Brown Dwarfs Can Have Storms Too

December 13, 2015

Astronomers have discovered a storm on the L-dwarf named W1906+40, similar to the Great Red Spot on Jupiter.

An illustration of L-dwarf W1906+40 and its storm. Credit: NASA/JPL-Caltech

Just as Jupiter has a long-lasting storm called Great Red Spot, some previous observations have indicated that brown dwarf stars could have short-lived storms as well. Brown dwarfs are so-called failed stars, because they are not massive enough to sustain atomic fusion like our Sun is. Now, a new study shows that a type of brown dwarf, L-dwarf, can have a weather spot lasting for at least two years.

A team of astronomers led by John Gizis of the University of Delaware, Newark, USA, observed the Jupiter-sized brown dwarf for two years and found that it has dark storm just like our system’s biggest planet.

“The star is the size of Jupiter, and its storm is the size of Jupiter's Great Red Spot," said Gizis. "We know this newfound storm has lasted at least two years, and probably longer." 

L-dwarf in question is named W1906+40. It was first discovered by WISE telescope in 2011, in the same area where Kepler Space Telescope was looking at. Kepler looks for drops in stars’ luminosity - an indication an exoplanet is orbiting a star, blocking the fraction of its light. Gizis and his team searched through Kepler data and realised that the dip in light coming from W1906+40 couldn’t come from an exoplanet and instead presumed it was caused by star spots, which we call sunspots when they occur on the Sun’s surface.

A follow-up observation with Spitzer Space Telescope revealed that dips in luminosity are caused by giant storm in the atmosphere of W1906+40. Sunspots are caused by fluctuations of Sun’s magnetic field, but observations with Spitzer, which detects infrared light, have shown that the dark patch was not caused by the L-dwarf’s magnetic field.

The storm, that has been raging for at least two years, is located near the north pole of the brown dwarf and it rotates around the star in nine hours. Such a long lasting storm has never been observed on objects that are not planets.

"The L-dwarf's clouds are made of tiny minerals," said Gizis. "We don't know if this kind of star storm is unique or common, and we don't why it persists for so long."

The study, offering the best evidence to date that brown dwarfs can have storms too, appears in The Astrophysical Journal.

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