Our Sun and eight planets with their moons make up what astronomers call the “Solar System”. Although our Solar System is not the only one in the galaxy, scientists have not yet found the one like it. Each planet in the Solar System is as unique as the system in which it orbits. As a matter of fact, eight planets have very few attributes in common. They similarly orbit around the Sun, and they have largely the same chemical compositions. Beyond those two properties, the planets contrast far more sharply than they neatly compare.
The Sun’s gravity and magnetic field, called the “heliosphere”, envelop the major planets and all the dwarf planets in the Solar System. Although we frequently represent the planets’ orbits as circular, the major planets actually trace cosmic ellipses as they rotate around the Sun.
The planets take their names from Roman gods and goddesses. Of course, “Jupiter”, by far the largest of the eight planets, is named for the King of the Roman gods. Saturn, frigid and ice-bound almost beyond measure and imagination, paradoxically carries the name of the Roman god of agriculture. Mars, relatively small and desolate, carries the name of the Romans’ war god.
Until 1977 scientists thought only Saturn had “rings” – vast planes of ice and rocks suspended in orbit around them. Further investigation has shown that Uranus and Neptune also have ring systems. Naturally, their ring systems are not so pronounced as Saturn’s, because they are proportional to the two much smaller planets.
Astronomers refer to the bodies we generally call “moons” as “satellites”, and our moon has generally the same characteristics and properties as the other 139 satellites in the Solar System: it orbits the Earth as the Earth orbits the Sun, held in its elliptical pattern around the third planet by gravity and magnetism much like the Sun holds the planets.
In the last several years debate has raged over Pluto’s status: does it qualify as a planet, or does it fall into some other category of celestial objects?
In the course of the debate over Pluto the International Astronomical Society (IAS) the governing body that sets standards for measurements, observations, and discoveries changed the definition of and criteria for a planet. In order to meet official planet standards, a celestial body must orbit the Sun, have sufficient gravity to maintain a uniformly spherical shape, and clear its own orbit. After the IAS established its current standards, Pluto no longer met the requirements. Astronomers, after changing their assessments several times, finally classified Pluto as a “dwarf planet”.
Pluto travels in a little cluster of celestial objects very much like it, and astronomers developed an official classification for the whole group, calling these objects “plutoids”. They have gravity and hold their shape as they orbit the Sun, but they have not cleared their orbits. Many astronomers have become fascinated with the plutoids, arguing that insight into their development and evolution will contribute to proving “The Big Bang Theory”.