Methane Aurora | by Brian Koberlein

NASA, ESA, CSA, Leah Hustak (STScI)

Brown dwarfs span the road between planets and stars. By definition, a star have to be large sufficient for hydrogen fusion to happen inside its core. This places the minimal mass of a star round 80 Jupiters. Planets, even massive gasoline giants like Jupiter, solely produce warmth by means of gravitational collapse or radioactive decay, which is true for worlds as much as about 13 Jovian plenty. Above that, deuterium can endure fusion. Brown dwarfs lay between these two extremes. The smallest brown dwarfs resemble gasoline planets with floor temperatures much like Jupiter. The biggest brown dwarfs have floor temperatures round 3,000 Okay and look basically like stars.

Due to this, it may be tough to review brown dwarfs, significantly ones that don’t orbit different stars. With out a lot mirrored or emitted mild, we are able to’t simply analyze their spectra to find out their composition. Luckily, some brown dwarfs do emit radio mild due to their robust magnetic fields.

Planets resembling Earth and Jupiter have robust magnetic fields, and this implies they will entice ionized particles resembling hydrogen. These charged particles then spiral alongside the magnetic area traces till they collide with the planet’s higher ambiance, producing glowing aurora. On Earth, we see them because the Northern Lights. For brown dwarfs, we are able to’t see the seen mild of their aurora, however we can detect their radio glow.

Just lately a workforce appeared on the auroral mild from a brown dwarf often known as W1935. ^{It’s a chilly brown dwarf 47 light-years from Earth with a floor temperature of simply 200 °C. Throughout the spectra the workforce discovered mild emissions from methane. Whereas the presence of methane was anticipated in chilly brown dwarfs, the truth that the methane emitted mild was not. This implies the ambiance of W1935 seemingly has a thermal inversion, the place the higher ambiance is hotter than the decrease layers.}

That is true for the ambiance of Earth however is pushed by photo voltaic radiance. W1935 doesn’t orbit a star, so how can its higher ambiance get so heat? One attainable clarification is that the brown dwarf has an undetected small companion. This companion could possibly be ejecting materials much like the best way Saturn’s moon Enceleadus ejects water vapor. As soon as ionized within the vacuum of area, it will turn out to be trapped by the magnetic fields of W1935, ultimately colliding with the brown dwarf’s higher ambiance and giving it a little bit of thermal heating.

This discovery exhibits that even the smallest brown dwarfs defy simple classification. Although they resemble planets, they could have their very own planetary system like a star.