Sedna is the name given by astronomers for the newest planet to be discovered orbiting the Sun. Although the details of the size of the planet are still being sought, it is thought to be about 1300 miles in diameter and could be of a similar size to Pluto. Given the Pluto is generally accepted to be a planet in its own right, Sedna becomes the tenth planet to be found orbiting the Sun.
Perhaps it may help to explain the lack of companions out at this distance, although it is probably too small in practice to have swept the Belt clean by itself.
However, although it is located here, it may actually be an object from the distant Oort Cloud of comets. At the moment it is nearing perihelion, which has brought it into the Kuiper Belt. It is the most distant object to have been located orbiting the Sun. It appears to be following an elliptical orbit of about 10,500 years duration, taking out to a phenomenal 900 AU when at aphelion. Size-wise, it is too small to be a 'Nibiru' type-entity, irrespective of whether Nibiru turns out to be a terrestrial planet or a gas giant. Nor is this rocky, icy world anything approaching a 'Dark Star', even though it is red (presumably die the deposit of organic material on its surface). But it is a thrilling discovery, even so.
Dr David Whitehouse, writing for the BBC, states that 'astronomers at the Tenagra Observatory in Arizona were asked to provide positional information so that an orbit could be determined for Sedna'. It has also been imaged by the Hubble Space Telescope.