“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.