Sedna is currently about 13 billion km from Earth |
It was first seen by astronomers using California's Palomar Observatory, and has been given the name "Sedna" after the Inuit goddess of the ocean.
Observations show it measures about 1,180-2,360km (730-1,470 miles) across, making it similar in size to Pluto.
Astronomers now say they have evidence that Sedna has its own moon, although this needs to be confirmed, and is also very red in colour.
There is likely to be some debate about whether it qualifies as a true planet, but some scientists are already saying it re-defines our Solar System.
Further than Pluto
Sedna, or 2003 VB12, as it was originally designated, is the most distant object yet found orbiting our Sun. It is three times further away than Pluto (average distance to the Sun is 5.9 billion km or 3.6 billion miles).
It was discovered using the Mt Palomar facility in November by astronomers from the California Institute of Technology, Yale Observatory and the Gemini Observatory.
Dr Mike Brown of the California Institute of Technology, US, leader of the research team that found the body, said he did not believe it was a true planet.
He suggested this "planetoid" is about half rock and half ice mixed together, but further work is needed to verify this.
The scientists say that its rotation on itself is relatively slow, suggesting it could have a satellite in orbit around it.
Follow-up studies by the Tanagra Observatory have measured the thermal radiation coming from Sedna to determine how hot it is, and therefore provide some estimate of its size.
Researchers believe that Sedna's surface temperature is about -240 degrees Celsius (-400 degrees Fahrenheit).
This estimate is uncertain but the object is likely to be between half the diameter of Pluto (2,360km or 1,470 miles) and Pluto's size; though some astronomers think it could be larger than the ninth planet itself.
From the observations made so far, astronomers have determined Sedna's orbit to be a very large one.
It is currently 90 times the Earth-Sun distance away (149 million km or 93 million miles), but its orbit can take it 10 times further away still.
Small worlds
Although Sedna could be a so-called Kuiper Belt object, its discoverers are unsure if it is as they consider it to be unlike any other object yet found.
