A Brief History of the Solar System

Abstract

From the year 1930 to the year 2006, nine objects in the solar system were considered as planets. The planets Mercury, Venus, Earth, and Mars are known as terrestrial or rocky planets because they have a solid surface like that of the Earth. The planets Jupiter, Saturn, Uranus, and Neptune are known as Jovian planets. They all are giant gaseous objects. Not much was known about Pluto. Even the mass of this planet was not known until the end of the last century. Now we know that Pluto is a rocky object and it is much smaller than even our Moon. The terrestrial planets are also known as the inner planets and the Jovian planets are known as the outer planets because an asteroid belt between Mars and Jupiter separates them. This asteroid belt has many small objects orbiting around the Sun and some of them are as large as Pluto. All the planets except Pluto are orbiting in the same plane. We also know that the planets are not the only members of the solar system. There are a large number of satellites, comets, asteroids, and planetesimals (small, rocky celestial objects formed during the birth of the solar system). In 2006, the general assembly of the International Astronomical Union, the “Parliament” or “Senate” of astronomers, decided a definition of the planets and resolved that Pluto should no more be considered as a planet. So, if we go by that definition, then the number of planets reduces to eight. Pluto and other objects similar to the size or mass of Pluto are now considered as Dwarf Planets. We shall discuss it later on.

This world was once a fluid haze of light,

Till toward the centre set the starry tides,

And eddied into suns, that wheeling cast

The planets: then the monster, then the man.

–Alfred Tennyson

(In The Princess, 1847)