Planets & Dwarf Planets in our Solar System

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Our SOLAR SYSTEM refers to the Sun in its centre with nine eight planets and their satellites (i.e., moons), asteroids, comets and meteoroids orbiting around the Sun. It includes drifting particles called interplanetary dust and electrically charged gas (called plasma) that together make up the interplanetary medium.

Our solar system is elliptical in shape (i.e., egg-shaped) and is a tiny part of a galaxy known as the Milky Way.

The nine eight planets in our solar system are commonly divided into two groups:
  1. Inner Planets - Mercury, Venus, Earth, and Mars.

    The inner planets are small and are composed primarily of silicate rock and iron.

  2. Outer Planets - Jupiter, Saturn, Uranus, Neptune and Pluto.

    The outer planets (except Pluto) are much larger but not very dense, and have deep gaseous atmospheres of hydrogen, helium, ammonia and methane. Methane in the atmospheres of Uranus and Neptune makes these planets a bright blue-green.
Here's a mnemonic to remember the order of the nine eight planets from the Sun:

"My Very Excellent Mother Just Served Us Nine Pizzas"

The first letter of each word represents a planet - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

On August 24, 2006, the International Astronomical Union (IAU) formally downgraded Pluto from an official planet to a "dwarf planet". According to the new rules, a planet must ...
  1. orbit around the Sun
  2. have sufficient mass for its self-gravity to overcome rigid body forces to take on a nearly round shape
  3. have cleared other objects in its orbital path around the Sun
    (i.e., so there are no similar objects at roughly the same distance from the Sun)
Pluto failed the third criteria - it orbits among the icy wrecks of the Kuiper Belt and is really "just" one of many Kuiper Belt Objects (KBOs). Eris and Ceres (in the asteroid belt) are the other two members of this dwarf planet classification.

BBC News: Backlash against the decision to strip Pluto of its status as a planet.
Mouseover the words to learn more about the planets in the solar system.
Original image source: National Aeronautics and Space Administration

Planets - Facts & Figures   |   Origin of the Solar System   |   World Book at NASA (more info on individual planets)
Astronomy and space science - Test   |   Solar System Crossword   |   Interactive Assessment Worksheets © by Alan & Hui Meng