Called Sedna for the Inuit goddess of the sea, the object is the most distant body orbiting the sun, and lies billions of miles away from Earth.
The team that discovered the mysterious-looking mass believe that Sedna is the first evidence of the long-hypothesised "Oort cloud," a faraway repository of small icy bodies that supplies the comets that streak by Earth.
Sedna is much closer than astronomers would have expected based on their projections about the Oort cloud, suggesting that it could be part of an "inner Oort cloud" that may have been formed by gravity from a rogue star near the Sun in the solar system's early days.
"This is the first good direct evidence that the Sun formed in a cluster of stars," said Michael Brown, an associate professor at the California Institute of Technology in Pasadena, California, who led the team that identified Sedna.
Brown and two colleagues in Hawaii and New York first spied the red mass in November 2003, but only finalised their observations recently, according to the National Aeronautics and Space Agency (NASA) which sponsored their work.
"We still don't understand what is on the surface of this body," said Chad Trujillo, an astronomer with the Gemini Observatory in Hawaii. "It is nothing like what we would have predicted or what we can explain."
Trujillo has begun to examine the object's surface with one of the world's largest optical/infrared telescopes on Mauna Kea, Hawaii, but what the trio has been able to determine thus far is that Sedna is a planetoid a little smaller in size than Pluto with a highly elliptical orbit.
Sedna moves in a 10,500-year orbit around the Sun, approaching it only briefly.
The planetoid will become brighter over the next 72 years, then dim as it moves farther away on its 10,500-year trip to the farthest reaches of the solar system.
"The last time Sedna was this close to the Sun, Earth was just coming out of the last ice age," said Brown. "The next time it comes back, the world might again be a completely different place."
At its most distant, it is 130 billion kilometers (84 billion miles) from the Earth, or 900 times the distance between the Earth and the Sun.
Brown estimates Sedna's size at somewhere between Pluto and the planetoid Quaoar, measuring about 1,700 kilometers (1,000 miles) in diameter.
On that reckoning, it does not rise to the level of a planet. "It is not massive enough," he told reporters during a teleconference from NASA's Jet Propulsion Laboratory Monday.
There is some debate about what exactly constitutes a planet, and not all astronomers agree that Pluto merits that definition either.
Brown, Trujillo and David Rabinowitz from Yale University first observed Sedna on November 14, 2003 using the 48-inch (122 centimeter) Samuel Oschin Telescope at Caltech's Palomar Observatory near San Diego.
Within days, astronomers in Chile, Spain, Arizona and Hawaii had made similar sightings.