What is a TNO?

Question inspired by Mark's comments on my G+ post.

In astronomy, the initialism TNO stands for Trans-Neptunian Object. So what's one of those?

Well, like the name implies, they're objects that are beyond Neptune. Specifically, they are objects (dwarf planets, comets, etc) that orbit the Sun with an average orbital radius that is greater than that of Neptune (which is about 30 AU).

The first TNO was discovered back in 1930, and that's the dwarf planet we know as Pluto. Pluto is the second largest known TNO, being pipped at the post by Eris, which is in the region of 20 km wider. Next in line are Makemake and Haumea. The most distant known TNO is Sedna, with an average orbital radius of over 500 AU, but an aphelion of nearly 1000 AU.

There are two types of TNO, based on their location:

  • Classical objects: These are TNOs that exist within the Kuiper belt, a region between 30 and 55 AU from the Sun, and orbit in the ecliptic (that is, the same plane as the planets of the solar system), usually with nearly circular orbits. These objects are further classified into two subgroups depending on whether they have an orbital resonance with Neptune or not.
  • Scattered Disk objects: These TNOs orbit the Sun further away than Kuiper belt objects and generally have orbits that are more eccentric and/or out of the ecliptic plane.

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