The discovery of the cosmic "snack" in galaxy NGC 4845, 47 million light-years away, was made by the European Space Agency's Integral space observatory with follow-up observations from ESA's XMM-Newton, NASA's Swift and Japan's MAXI X-ray monitor on the International Space Station, a release from ESA's Paris headquarters reported Tuesday.
Astronomers using Integral to study a different galaxy noticed a bright X-ray flare coming from another location, with XMM-Newton, Swift and MAXI tracing it to NGC 4845, which brightened by a factor of a thousand in January 2011 then subsided over the course of the year.
"The observation was completely unexpected, from a galaxy that has been quiet for at least 20–30 years," Marek Nikolajuk of the University of Bialystok in Poland said.
The X-ray emissions came from a halo of material around the galaxy's central black hole as it tore apart and fed on an object of 14–30 Jupiter masses, researcher said.
While this size range corresponds to brown dwarfs -- substellar objects not massive enough to fuse hydrogen in their core and ignite as stars -- the researchers note it could have had an even lower mass, just a few times that of Jupiter, placing it in the range of gas-giant planets.
Free-floating planetary-mass objects of this kind may occur in large numbers in galaxies, ejected from their parent solar systems by gravitational interactions, astronomers said.
"Estimates are that events like these may be detectable every few years in galaxies around us, and if we spot them, Integral, along with other high-energy space observatories, will be able to watch them play out just as it did with NGC 4845," ESA Integral project scientist Christoph Winkler said.