Astronomers think they have found the 10th rock from the Sun. Naturally there is still a great deal of argument about this, but there seems to be reliable word that a body at least 1.5 times the size of Pluto has been found in the Kuiper Belt. It is estimated at 2,100 miles in diameter, and revolves around Sol at 97 a.u., about three times the distance out than Pluto (and a.u., or astronomical unit, is taken as the distance from the Earth to the Sun). Its orbit is inclined a surprising 45 degrees from the plane of our own, which is one reason its been so hard to find.
Yahoo sums up the breaking news from Space.com in this story.
Persephone is the name sf writers have floated from time to time as the next major planet to be found out from Pluto, if one should exist. It falls neatly into our theme of naming planets for the Greek pantheon; she was queen of the underworld in the Greek mythos and formed the mythological basis for the changing of the seasons, as she spent six months in the underworld and six months up top. James Blish, in the Cities In Flight cycle, called this notional planet by its Roman name, Proserpina.