For thousands of years, from our small world, we have looked upon the cosmic ocean. Early star watchers thought that the Earth was in the middle of the solar system and that the sun and all the other stars spun around the Earth. Copernicus showed that Earth and the other planets in our solar system revolved around our sun. Early astronomers perceived dots of light that seemed to move in the midst of the stars. They called these objects “planets,” meaning nomads, and titled them after Ancient Roman holy beings Jupiter, ruler of the divinities; Mercury, an emissary of the gods; Venus, the deity of love and beauty, Mars, the god of war and Saturn, dad of Jupiter and god of cultivation. The astronomers also detected Comets with twinkling tails, and meteors or shooting stars seemingly falling from the heavens.
Fig.1: Our Solar System as we know it,was fashioned 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud.
Subsequently, the telescope was invented, and three more planets were revealed in our solar system: Uranus in 1781, Neptune in 1846, and now demoted to a dwarf planet, Pluto was discovered in 1930. Moreover, there are thousands of smaller bodies such as comets and asteroids. Almost all asteroids orbit in a section sandwiched between the orbits of Mars and Jupiter while the comets lie in the Oort Cloud, outside the orbit of Pluto.
Mercury, Venus, Earth and Mars are closest to the sun and are termed as the terrestrial planets because they have compact stony surfaces. Saturn, Jupiter, Neptune and Uranusare the four huge planets (Jovian Planets) past the trajectory of Mars and are called gas giants. Petite, distant, Pluto has a hard but frostier surface than the other planets.
Fig.2: Mass of Planets (Units of Earth Mass)
Just about every planet and even some of the moons have an atmosphere. Our world’s atmosphere is mainly made up of oxygen and nitrogen. Venus has a thick atmosphere of carbon dioxide, with hints of poisonous gases, for instance, sulfur dioxide. Mars has a carbon dioxide atmosphere which is very thin. Jupiter, Saturn, Uranus, and Neptune are mainly helium and hydrogen.
Fig.3: Average Density of Planets (g/cc)
The Solar System furthermore comprises of smaller objects. The asteroid belt, lying between the trajectories of Mars and Jupiter, generally contains objects composed, like the terrestrial planets, of rock and metal. Outside Neptune’s orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects made up mostly of ice, and beyond them a recently discovered population of sednoids. Within these populations are several dozen to perhaps tens of thousands of objects so huge that their gravity has rounded them. Such bodies are regarded as dwarf planets. Well-known dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. Additionally various other small-bodies, including comets and interplanetary dust, freely travel between regions. Six of the planets, no less than three of the dwarf planets, and several of the smaller bodies are circled by natural satellites, usually called “moons” after the Moon. Each of the outer planets is encircled by planetary rings of dust and other small objects.
Fig.4: Classification of our Solar System
Inferior Planets: Planets inside the orbit of the earth. Eg.Mercury and Venus
Superior Planets: Planets outside the orbit of the earth. Eg.Mars, Jupiter, Saturn, Uranus, and, Neptune.
Inner Planets (Terrestrial Planets): The term is used for planets inside the asteroid belt. Eg.Mercury, Venus, Earth, and Mars
Outer Planets: Planets outside the asteroid belt. Eg.Jupiter, Saturn, Uranus, and Neptune.
Gas Giant (or Jovian) Planets: Planets sharing the gaseous structure of Jupiter. Eg.Jupiter, Saturn, Uranus, and Neptune.
Did you know!?
When Pluto is in proximity to the sun, it has a thin atmosphere, but when it travels to the exterior regions of its trajectory, the atmosphere freezes and collapses to the planet’s surface. Thus, in this manner, Pluto acts like a comet!