Brown Dwarfs May Grow Rocky Planets
Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have for the first time found that the outer region of a dusty disc encircling a brown dwarf star contains millimeter-sized 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.
ALMA’s 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. This discovery, and that of the millimeter-size grains, suggest that the disc is much more similar to the ones around young stars than previously expected.
Ricci and his colleagues made their finding using the partially completed ALMA telescope in the high-altitude Chilean desert. ALMA is a growing collection of high precision, dish-shaped antennas that work together as one large telescope to observe the universe with groundbreaking detail and sensitivity. ALMA “sees” the universe in millimeter-wavelength light, which is invisible to human eyes. Construction of ALMA is scheduled to finish in 2013, but astronomers began observing with a partial array of ALMA dishes in 2011.
ALMA collected light with wavelengths around a millimeter, emitted by disc material warmed by the brown dwarf. The grains in the disc do not emit much radiation at wavelengths longer than their own size, so a characteristic drop-off in the brightness can be measured at longer wavelengths. ALMA is an ideal instrument for measuring this drop-off and thus for sizing up the grains. The astronomers 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 millimeter or more in size.
“ALMA is a powerful new tool for solving mysteries of planetary system formation,” commented Leonardo Testi from ESO, a 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!” he said.
In the near future, the completed ALMA telescope will be powerful enough to make detailed images of the discs around Rho-Oph 102 and other objects. Ricci explained, “We will soon be able to not only detect the presence of small particles in discs, but to map how they are spread across the circumstellar disc and how they interact with the gas that we’ve also detected in the disc. This will help us better understand how planets come to be.”