A mysterious object discovered near a brown dwarf doesn't fit into any known astronomical category.

The newly discovered mystery companion forms a binary system with the brown dwarf, located 460 light-years away in the Taurus star-forming system. The object is too light to be another brown dwarf, but it's too young to have formed by accretion, the way a typical planet does.

"Although this small companion appears to have a mass that is comparable to the mass of planets around stars, we don't think it formed like a planet," said astronomer Kevin Luhman of Penn State University, co-author of the study April 5 in The Astrophysical Journal. "This seems to indicate that there are two different ways for nature to make small companions."

Luhman's team made the discovery with the Hubble Space Telescope and the Gemini Observatory.

The new object and its companion brown dwarf are orbiting as a binary pair, 15 astronomical units from each other. If they were superimposed on our solar system, the companion would be orbiting midway between Saturn and Uranus. The oddball object's mass is somewhere between five and 10 Jupiter masses, making it too small to fuse deuterium. The International Astronomical Union currently uses this fusion line, which occurs at about 13 Jupiter masses, as the defining characteristic of a brown dwarf.

But the object appears to be around the same age as its binary partner, which doesn't fit conventional ideas about planet formation. Traditional theories describe planets forming from the gaseous disk that swirls around the equator of a newly formed star. Particles in the gas and dust cloud collide, and gradually accrete into larger objects, eventually becoming planets. These rocky planets can grow into sizes up to 10 Earth masses before they become gas giants.

And 1 million years is much shorter than the expected time for a planet to be born this way. Planets can form this quickly when there is a gravitational instability in the gaseous disk, but the brown dwarf's disk probably didn't have enough material to form a planet larger than a single Jupiter mass.

"It looks like this new system formed by the collapse and fragmentation process that forms binary star systems," Alan Boss, president of the IAU Commission on Extrasolar Planets said in an e-mail to Wired.com. Boss theorized that these sorts of planet-sized objects exist in a paper published in 2001.

"While people like to use the 'p-word' to describe objects with masses below 13 Jupiter masses, given the attention given to exoplanets these days, they should more properly be called 'sub-brown dwarfs,'" Boss said.

Because this strange object seems more likely to have formed the same way as its binary partner, the brown dwarf, Luhman believes it is probably best classified as a very small brown dwarf.

"This object, because it formed like a star, its composition is probably the same throughout," Luhman said. This homogenous composition is in stark contrast to the innards of gas giants, like Jupiter, which probably have a heavy-element rocky core surrounded by a gaseous shell composed mainly of hydrogen and helium.

The presence of another nearby binary system, of a red star and a brown dwarf, supports Luhman's theory. It seems to have been formed around the same time as the mystery pair, indicating that all four may have formed the same way, as stars.

"This configuration – two tight pairs that are widely separated from each other – is called a hierarchical configuration and is commonly seen in quadruple star systems," Luhman said.

Images: 1) NASA, ESA, K. Todorov, K. Luhman, Penn State University. 2) Artist's rendering from Gemini Observatory/L. Cook.