'Brown dwarfs could host rocky planets'

Brown dwarfs - which inhabit a kind of fuzzy line between stars and planets - could host rocky planets since they have dusty discs encircling them, researchers say.

Astronomers for the first time have found that the outer region of a dusty disc encircling a brown dwarf contains millimetre-sized solid grains like those found in denser discs around newborn stars.

The surprising finding by the Atacama Large Millimeter Array (ALMA) challenges theories of how rocky, Earth-scale planets form, and suggests that rocky planets may be even more common in the Universe than expected.

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.

These tiny grains, known as cosmic dust, are similar to very fine soot or sand. However, in the outer regions around a brown dwarf ¿ a star-like object, but one too small to shine brightly like a star astronomers expected that grains could not grow because the discs were too sparse, and particles would be moving too fast to stick together after colliding.

Prevailing theories say that any grains that manage to form should move quickly towards the central brown dwarf, disappearing from the outer parts of the disc where they could be detected, European Southern Observatory (ESO) reported.

"We were completely surprised to find millimetre-sized grains in this thin little disc," said Luca Ricci of the California Institute of Technology, USA.

"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. 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," he said.

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