Mysterious ‘Rogue Planet’ Roams the Stars Alone, Not So Far From Earth

Researchers have detected a rogue planet traveling the void between stars some 20 light years away. Rogue planets or brown dwarfs (which this might be) aren’t exactly rare, cosmologically speaking, but they tend to be very difficult to see. And yet, the way we found this particular planet/brown dwarf suggests we might locate other similar stellar objects through an application of the same technique.

SIMP J01365663+0933473 is either a brown dwarf or a planet — initially, it was thought to be a brown dwarf, but later mass estimates suggest it’s 12.7x the mass of Jupiter, which puts it right on the cusp of the planet / brown dwarf distinction. Brown dwarfs are typically thought to begin at 13 Jupiter masses (MJ), which is why SIMP J01 (etc) was originally thought to be a brown dwarf. In fact, it’s not clear this is a settled issue — some of the write-ups on this story explicitly refer to SIMP J01365663+0933473 as a brown dwarf, while others call it an enormous planet. The National Radio Astronomy Observatory site refers to a discovery late last year that suggests the planet is only 200 million years old and is too young to be a star. But regardless, it’s one for the record books.

First, it’s relatively close, at just 20 light years away. It possesses an enormous magnetic field some 200x stronger than Jupiter’s. It has 12.7x Jupiter’s mass, but it wouldn’t actually be much larger than Jupiter itself. Brown dwarfs, even those vastly more dense than Jupiter (60-90MJ), aren’t actually all that much larger than our fifth planet, and brown dwarf sizes only appear to vary by 10-15 percent. This object, for example, is just 1.22x the diameter of Jupiter, despite its mass.

Astronomers at the National Science Foundation’s Karl G. Jansky Very Large Array used the radio telescope to detect the rogue planet, making this the first time we’ve identified a planetary-mass object using radio astronomy.

One feature of some brown dwarves — and of this planet — are exceptionally strong auroras. Auroras on Earth are created by the interaction of our own magnetic field and the solar wind from the Sun. There isn’t a sun near SIMP J01365663+0933473 to be interacting with its magnetic field, but it’s possible that the planet could have a moon in orbit interacting with and modifying its field. Jupiter and Io, for example, are known to have this kind of relationship. Its surface temperature is remarkably low for an object of this size, estimated at 825C or roughly 1500F. The Sun’s temperature, in contrast, is roughly 5,500C.

“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,” said Melodie Kao, who led this study while a graduate student at Caltech, and is now a Hubble Postdoctoral Fellow at Arizona State University. “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. We think these mechanisms can work not only in brown dwarfs but also in both gas giant and terrestrial planets.”

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