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Are brown dwarfs stars or planets? Boffins find evidence for proto-suns in a solar system
Finding a twin brown dwarfs around a giant star just adds to the confusion
The discovery of two oddball brown dwarfs orbiting a giant orange star has reignited the question of whether brown dwarfs are feeble stars or bloated planets.
V Ophiuchi (pronounced Nu Ophiuchi) is a large, cool K-type star surrounded by two brown dwarfs. One is 22.2 times the mass of Jupiter, and the other is even heavier at 24.7 times the mass of Jupiter. Brown dwarfs are often found orbiting stars or floating through the Milky Way alone, so it’s not too uncommon to spot two circling a larger a star.
What is surprising, however, is the structure of both of the brown dwarfs’ orbits. They are locked around their parent star v Ophiuchi in such a way that their orbital periods are almost in perfect ratios of one another. The lighter brown dwarf closer to the star completes an orbit in 530 days, whilst the heavier one further away finishes a lap in 3185 days. It means that as soon as the outer object finishes one orbit, the inner one has already gone past its star six times.
A team of researchers led by the Centre for Astronomy of Heidelberg University describe this as a 6:1 resonant orbital configuration. It raises the question of whether brown dwarfs are created from the gravitational collapse of interstellar clouds like typical stars, or if they’re formed in protoplanetary disks around other stars like planets.
“The 6:1 resonance is a strong indication for the latter scenario,” said Andreas Quirrenbach, first author of the paper published in Astronomy & Astrophysics and a research scientist at the University of Heidelberg. “Only then could the orbits of the newly developing brown dwarfs adjust to a stable resonance over millions of years.”
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There are an estimated 100 billion brown dwarfs sprinkled across the Milky Way. They lack enough mass to burn hydrogen in their cores, and, instead, have lukewarm surfaces and emit a lackluster light.
“Brown dwarfs are neither stars nor planets,“ said Eileen Gonzales, a graduate student studying low mass stars at the City University of New York, who was not involved in the study, told The Register. “They are their own category of objects that have masses that lie in between those of stars, which are more massive, and planets, which are less massive.”
Although it’s the first time that astronomers have found evidence that brown dwarfs might have formed from protoplanetary disks, it doesn’t provide conclusive evidence that they’re stars, Gonzales said.
“This paper doesn't show any evidence that brown dwarfs are planets. In fact, this work tries to answer the question of how brown dwarfs form. This paper makes a clear argument on the idea that these two brown dwarfs formed in the planetary disk of the star ν Ophiuchi, but they make no claim on which mechanism - either core accretion, the formation mechanism for planets, or gravitational instability, the formation mechanism for stars.”