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Brown dwarf boasts ‘Northern Lights’

Posted July 31, 2015

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 host auroras just like Earth.

Illustration showing a brown dwarf's aurora Image: Chuck Carter and Gregg Hallinan/Caltech

Illustration showing a brown dwarf’s aurora
Image: Chuck Carter and Gregg Hallinan/Caltech

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, reported in Nature, provide further evidence that suggests these stars act more like super-sized planets.

‘In science, new knowledge often challenges our understanding,’ said Dr Garret Cotter of Oxford University’s Department of Physics, an author of the paper. ‘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.’

Co-author Dr Stuart Littlefair, from the University of Sheffield’s Department of Physics and Astronomy, said: ‘Brown dwarfs span the gap between stars and planets and these results are yet more evidence that we need to think of brown dwarfs as beefed-up planets, rather than ”failed stars”.

‘We already know that brown dwarfs have cloudy atmospheres – like planets – although the clouds in brown dwarfs are made of minerals that form rocks on Earth now we know brown dwarfs host powerful auroras too.’

Auroral displays result when charged particles manage to enter a planet’s magnetic field. Once within the magnetosphere, those particles get accelerated along the planet’s magnetic field lines to the planet’s poles where they collide with gas atoms in the atmosphere, producing the bright emissions associated with auroras.

Dr Cotter said: ‘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.’

Source: University of Oxford

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