Eighteen celestial objects were caught loitering recently in a young stellar cluster near Orion’s belt, rather than orbiting stars as most planets do, reports a team of astronomers in the Oct. 6 Science. The astronomical community is divided over what to do with these misfit youths.
Based on our own solar system, a massive body is defined as a planet if it has no nuclear reaction in its interior and orbits a star, says Maria Rosa Zapatero Osorio, who led the team at the Astrophysics Institute of the Canary Islands. But that definition leaves the new-found free-floaters, which have planet-like interiors, in semantic limbo. They hover in the star cluster without orbiting anything.
While some astronomers would like to initiate a change in policy to allow planets the freedom to drift when young, “most astronomers still insist that planets have to be in orbit about stars,” says Jibor Basri of the University of California at Berkeley.
At five to 15 times the mass of Jupiter, these new kind of isolated giant planets, as the Canary Island team calls them, breach the size of brown dwarfs. A failed star, a brown dwarf burns deuterium in its core and not hydrogen, as other stars do. It must be at least 13 times the mass of Jupiter before it can burn deuterium, but never reach 75 times Jupiter’s mass, the point at which hydrogen fusion begins.
Location of the Orionis star cluster
(region around the brightest star in the
bottom right side of the figure) and
finding charts (45” by 45”) for three
spectroscopically confirmed, very
young cluster membranes with masses
between 5 to 15 times the mass of Jupiter.
