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Brown dwarfs may grow rocky planets

Sarah Cruddas
Dec 2, 2012, 8:00 UTC

Sen—Astronomers have discovered that the outer region of a brown dwarf - a star-like object, but one too small to shine brightly like a star - contains millimetre sized solid grains. Known as cosmic dust and similar to soot or sand, it is normally found around new stars. The findings were made by a team using the Atacama Large Millimetre/submillimetre Array (ALMA). The discovery challenges current theories of how rocky, Earth like planets form and could mean that rocky planets are even more common in the universe than previously thought.

“Solid grains of that size shouldn’t be able to form in the cold outer regions of a disc around a brown dwarf, but it appears that they do" said Luca Ricci of the California Institute of Technology, who led the team of astronomers based in the United States, Europe and Chile. "We can’t be sure if a whole rocky planet could develop there, or already has, but we’re seeing the first steps, so we’re going to have to change our assumptions about conditions required for solids to grow”.

The findings are important because astronomers expected that grains around brown dwarf stars could not grow, because the discs were too sparse and particles would be moving too fast to stick together after colliding. Rocky planets are thought to form through the random collision and sticking together of what are initially microscopic particles in the disc of material around a star. Current theories are that if the grains were able to form, they would move quickly towards the central brown dwarf, disappearing from the outer parts of the disc where they could be detected.

The increased resolution of ALMA compared with previous telescopes also allowed the team to pinpoint carbon monoxide gas around the brown dwarf. This is the first time that cold molecular gas has been detected in such a disc. This discovery, combined with the millimetre-size grains, suggest that the disc is much more similar to the ones around young stars than previously expected. 

The ALMA telescope was pointed by astronomers to the young brown dwarf star ISO-Oph 102, which also goes by the name Rho-Oph 102, in the Rho Ophiuchi star-forming region of the constellation of Ophiuchus. The star is 60 times the mass of Jupiter, but is still small, at 0.06 times the size of the Sun. It has too little mass to ignite the thermonuclear reactions which causes stars to shine, so instead it emits heat released by its slow gravitational contraction and shines with a reddish colour.

Artist illustration of dust particles clumping together

Artist illustration of grains of cosmic dust in the disc around the brown dwarf. Credit: ALMA (ESO/NAOJ/NRAO)/L. Calçada (ESO)

The astronomers used ALMA to collect light with wavelengths around a millimetre, emitted by disc material warmed by the brown dwarf. The team then compared the brightness of the disc at wavelengths of 0.89 mm and 3.2 mm. The drop-off in brightness from 0.89 mm to 3.2 mm was not as steep as expected, showing that at least some of the grains are a millimetre or more in size.

The ALMA telescope is currently only partially completed, it consists of a growing collection of high precision, dish-shaped antennas that work together as one large telescope to observe the Universe in millimetre-wavelength light, which is invisible to human eyes. Once completed in 2013, ALMA will be powerful enough to make detailed images of the discs around Rho-Oph 102 and other objects.