The distant object that some astronomers think could be the Solar System's 10th planet may have a moon.
The new planetary candidate, which has been named Sedna, rotates more slowly on itself than expected, suggesting it may have a satellite orbiting it.
One of the scientists who found Sedna has been giving further details of its discovery at a news conference.
Observations show it measures less than 1,700km (about 1,000 miles) in diameter, which is smaller than Pluto.
"We think that there's evidence there is a satellite around Sedna," said Dr Mike Brown, of the California Institute of Technology, US, and leader of the research team that found the body.
"We're hoping in the very near future to get some observations from the Hubble Space Telescope that should put that question to rest."
Sedna was first seen on 14 November 2003 with the 48-inch Samuel Oschin Telescope at California's Palomar Observatory. Astronomers from the California Institute of Technology, Yale University and the Gemini Observatory were involved in the discovery.
Sedna, which is named after the Inuit goddess of the ocean, is both very shiny and very red - the reddest object in the Solar System after Mars.
Further than Pluto
Sedna, or 2003 VB12, as it was originally designated, is the most distant object yet found orbiting our Sun. It is currently three times further away than Pluto (average distance to the Sun is 5.9 billion km or 3.6 billion miles).
"The Sun appears so small from that distance that you could completely block it out with the head of a pin," said Dr Brown.
He added that, in his view, the object's apparently small size suggested it should not be classified as a true planet.
Sedna is currently about 13 billion km from Earth
Dr Brown suggested the "planetoid" was probably half-rock and half-ice, but further work was needed to verify this.
Follow-up studies by the Tanagra Observatory have measured the thermal radiation coming from Sedna to help provide some estimate of its size.
Researchers believe that Sedna's surface temperature is about -240 degrees Celsius (-400 degrees Fahrenheit).
The object is usually even colder, because it approaches the Sun only briefly during its 10,500-year solar orbit.
At its most distant, Sedna is 130 billion km (84 billion miles) from our star, which is 900 times Earth's solar distance (149 million km or 93 million miles).
Although Sedna could be a so-called Kuiper Belt object, its discoverers doubt this. The object's elliptical orbit is unlike anything previously seen by astronomers.
It could be the first detection of the long-hypothesized "Oort cloud", a faraway repository of small icy bodies that supplies some of the comets that streak by Earth.
KUIPER BELT AND OORT CLOUD
KB bodies orbit beyond Neptune, probably in a disc
More than 400 such icy objects currently known
Believed to be remnants of Solar System formation
Spherical Oort cloud extends perhaps 3 light-years
At extreme of Sun's gravitational and dynamic influence
Contains comets thrown inwards by passing stars
The cloud is believed to surround the Sun and extend outward halfway to the nearest star. But Sedna is 10 times closer than the predicted distance of the Oort cloud.
Dr Brown said this "inner Oort cloud" might have been formed by gravity from a rogue star near the Sun in the Solar System's early days.
"What we think must have happened is that early in the history of the Solar System there must have been many, more stars very close to the Sun than there are now," he added.
"And the reason that would have happened is if when the Sun was born, it was born in a cluster with many, many other stars tightly bound together."
In recent years, astronomical work has thrown up several big objects beyond Neptune.
Quaoar, found in 2002, is about 1,200km (745 miles) across. Ixion, discovered in 2001, is 1,065 km (660 miles) wide. Varuna, detected in 2000, has a diameter of approximately 900 km (560 miles).
And only in February this year, scientists announced the discovery of the object 2004 DW, which is thought to be 1,600km (995 miles) across.
The new discovery will reignite the debate about what constitutes 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. These scientists would like to see it demoted.
So far, astronomy's governing body, the International Astronomical Union, has refused to do so.