Named Sedna after the Inuit goddess of the ocean, the lump of rock and ice was found by the Spitzer Space Telescope.
Observations show it is about 2000km across and it may even be larger than Pluto which comes in at 2250km.
There is likely to be debate about whether it qualifies as a true planet, but some astronomers are already saying it re-defines the Solar System.
The Hubble Space Telescope has also seen it. Details will be announced by the US space agency Nasa later on Monday.
How it was found
Sedna is the largest object found circling the Sun since the discovery of Pluto in 1930.
It was found during the course of a survey led by Dr Michael Brown of the California Institute of Technology.
Scientists are only midway through this three-year project. Preliminary calculations suggest that it is 10bn kilometres from Earth in a region of space known as the Kuiper Belt.
Defining a planet not so easy,
but does size matter?
The Belt contains hundreds of known objects and astronomers believe there are many more awaiting discovery.
Most are small worlds of rock and ice but some, like Sedna, could be as large as or larger than Pluto.
The importance of the discovery is that it is the first such world found in its normal orbit.
Other similar though smaller worlds, like Quaoar and Varuna, originated in the KB but have since been perturbed into different orbits.
Following the object's discovery, astronomers at the Tenagra Observatory in Arizona were asked to provide positional information so that its orbit could be determined.
The new discovery is likely reignite the debate about what is a planet.
One group of astronomers believes that Pluto is not a true planet but merely one of the largest of a vast number of minor objects in the outer Solar System.
The alternative standpoint is that Pluto is a planet and those who believe that will have to classify Sedna as the tenth.