Astronomers have now observed that the dust disc orbiting a brown dwarf contains millimeter-sized solid particles – the same “grains” found in the denser discs surrounding new stars.The new data suggests that rocky planets, such as Earth, may be more common throughout the univers than previously thought.

Prevailing hypotheses held that brown dwarf accretion discs were too sparse, and the particles moving too fast to form larger objects. Also, any grains that manage to form were expected to move quickly towards their host brown dwarf. The new research, published this week in The Astrophysical Journal Letters, challenges theories of how rocky planets form.

“We were completely surprised to find millimetre-sized grains in this thin little disc,” said Luca Ricci, an astronomer at the California Institute of Technology who led the research. “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.”

The new observations were made using the Atacama Large millimeter/submillimeter Array (ALMA) in Chile. The researchers say its increased resolution helped them find carbon monoxide around a brown dwarf called Rho-Oph 102, an object around 60 times the mass of Jupiter, but too small to become a true star. It was the first time cold molecular gas and small grains have been detected around a brown dwarf, suggesting that the disc is similar to those found around new stars.

“ALMA is a powerful new tool for solving mysteries of planetary system formation,” said Leonardo Testi, an European Southern Observatory (ESO) astronomer and member of the research team. “Trying this with previous generation telescopes would have needed almost a month of observing – impossibly long in practice. But, using just a quarter of ALMA’s final complement of antennas, we were able to do it in less than one hour!”

In the future, astronomers hope to use ALMA, which collects light with wavelengths around a millimeter, to map how the small particles and gas are distributed across a circumstellar disc and how they interact.

[Image courtesyALMA (ESO/NAOJ/NRAO)/M. Kornmesser (ESO)]