Size Doesn’t Matter





Source: Hubblesite STScI-2006-16

Dwarf planets may be too small to be considered full-fledged planets, but that doesn’t make them any less interesting or important. With the exclusion of Pluto from the list of planets, which has sparked quite a debate and controversy which still continues to divide the scientists, the spotlight has been shifted more toward the dwarf planets, which haven’t received much attention until now. Astronomers suspect that there could be as many as 200 dwarf planets in the Solar System, mostly located in the Kuiper Belt. The difference between the “regular” planets and dwarf planets is not that easy to spot, so we’re going help you to see what’s what.

As defined by the International Astronomical Union, the planet has to be in orbit around the Sun, have enough gravity to pull its mass into a spherical shape, and to clear its orbit from other, smaller object. It is the last requirement that draws the line between planets and dwarf planets. The gravity of the planet either traps smaller object that get in its way, or pushes them away. Not having sufficient gravity to make this happen, dwarf planets are not categorized as full-fledged planets. As of 2008, the are five dwarf planets recognized by the International Astronomical Union, and they are named Pluto, Ceres, Eric, Haumea and Makemake.

We’ve already written about Pluto, so you might want to back and read about it at, if you haven’t done so already. Next in line is Ceres, the earliest known and the smallest dwarf planet, discovered in 1801 by the Sicilian astronomer Giuseppe Piazzi. Ceres is so small that it is classified both as dwarf planet and an asteroid. It is the closest to Earth of all the dwarf planets, with a diameter of just 950 kilometers, and a mass that is only 0.015 percent than that of Earth. It takes Ceres 4.7 Earth years to complete a single revolution around the Sun, and its day is 9.1 hours long.

Eris is the largest discovered dwarf planet, with a diameter of 2326 kilometers, just slightly larger than Pluto. Its orbit is very unusual, as it crosses paths with both Pluto and Neptune. It takes Eris 557 years to orbit around the Sun, and its day lasts for 25.9 hours. 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.

Source: ESO/L. Calçada

The most recently discovered dwarf planets in the Solar System are Haumea and Makemake. Haumea is interesting because of its ellipsoid shape, barely meeting the criteria for hydrostatic equilibrium. However, its irregular shape is due to its rapid rotation, not a lack of mass. Its day last only 3.9 hours, and as for its orbit around the Sun, Haumea takes nearly 282 Earth years to complete a single revolution. Makemake, discovered in 2005, orbits at 45.3 times Earth’s distance and takes more than 305 years to complete a circuit of the sun. Its day is 22.5 hours. Makemake’s average diameter is 1,420 km.

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