Sedna is a large Trans-Neptunian Object discovered in 2003. Here's how an artist thinks Sedna might look. Click on image for full size (31K JPEG) Image courtesy NASA/JPL-Caltech/R. Hurt (SSC-Caltech)

There are many icy and rocky planetoids on the outer edge of our Solar System. As a group, all bodies that orbit, on average, further from the Sun than the 8th planet Neptune are called Trans-Neptunian Objects (TNOs).

There are different classes of TNOs. The region of space just outside Neptune's orbit, but largely near the same plane in which the planets revolve, is called the Kuiper Belt. Much further out is the Oort Cloud, which is scattered into a gigantic sphere around the Sun. Some astronomers recognize a third class of TNOs called scattered disc objects (SDOs), which may be in transition between the Kuiper Belt and the Oort Cloud. SDOs have orbits intermediate between the Kuiper Belt and the Oort Cloud, in terms of both size of orbit and amount of tilt of orbital plane.

This region is the main source of comets that visit the inner Solar System. When the orbits of TNOs are disturbed, the objects can be flung out of the Solar System or they can plunge inward towards the Sun.

TNOs hold clues to the origin and evolution of our Solar System. Some Kuiper Belt Objects (KBOs) may have formed near their current positions, and may thus be deep frozen samples of material from the earliest days of our Solar System. Many KBOs, Oort Cloud objects, and SDOs have been scattered into their current locations by past gravitational interactions with large planets, particularly Jupiter. Their orbits provide clues about possible changes in positions of the gas giant planets throughout the history of our Solar System.

The planet Pluto and its large moon Charon are examples of the Kuiper Belt subset of TNOs. Quaoar, Orcus, and Varuna are some other KBOs you might have heard about. The "tenth planet", 2003 UB₃₁₃ (nicknamed "Xena"), seems to be a scattered disc object. Sedna might be an extreme innermost fringe member of the Oort Cloud.

Sedna is a big Trans-Neptunian Object. Here's what an artist thinks Sedna might look like. Click on image for full size (31K JPEG) Image courtesy NASA/JPL-Caltech/R. Hurt (SSC-Caltech)

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.

Sedna is a Trans-Neptunian Object. Here's what an artist thinks Sedna might look like. Click on image for full size (31K JPEG) Image courtesy NASA/JPL-Caltech/R. Hurt (SSC-Caltech)

There are lots of small worlds at the edge of our Solar System. They are made of ice and rock. They are further away from the Sun than most planets. They are further away than the 8th planet, Neptune. They are called Trans-Neptunian Objects (or TNOs for short).

Some TNOs are only a little farther out than Neptune. They are in an area called the Kuiper Belt. It is sort of like the asteroid belt. They are called Kuiper Belt Objects (or KBOs for short). The planet Pluto is actually a Kuiper Belt Object. So is Quaoar.

Some TNOs are much, much further away. They are way out past Neptune. They orbit in an area called the Oort Cloud. Sedna might be from the Oort Cloud.

Some objects are in between the Kuiper Belt and the Oort Cloud. 2003 UB₃₁₃ is a TNO that was discovered in 2005. Its nickname is "Xena". Xena might be bigger than Pluto. Xena has a strange orbit. Its orbit is in between a Kuiper Belt orbit and an Oort Cloud orbit.

Most comets come from the Oort Cloud and the Kuiper Belt.