Sedna is a big Trans-Neptunian Object. Here's what an artist thinks Sedna might look like.
Click on image for full size
Image courtesy NASA/JPL-Caltech/R. Hurt (SSC-Caltech)
Trans-Neptunian Objects (TNO)
The outer edge of our Solar System is not empty. There are many huge spheres of rock and ice out near Pluto's orbit and beyond. Astronomers have a name for everything further out than the planet Neptune. They call these icy worlds Trans-Neptunian Objects (TNOs).
The planet Pluto is a Trans-Neptunian Object. It is in a region called the Kuiper Belt. There are many other giant ice balls in the Kuiper Belt. The Kuiper Belt is just outside of Neptune's orbit. Quaoar is another large Kuiper Belt Object.
Some TNOs are much, much further out than the Kuiper Belt. They are part of the Oort Cloud. Sedna might be from the Oort Cloud.
Scientists think they have found at least one TNO bigger than Pluto. This "tenth planet" is officially called 2003 UB313, but it has the nickname "Xena". Xena might be a scattered disc object, another type of TNO. It may be kinda in between a KBO and something from the distant Oort Cloud.
Most comets come from the Oort Cloud and the Kuiper Belt.
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