To most of us, the difference between a star and a planet is pretty clear. But officially it's surprisingly murky, thanks to a middle-ground celestial object known as a brown dwarf, which could be classed as either the largest possible planets or the smallest possible stars. Now astronomers from Heidelberg University have found evidence of brown dwarfs forming like planets, as opposed to stars.

Stars and planets are born in similar ways but usually under different circumstances. If you have a cloud of dust and gas, a star can be born when enough material gathers in one spot that the pressure and heat kickstarts hydrogen nuclear fusion in its core. Planets often form out of the leftover material as it swirls in a disk around a star, but they don't collect enough mass to ignite into stars themselves.

Brown dwarfs seem to straddle the line between the two. They're massive enough for some nuclear fusion to fire up in their cores, at least temporarily, but they don't have enough mass to ignite hydrogen and give off their own light. Official definitions are unclear, but one proposed limit for brown dwarfs is 10 times the mass of Jupiter – any smaller and it's a regular old planet.

But the real question remains: how do brown dwarfs form? Are they basically huge planets, or "failed" stars? They seem to be a bit of both. Some have been seen whizzing through the cosmos on their own as "rogue planets," indicating they formed from interstellar molecular clouds like stars. Others have been spotted orbiting stars, while some even have planets orbiting them.

In the new study, the Heidelberg team focused on v Ophiuchi, a star with a mass about two and a half times that of the Sun that lies about 150 light-years away. Strangely enough, this star is being orbited by not one, but two brown dwarfs.

The team studied the radial velocity of v Ophiuchi over the course of 11 years, and found that the two brown dwarfs are orbiting in an odd pattern. One takes about 530 days per orbit, while the other takes 3,185. That puts them in a 6:1 resonant configuration – meaning the inner brown dwarf completes six laps for every one orbit of the outer object.

The significance of that configuration is that it suggests the objects formed in the protoplanetary disk around the star – the way planets do. The team says this is the first direct evidence that brown dwarfs can form this way.

"The 6:1 resonance is a strong indication for the (planet-like) scenario," says Andreas Quirrenbach, lead researcher on the study. "Only then could the orbits of the newly developing brown dwarfs adjust to a stable resonance over millions of years."

Of course, not all brown dwarfs necessarily form this way. They can almost certainly be made as "failed stars" too. The researchers say that further studies need to be conducted on these bizarre objects, which may lead to new classifications.

The research was published in the journal Astronomy & Astrophysics.

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