Auroras—more commonly known as the northern and southern lights—are often counted among the wonders of the world. But they are actually wonders on other worlds, too. Astronomers have detected these ethereal curtains of light in the atmospheres of other planets and now, according to a study published in Nature on July 30, a substellar object called a brown dwarf. This object hosts the first known auroras beyond the solar system. (Scientific American is part of Nature Publishing Group.)
Auroras are produced when the solar wind—a stream of plasma comprising charged particles that flow outward from the sun—becomes entangled in a planet’s or other celestial object’s magnetic field. The field tugs these particles toward the object’s poles where they smash into atoms in the atmosphere. These collisions transfer energy from the charged particles to the atoms, and then the so-called “excited” atoms release that energy as radiation. The type of radiation emitted, such as visible light or radio emission, depends on the atoms that make up the atmosphere.
A team of astronomers led by Gregg Hallinan, an astronomer at California Institute of Technology, recently spotted auroras on a brown dwarf dubbed LSR J1835+3259. They observed the phenomenon using the Very Large Array radio telescope in New Mexico and the Hale Telescope in California. Charged particles flare with radio emission as they travel along the brown dwarf’s magnetic field lines toward its atmosphere, where they collide with hydrogen atoms that subsequently release flashes of visible red light. The auroras on LSR J1835+3259 are the brightest astronomers have ever seen. Because the brown dwarf does not orbit a star that can bombard it with solar wind, some other unidentified object like an orbiting planet must provide the charged particles that power the auroras.
The team’s next step is to explore the variety of brown dwarf auroras out there, Hallinan says. He and his colleagues will survey brown dwarfs for auroras to determine what types of radiation they emit and how auroral features depend on brown dwarf characteristics like temperature. Hallinan suggests scientists might also be able to detect auroral activity on planets around other stars. Meanwhile LSR J1835+3259 has just joined a very exclusive club of known celestial objects that shimmer with auroras. Let’s take a look at some of the other known aurora light shows out there.