An object at the limit between planet and brown dwarf with a very strong magnetic field

Artist's concept of SIMP J01365663+0933473 (Image Caltech/Chuck Carter; NRAO/AUI/NSF)

An article published in the “Astrophysical Journal” describes a study on the magnetic fields of five brown dwarfs, objects at the limit between the planet and the star, cold even by the standards of their category. A team of researchers used the Very Large Array (VLA) radio telescope to examine the brown dwarfs chosen due to their radio wave emissions. The one cataloged as SIMP J01365663+0933473 is especially interesting because it’s at the limit between the planet and the brown dwarf and has a magnetic field over 200 times stronger than Jupiter’s.

In July 2017 an article published in the journal “Astronomy & Astrophysics” described the discovery of a star cataloged as EBLM J0555-57Ab, at the limit of the sustainability of hydrogen nuclear fusion. Its mass slightly exceeds the upper limit of the mass for a brown dwarf, the lower limit was established in 13 times the mass of the planet Jupiter but it’s a rule of thumb, based on observations and not on the rigor of a mathematical formula. Below that mass the fusion of deuterium stops and for that reason it’s defined as “deuterium-burning limit”.

Initially, astronomers thought that brown dwarfs didn’t emit radio waves but the first object of this type was only discovered in 1995, so they’ve been studied for a relatively short time. in 2001 the VLA detected emissions in that range of frequencies coming from a brown dwarf. This opened the door to further study possibilities of these objects, including this one, conducted by a team led by Melodie Kao, who at the time was a graduate student at Caltech, and is now a Hubble Postdoctoral Fellow at Arizona State University.

SIMP J01365663+0933473 is about 20 light years away from the Earth. It’s practically in the neighborhood from an astronomical point of view but because of its limited mass it was discovered only in 2006. In 2016 it was included in the group of five brown dwarfs to be examined by Melodie Kao’s team but in 2017 another team – though the two teams have in common the presence of Adam Burgasser – announced that the brown dwarf was part of a group of very young stars, with an estimated age of around 200 million years. Its mass was estimated at 12.7 times that of Jupiter with a radius 1,22 times that of Jupiter and a surface temperature of 825° Celsius (more than 1500° Fahrenheit). The estimated mass is just below the deuterium-burning limit.

Melodies Kao explained that those estimates were announced when her team had just finished analyzing the new data collected using the VLA. This made SIMP J01365663+0933473 even more interesting beyond the possibility of it being reclassified as a rogue planet because detections at various radio frequencies indicated that its magnetic field is more powerful than initially measured, over 200 times the planet Jupiter’s.

A further reason of interest for SIMP J01365663+0933473 is given by the fact that the researchers detected its auroral emissions. Strong aurorae were detected over the years in some brown dwarfs and this raised the question of their origin. The Earth’s aurorae are generated by the interaction with the solar wind but what can generate it in a brown dwarf without companions or in a rogue planet? A possibility is an object smaller and invisible from the Earth but with a magnetic field that interacts with the larger object the way Io, one of Jupiter’s moons, interacts with the planet.

Getting answers is important to better understand the magnetic dynamo mechanisms in brown dwarfs and in planets of various kinds as they’re not limited to gas supergiants. Gregg Hallinan of Caltech, another of the authors of this study, pointed out that the possibility of detecting radio emissions from aurorae could be used to detect exoplanets, including rogue ones.

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