The Solar System | Mercury | Venus | Earth | Mars | Ceres | Jupiter
Saturn | Chiron | Uranus | Neptune | Pluto
The Kuiper Belt and the Oort Cloud | Quaoar | Eris | Sedna

The media-dubbed 'Tenth Planet', recently named Sedna after the Inuit goddess of the sea, was discovered by the Samuel Oschin Telescope, Mount Palomar facility, California, USA, in November 2003. The discovery was attributed to the team at the facility consisting of Mike Brown, Chad Trujillo and David Rabinowitz.

How it got its Name

There was originally some uncertainty about the name Sedna. The Inuit goddess was chosen because of the planet's frigid temperatures. This name which was provided by its discoverers and proclaimed throughout worldwide media outlets was heavily criticised as it did not fit in with the Graeco-Roman theme of the other planets. However, the name was accepted by the International Astronomical Union on 28 September, 2004.

How far Away is it?

Sedna was the most distant object yet discovered within our Solar System in 2003. It is twice the distance from the Sun of any other known object in the Kuiper Belt¹, and three times as far as Neptune or Pluto. However, unlike many of the other planetary bodies, Sedna has an extreme elliptical orbit, far greater than that of Pluto.

This orbital pattern resembles more of a comet than a planet, throwing into dispute whether it is even a planetary object at all. Many scientists now believe that Sedna is rather a member of the Oort Cloud², rather than the inner ring of astrological bodies around the Sun.

The reason why this object has only been discovered now is that there is such a vast expanse of sky to search, that the sky has to be searched in very small segments to see anything as the pinpoints of light are incredibly small. As well as this, only recently have modern telescopes been able to take in enough light to see such small objects. It is really only through systematic analysis of different sectors, or just blind luck, that any objects are found. Indeed, after this discovery, images of the planet were found on pictures taken as far back as 1996, though it was not then thought to be anything more than a far-off star or a stray comet.

Essential Facts

• Estimated Equatorial Radius: 750km
• Estimated Length of Day: 840 hours
• Period of Revolution about Sun: 10,500 years
• Constitution: 50% rock, 50% ice
• Surface Temperature: -240°C
• Distance from the Sun: 90 AU

Current estimates show that the planet consists of 50% rock and 50% ice mixed together, but this research is far from conclusive. The planet's colour is said to be red and very shiny, however, because this combination is very unusual, the estimates as to what the object is actually made of is as yet unknown. Researchers believe that Sedna's maximum surface temperature is around -240°C (-400°F). The object, however, may drop to an even lower temperature as it has a highly elliptical solar orbit.

Due to the vastness of the object's distance from the Sun, some researchers believe that Sedna was captured by our Solar System from another star moving past around 100 million years ago.

Is Sedna a Dwarf Planet or Planetoid?

The classification of 'planet' is still under debate, though its discoverers still maintain it to be a 'planetoid'. The main question is that if Sedna is called a dwarf planet, then should other bodies, such as Quaoar also be considered thus? Indeed, what classifications are necessary to scientifically define an object as a 'planet'?

The California Institute of Technology has issued an article that argues that a planet should be defined by its gravitational rounding. In other words:

Any object which is round due to its own gravitational pull and which directly orbits the Sun is called a planet.
– California Institute of Technology

From this definition the word 'planetoid' has been coined, which neatly describes all of the accepted eight planets in the Solar System, as well as some other large bodies in the Solar System, such as Quaoar, and the asteroid Ceres, which was originally called a planet at the time of its discovery.

However, the 'Population Classification' theory states that planets must be stand-alone objects in space, and cannot be members of a group of similarly-sized objects. If this theory is accepted, then many objects would no longer be classified as planets. Many find this proposal unsatisfactory, and base their arguments on historical grounds. But should planets be classified historically or scientifically? Indeed, this theory would allow for a fluid change of classification, as more information is gathered about the local geography of planetoid space.

A debate also rages over whether this object should be considered a body in itself, or attached to either the Kuiper Belt or the Oort Cloud. Arguments that it could be a Kuiper Belt object were soon to be discounted once the orbital period was discovered; showing the object never enters any sector of the Kuiper Belt. The general consensus now is that it is an inner Oort Cloud object.

Has the 'Tenth Planet' Been Found?

Upon the discovery of Sedna, many scientists became excited about the prospect of this object possibly being the missing Planet X that was theorised before the discovery of Pluto. However, due to the apparent size of this object, the hopes that this might be the missing planet were dashed. It is thought that the missing planet would need to be much larger than Sedna. When one of the Voyager spacecraft went past Neptune, it did very detailed measurements of the exact location and speed of the planet. It turns out there was never an anomaly in Neptune's orbit, and no mysterious Planet X.

Tenth Planets in Fiction

In the meantime, we can make do with the many 'Tenth Planets' to be found in fiction — for instance: Rupert was the tenth planet discovered in Douglas Adams' Mostly Harmless, the fifth book in his famous Hitchhikers Guide to the Galaxy series. In the BBC TV series Doctor Who, 'The Tenth Planet' of Earth's solar system was called Mondas, the birthplace of the evil Cybermen.

The h2g2 Tour of the Solar System

Take the h2g2 Shuttle for your whistle-stop tour of the Solar System.

• Next Stop (Inward Journey):
• Eris
• Next Stop (Outward Journey):
• The Oort Cloud