Auroras spotted on brown dwarfs outside solar system

Spectacular auroras, often seen at higher latitudes on Earth, have been detected outside our solar system for the first time.

The auroras were found in the atmospheres of several failed stars called brown dwarfs.

The findings, reported in the journal Nature, indicate brown dwarfs may be far more planet-like than astronomers previously thought.

"This is the first time auroras have been detected on brown dwarfs," says the study's lead author Gregg Hallinan of the California Institute of Technology.

• Summary of the paper in Nature

Brown dwarfs don't have enough mass to reach the necessary core temperatures and pressures for nuclear fusion, the process which makes stars like our Sun shine.

Instead, brown dwarfs fill the gap between the smallest stars called red dwarfs, and the largest planets, the gas giants like Jupiter.

All planets with magnetic fields produce auroras when charged particles in the solar wind interact with the planet's magnetosphere, causing the particles to spiral down the planet's magnetic field lines and collide with atoms in the atmosphere.

The emissions are seen in optical wavelengths as spectacular curtains of light high in the atmosphere, and can also be detected in radio wavelengths beaming out like a lighthouse signal.

Over a five-year period Hallinan and colleagues used both optical and radio telescopes to search for the radio pulsations and optical signature of aurora beyond our solar system.

The international team of researchers detected radio and optical emissions from a rapidly rotating brown dwarf named LSR J1835+13259, located about 18.5 light years away.

"So when we saw radio emission from the brown dwarf, we speculated that it had auroras," says Hallinan.

Planet or star?

Over the past year since that first detection, the researchers found both optical and radio auroras on even smaller and cooler brown dwarfs.

"This tells us that brown dwarfs behave like giant planets, like super Jupiters, rather than like small stars," says Hallinan.

"So we think this phenomenon extends all the way down from brown dwarfs to planets."

This tells us that brown dwarfs behave like giant planets, like super Jupiters, rather than like small stars- Gregg Hallinan, California Institute of Technology

However, Hallinan admits questions still remain about the processes generating auroras on brown dwarfs.

"In our own solar system it's the solar wind for Earth, Saturn, Uranus and Neptune, but in Jupiter's case it's a different process," says Hallinan.

"Jupiter has a very powerful magnetic field, in fact from our perspective on Earth, if we could see the magnetic field on Jupiter, it would look larger than the Sun in the sky."

Several large moons orbit inside Jupiter's magnetosphere including the volcanic moon Io.

"Io's volcanoes spew sulfur dioxide into Jupiter's magnetosphere, and as Io passes through Jupiter's magnetic field it creates an aurora on Jupiter's surface," says Hallinan.

"Jupiter's rapid rotation also helps power this aurora."

However, the dissipated power from a brown dwarf aurora is at least 10,000 times larger than that produced in the magnetosphere of Jupiter.

"On brown dwarfs, we know there's no star, so there's no stellar wind, so maybe there's an Earth-sized planet sitting very close powering the aurora," says Hallinan.

"Or maybe just like Jupiter, the rapid rotation is powering the aurora in a different fashion."

Hallinan says auroras may be common features of large-scale magnetospheres, and could explain some weather phenomena previously detected on brown dwarfs.

Hallinan's team are planning further observations over the next year using optical telescopes including Hubble and Keck, as well as the Very Large Array radio telescope to try to resolve the mystery of what powers these spectacular alien sky shows.