Introduction: The Earth in the Universe

The universe

The universe has unlimited size and unknown shape. All the existing things include galaxies, planets, satellites, comets, and meteors. The solar system is located in Milky Way galaxy. The Milky Way galaxy is a medium sized galaxy, which consists of about 150 billion of stars. Astronomers believe that there are 1012 such galaxies in the universe. Astronomical unit (AU), Lightyear (LY) and parsec are used to measure the distance in the universe.

1 AU = 1.5 × 10¹¹ m
1 LY = 9.46 × 10¹⁵ m
1 parsec = 3.26 LY

Solar system

Only the sun is known star, which has a group, like solar family and only one planet the earth of it has suitable conditions for organisms to survive on it. The eight planets of the solar system are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Countless meteors, meteorites, and comets also revolve around the sun beside planet. Drifting particles called interplanetary dust and an electrically charged gas known as plasma exist in the space of the solar system.

According to international convention of astronomers held on 2005 AD following criteria should be fulfilled by revolving planets:

• A planet should have its own orbit around the sun.
• Its shape must be round.
• Its orbit must not cross the orbit of other planets.

Pluto was removed from the planet as it crosses the orbit of Neptune.