Objects in Space Beyond Neptune

Pluto was once categorized as a planet, because we didn’t have a clear definition of what a planet is and because the objects we called planets were basically all the large objects we found orbiting the Sun.

But then things changed. Around the year 2000, a team of astronomers headed by Mike Brown began discovering new objects beyond Neptune. In November of 2003, they discovered Sedna, in December of 2004, they discovered Haumea, in January of 2005, they discovered Eris, and in March of 2005, they discovered Make Make. And there have been many more.

Largest known trans-Neptunian objects

These new discoveries forced a re-examination of the definition of a planet. The inner eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune) all share some common characteristics:

  • They all orbit close to the same flat plane (known as the ecliptic plane)
  • Their orbits are not perfect circles, but their deviation from a circle (known as eccentricity) is low
  • Their gravitational dominance has swept their orbital neighborhood clear of most other objects
  • They are large enough to have assumed a round shape because of their gravitational force

The first two factors were not used in a new definition of “planet” but the last two were (along with the fact that they orbit the Sun), resulting in new categorizations that include the inner eight planets, “dwarf planets”, “trans-Neptunian objects”, and “small Solar System bodies”.

The new class of dwarf planets includes: Ceres (asteroid belt), Pluto, Haumea, Make Make, and Eris. More than a hundred other objects may become candidates for this status and as the Kuiper Belt area beyond the orbit of Neptune is explored, astronomers estimate that the total number will swell beyond 10,000.

Beyond the Kuiper Belt is a region known as the “Scattered Disk”. And beyond this is a spherical area called the “Oort Cloud” which contain as many as 2 trillion objects.

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