The Solar System
Within the observable universe there are hundreds of billions of galaxies. Each galaxy is comprised of stars, nebula, other astronomical objects, and dark matter. We live in a galaxy known as the Milky Way. The Milky Way is full of many stars and planets- somewhere between 200 billion to 400 billion! In this galaxy lies our solar system, consisting of our “Sun and all of those celestial bodies bound to it by gravity” (wikipedia, par. 1).
Our solar system includes eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. As of 2006, the term dwarf planet was adopted by the International Astronomical Union. According to the IAU, dwarf planets are celestial bodies that are orbiting the Sun which have large enough masses to be rounded by their own gravity (wikipedia, par. 1). There are currently five dwarf planets that orbit the Sun- Ceres, Pluto, Haumea, Makemake, and Eris.
The first four planets are known as terrestrial planets. They are dense planets made up of mostly rock and metal. The four outer planets are significantly more massive than the inner four planets and are mainly composed of hydrogen and helium. They are often referred to as Jovian planets or gas giants.
Separating the two groups of planets is the asteroid belt. The asteroid belt has tens of thousands of objects over 1km in diameter. Beyond Neptune lies another belt similar to the asteroid belt, called the Kuiper belt. It is often referred to as the outer solar system and contains many undiscovered celestial bodies. The Kuiper belt is home to Pluto, a dwarf planet that, until 2006, was classified as the ninth planet in our solar system.
Mercury is considered the first planet, as it is closest to the Sun. Greek astronomers once believed that Mercury was two separate objects. One was visible only at sunrise; the other visible only at sunset. They named these objects Apollo and Hermes (NASA). It was later “named by the Romans after the...