Dwarf planets are worlds that are too small to be considered full-fledged planets, but too large to fall into smaller categories.
In recent years, there's been a lot of hubbub about Pluto losing its status as one of the planets in our solar system. Pluto is no longer considered the ninth planet in the series of major planetary objects, but instead is now just one of the many so-called "dwarf planets."
Astronomers estimate that there could be as many as 200 dwarf planets in the solar system and Kuiper Belt. But the differences between planets and dwarf planets may not be obvious at first.
Dwarf planets of the solar system
The International Astronomical Union defines a planet as being in orbit around the sun, has enough gravity to pull its mass into a rounded shape (hydrostatic equilibrium), and has cleared its orbit of other, smaller objects. This last criterion is the point at which planets and dwarf planets differ. A planet's gravity either attracts or pushes away the smaller bodies that would otherwise intersect its orbit; the gravity of a dwarf planet is not sufficient to make this happen. [Meet the Dwarf Planets of the Solar System]
As of 2008, the IAU recognizes five named dwarf planets: Ceres, Pluto, Eris, Haumea, and Makemake.
Ceres is the earliest-known and smallest of the current category of dwarf planets. Ceres was discovered in 1801 by the Sicilian astronomer Giuseppe Piazzi, based on the prediction that the gap between Mars and Jupiter contained a missing planet. It is only 590 miles (950 km) in diameter and has a mass of just 0.015 percent that of Earth.
In fact, Ceres is so small that it is classified as both a dwarf planet and an asteroid, and is often named in scientific literature as one of the largest asteroids in the solar system. NASA's unmanned Dawn mission will perform an exploration of Ceres after it arrives at the asteroid/dwarf planet in 2015.
Pluto is the most well-known of the dwarf planets. Since its discovery in 1930 and until 2006, it had been classified as the ninth planet from the sun. Pluto's orbit was so erratic, however, that at times it was closer to the sun than the eighth planet, Neptune. In 2006, with the discovery of several other rocky bodies similar in size or larger than Pluto, the IAU decided to re-classify Pluto as a dwarf planet.
Despite its small size, 0.2 percent the mass of Earth and only 10 percent the mass of Earth's moon, Pluto's gravity is enough to capture four moons of its own. The pairing between Pluto and its largest moon, Charon, is known as a binary system, because both objects are orbiting around a central point which is not within the mass of Pluto.
Eris is the largest of the dwarf planets, with a mass 27 percent larger than that of Pluto and a diameter of approximately 1,400 to 1,500 miles (2,300 to 2,400 km). It was the discovery of Eris that prompted the IAU to reconsider the definition of a planet.
The orbit of Eris is very erratic, crossing that of Pluto and nearly intersecting the orbit of Neptune, but is still more than three times larger than Pluto's orbit. It takes 557 years for Eris to orbit the sun. At its furthest point from the sun, a point that is also called its aphelion, Eris and its satellite are the furthest natural objects contained by the solar system.
Haumea and Makemake
Haumea is unique because of its ellipsoid shape, only just meeting the hydrostatic equilibrium criteria for dwarf planet status. The elongated shape of the dwarf planet is due to its rapid rotational spin, not a lack of mass, which is about one-third the mass of Pluto.
Makemake is also a unique dwarf planet, being the only one of the Kuiper Belt Objects to lack a satellite. This lack makes it more difficult to measure the mass of the dwarf planet, though its diameter is known to be about two-thirds that of Pluto. Makemake is also of value to the astronomical community, as it is another reason for the reconsideration of the definition of a planet. Its comparable mass and diameter to Pluto would grant it planet status if Pluto wasn't also stripped of that title.
Dwarf planets as "plutoids"
Pluto, Eris, Haumea, and Makemake are all known as "plutoids," unlike the asteroidal dwarf planetoid Ceres. A plutoid is a dwarf planet with an orbit outside that of Neptune. Plutoids are sometimes also referred to as "ice dwarfs" due to their diminutive size and cold surface temperatures.
The outer planets show evidence of interaction with plutoids. The largest moon of Neptune is likely a captured plutoid, and it is even possible that the odd tilt of Uranus on its axis is due to a collision with a plutoid. Similarly to dwarf planets, there are potentially hundreds of plutoid objects in the solar system that have yet to be given official status.
Most people remember that Pluto was stripped of its status as a major planet in the Solar System, but they do not understand about the re-classification of it as a dwarf planet. Pluto and the other dwarf planets contain many interesting secrets about the formation of the solar system and our own planet Earth.
References and Further Reading
These sites were used in the writing of this article, and contain interesting and useful information on the subject of dwarf planets.