A team of astronomers from the United States and Europe has discovered that brown dwarfs – the so-called failed stars – host powerful auroras just like Earth.

Brown dwarfs are relatively cool, dim objects that are difficult to detect and hard to classify.

These objects are too massive to be planets, yet possess some planet-like characteristics; they are too small to sustain hydrogen fusion reactions at their cores, a defining characteristic of stars, yet they have star-like attributes.

“Brown dwarfs span the gap between stars and planets. We already know that they 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,” said Dr Stuart Littlefair from the University of Sheffield, UK, co-author on the study published in the journal Nature.

Dr Littlefair and his colleagues conducted an extensive observation campaign of LSRJ 1835+3259, a brown dwarf located 18.6 light-years away.

Using the National Radio Astronomy Observatory’s Very Large Array, they detected a bright pulse of radio waves that appeared as the brown dwarf rotated around. The object rotates every 2.84 hours, so the team was able to watch nearly 3 full rotations over the course of a single night.

Next, the scientists used Palomar’s Hale Telescope to observe that the brown dwarf varied optically on the same period as the radio pulses. Focusing on one of the spectral lines associated with excited hydrogen – the h-alpha emission line – they found that the object’s brightness varied periodically.

Finally, they used W.M Keck Observatory’s telescopes to measure precisely the brightness of the brown dwarf over time – no simple feat given that these objects are many thousands of times fainter than our Sun.

The team was able to establish that the hydrogen emission is a signature of auroras near the surface of LSRJ 1835+3259.

“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,” said co-author Dr Garret Cotter from the University of Oxford, UK.

“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.”

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G. Hallinan et al. 2015. Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence. Nature 523, 568-571; doi: 10.1038/nature14619