Brown dwarfs could grow rocky planets
Astronomers have detected that the outer region of a dusty disc encircling a brown dwarf - a star-like object, contains millimetre-size solid grains like those found in denser discs around newborn stars.
The surprising finding challenges theories of how rocky, earth-scale planets form, and suggests that rocky planets may be even more common in the universe than expected.
"We were completely surprised to find millimetre-sized grains in this thin little disc," said Luca Ricci of the California Institute of Technology, who led a team of astronomers from the US, Europe and Chile.
"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.
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, according to a California statement.
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.
Also, 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.
ALMA's (Atacama Large Millimetre/submillimetre Array) increased resolution compared to previous telescopes also allowed the team to pinpoint carbon monoxide gas around the brown dwarf -- the first time that cold molecular gas has been detected in such a disc.
The astronomers pointed ALMA at the young brown dwarf ISO-Oph 102, also known as Rho-Oph 102, in the Rho Ophiuchi star-forming region in the constellation of Ophiuchus (The Serpent Bearer).
With about 60 times the mass of Jupiter but only 0.06 times that of the Sun, the brown dwarf has too little mass to ignite the thermonuclear reactions by which ordinary stars shine.
However, it emits heat released by its slow gravitational contraction and shines with a reddish colour, albeit much less brightly than a star.