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Types of Planets

The planets of our own solar system were not fully identified until the early 20th century, when Pluto was discovered and labelled as the ninth planet. The last decade of the 20th century shifted the focus of planetary studies from strictly our own solar system to finding extra-solar planetary bodies. Since the search for other worlds began, more than 230 planets have been identified, including huge gas giants.

Terrestrial Planets
- Terrestrial planets are solid celestial bodies that are primarily composed of silicates and/or metals. These include the inner planets of our solar system: Mercury, Venus, Earth and Mars. They all have a metal core and are surrounded by silicates. Astronomers have theorized that terrestrial planets made of other materials, such as carbon, may exist elsewhere in the universe. They predict that these planets will be larger than the Earth and may have more than five times the mass of our planet. Scientists believe carbon-based planets would be around 12,000 miles in diameter.
Gas Giants
- The best-known gas giant planets are those in our own solar system. Jupiter is the largest of these; it is 1,000 times larger than Earth. Saturn, Uranus and Neptune are also gas giants. Other gas giants have been discovered beyond our own solar system; they are made of the lighter elements: hydrogen and helium. Gas giants exert intense gravitational forces on the space surrounding them. They feature turbulent atmospheric activity and are much larger than other kinds of planets. Planets that are made of pure hydrogen could be 35,000 to 40,000 miles in diameter.
Planetoids
- Planetoids are large, solid celestial objects. They are larger than asteroids, but smaller than terrestrial planets. Planetoids exist in an area surrounding the solar system called the Kuiper Belt. Astronomers have observed many objects of planetoid size in this region of the solar system. They believe these are remnants of the materials that originally formed our solar system.
Dwarf Planets
- Dwarf planets are small celestial bodies that orbit in or near the Kuiper Belt. As of July 2011, scientists have found 10 dwarf planets in addition to reclassifying our ninth planet, Pluto, as a dwarf planet. Dwarf planets are different than asteroids and planetoids in that they are more developed. However, they lack the strong gravitational pull of regular planets. Eris and Ceres are two other dwarf planets outside of our solar system.

Astronomy