L-dwarfs are a special subset of tiny stellar objects that possess both star-like and planet-like characteristics. Known colloquially as “failed stars,” brown dwarfs are too massive to be classified as planets, but they are too small to be clearly defined as stars. They form a bridge between planets and stars and can weigh-in at many times the mass of Jupiter (although their physical size is approximately that of Jupiter). They are celestial mongrels in a way; they have qualities of both stars and planets, but can be clearly defined as neither.
For example, although some of the more massive brown dwarfs (such as M- and L-dwarfs) can experience some low-level fusion in their cores (a star-like quality), it’s not enough to raise the object’s temperature beyond a couple of thousand degrees. Therefore, their atmospheres can become stratified (layered) and possess very planet-like phenomena such as clouds and, in this case, powerful storms.
NASA’s Wide-field Infrared Survey Explorer discovered W1906+40 in 2011 and astronomers realized that the object was within the field of view of NASA’s exoplanet-hunting Kepler Space Telescope. Usually, Kepler will look out for “transits” of exoplanets that orbit in front of their host stars — the slight dimming caused by the planet blocking star light causes a dip in brightness. But sometimes “starspots” can also be detected by Kepler — basically huge dark patches of magnetic activity in the uppermost stellar layers.
So, using Kepler, although the light generated by W1906+40 is faint, astronomers detected a huge dark patch rotate with the L-dwarf’s spin. Could it just be another star sporting a vast, dark cluster of star spots, like our sun does during periods of high magnetic activity?
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