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28th November 2015

Research by Dr Garret Cotter, Fellow in Physics, shows brown dwarfs share characteristics with planets

Research by Dr Garret Cotter, recently appointed Fellow in Physics at Exeter College, has shown that brown dwarfs show behaviour more akin to planets than stars.

Brown dwarfs, sometimes called “failed stars”, are too massive to be planets but not massive enough to sustain a hydrogen fusion reaction in their cores like other stars. These faint objects are difficult to detect and remain hard to classify but now physicists from Oxford University and the University of Sheffield have revealed that they have dazzling aurora displays similar to Earth’s “Northern Lights”, suggesting they behave more like planets than stars.

The international team of researchers made the discovery by observing a brown dwarf 20 light years away using both radio and optical telescopes. Their findings provide further evidence that suggests these stars act more like super-sized planets.

Dr Cotter said, “In science, new knowledge often challenges our understanding. We know how controversial the situation was with Pluto, where astronomers had to look hard to try to decide if it is fundamentally one of the major planets of the solar system, or the first of the Kuiper Belt objects. Now, up at the other end of the size scale, we are challenged by seeing objects that traditionally would have been classified as stars, but seem to be showing more and more properties that make them look like super-sized planets.

“Several of the planets, like the Earth and Jupiter, have strong magnetic North and South poles, as if they have a strong bar magnet inside; these allow the aurorae to form when plasma is trapped near the North and South poles in the magnetic field. On the other hand, we know the sun has a very tangled magnetic field, chaotic all over the surface, causing sunspots where it has particularly intense patches. So the simple picture would be stars have sunspots, planets have aurorae. But here we’re well up into the mass range of the stars, and we are seeing aurorae. Why are we seeing phenomena we associate with planets? Clearly, we don’t yet fully understand how the ‘big magnet’ is formed inside such massive bodies.”

As well as providing new understanding of brown dwarfs, the research and techniques used in the study hold the promise that the faint radio emission from the aurorae can be used as a new way to detect and study exoplanets, planets orbiting stars other than our own sun.

To read more about the research click here. 

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