As if the menagerie of known worlds in the Milky Way weren’t strange enough, now there might be solar systems forming within three light-years of the supermassive black hole at the center of the galaxy. This harsh environment is a dangerous place for planets, which risk getting torn away by the black hole if they even form at all.
A newfound cache of 44 blobs of gas sits in a sea of molecular clouds swirling around the black hole, a monster weighing roughly as much as 4 million suns. The blobs are probably low-mass stars ringed by dusty, planet-forming disks, Farhad Yusef-Zadeh, an astrophysicist at Northwestern University in Evanston, Ill., and colleagues report in the March 10 Astrophysical Journal Letters.
Yusef-Zadeh and colleagues discovered the possible planet nurseries with the Very Large Array radio observatory near Socorro, N.M. A band of nearby massive stars bathes the disks with ultraviolet radiation, and the disks respond by emitting detectable radio waves. The fact that the blobs don’t also emit infrared light indicates that the stars that host these nascent solar systems aren’t very massive, Yusef-Zadeh says, which means disks can survive around them; stellar heavyweights would radiate lots of infrared light as well.
The gas blobs resemble some young stars in the Orion Nebula, which at about 1,300 light-years away is the closest stellar nursery to Earth. These stars are encircled by disks, many of which are being eroded by a bright, massive star in the nebula (SN Online: 3/13/14). Bow shocks, streams of charged particles caused by colliding gas streams, curve like the wake of a boat around some of these young stars. The radio maps of the galactic center show bow shocks forming around many of the gas clumps there as well, Yusef-Zadeh says, which means the two environments might have a lot in common.
Astrophysicist Reinhard Genzel is open to the idea of planets forming near the black hole, but he isn’t yet convinced. He suspects that the clumps of gas are ephemeral and temporarily arise out of all the turbulence in the region. “It’s a little premature at this point to say ‘Hooray, we have planets!’” says Genzel, of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. Gas clouds that could collapse to form stars in other parts of the galaxy don’t stand a chance near a supermassive black hole. The gravity of the black hole tends to stretch and squeeze gas clouds into long ribbons. Yusef-Zadeh notes that the squeezing could trigger star formation, though Genzel says there’s no evidence that the gas is dense enough to overcome the black hole’s pull.
If planets were to form, it’s not clear what would become of them. They would be caught in a gravitational tug-of-war between their host stars and the black hole. Not many people have thought about planet formation near a black hole, Yusef-Zadeh says. What kinds of planets could form? Would they survive? “These are all interesting questions,” he says, “but we don’t really know.”