To the casual observer, the difference between a planet and a star is pretty stark, but in reality things are a little murkier. A class of object known as a "brown dwarf" sits in the middle, with more mass than a planet but not enough to kickstart the nuclear fusion process that characterizes a star. One of these strange objects was recently downgraded to planet status (albeit a huge one), and now astronomers have discovered that it generates an enormous magnetic field, which could provide a new tool in the search for exoplanets.
The object of interest in the new study, known as SIMP J01365663+0933473, was first discovered in 2016, and at the time was believed to be an ancient, massive brown dwarf. But last year astronomers got a better look at it, determining its surface temperature was about 825° C (1,517° F) and bringing its age down to a mere 200 million years – a relative baby in astronomical terms.
But most importantly, it was found to not be a brown dwarf after all, but a very large planet. The difference between the two is still contentious, but a proposed definition is that an object with a mass of over 13 times that of Jupiter is a brown dwarf, because that's the point where deuterium fusion starts up in the core. At 12.7 Jupiter masses, SIMP just barely limbos under the bar.
"This object is right at the boundary between a planet and a brown dwarf, or 'failed star,' and is giving us some surprises that can potentially help us understand magnetic processes on both stars and planets," says Melodie Kao, lead researcher on the study.
Its size isn't the only surprising thing about the object. SIMP was found to have a magnetic field and play host to auroras, but how that's possible is still a mystery. Auroras in our own solar system and beyond are caused by charged particles from stars interacting with the magnetic field of a planet, yet brown dwarfs don't normally have stars blasting charged particles at them.
By observing the object using the Very Large Array (VLA) radio telescope, astronomers have now found that SIMP's magnetic field is far stronger than was previously detected – up to 200 times stronger than Jupiter's, in fact. That strength may challenge our understanding of the mechanisms that generate these fields, the researchers say.
"This particular object is exciting because studying its magnetic dynamo mechanisms can give us new insights on how the same type of mechanisms can operate in extrasolar planets – planets beyond our solar system," says Kao. "We think these mechanisms can work not only in brown dwarfs, but also in both gas giant and terrestrial planets."
Along with raising those questions, the team also says the object is significant because these observations mark the first time an exoplanet has been detected via radio telescope, and the first time such a planet's magnetic field has been measured.
Of the 3,774 exoplanets confirmed so far (plus hundreds more candidates), the majority were discovered through the transit method, where the light from a star is temporarily blocked out by an orbiting planet. But that won't work for brown dwarfs since these rogue planets roam the cosmos on their own. Using auroral radio emissions, as demonstrated in this study, could be a new way to find and study these mysterious loners.
The research was published in the Astrophysical Journal.
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