlanets are large, spherical bodies which orbit a star. Unlike stars, they do not have sufficiently high temperatures in their cores to initiate thermonuclear reactions. Astronomers speak of two general types of planets, terrestrials, which share characteristics with Earth, and the jovians, such as Jupiter. In 2006, the International Astronomical Union established a new definition of planet, which resulted in the ‘demotion’ of Pluto as a planet.
Asteroids (Minor Planets)We now know of hundreds of thousands of minor planets with most of them residing between the orbits of Mars and Jupiter. The largest, Vesta, was discovered with three other minor planets in the first years of the nineteenth century. Ceres was the first one discovered by Piazzi in 1801. (Ceres is now a dwarf planet, by the way) For the next hundred years, these objects were discovered at a rate of a dozen or so each year. With the advent of photography in the late nineteenth century hundreds were discovered annually. And more recently, automated telescopes are finding new objects by the thousands each year.
CometsComets are icy objects that spend most of their time far from the Sun in the dark depths of the Oort Cloud. Occasionally they are bumped into highly elongated elliptical orbits that cause them to fall into the inner solar system, where they grow their impressive tails and put on a grand show for a few weeks.
Kuiper Belt ObjectsThe existence of these icy objects was first suggested by Gerard Kuiper more than fifty years ago. Beginning in the 1990’s, dozens of these objects were discovered in the region of the solar system where Pluto resides. It became evident to many astronomers that Pluto was just one of hundreds of similar objects, resulting in a new definition of planet by the I.A.U. in 2006.
MoonsMoons are objects that orbit planets. Astronomers now count nearly 150 moons around the planets. The big ice giant planets have dozens of moons. We find a great variety of natural satellites in our solar system. Some of the larger moons were almost certainly formed at the same time as their parent planet. Others are obviously captured objects with wildly eccentric orbits. Still others orbit near the planet and have the job of keeping planetary rings in line.