Despite being only 0.9 arcsecond apart, the "dwarf planet" Pluto and its largest moon, Charon, appear well separated in this Hubble Space Telescope image snapped in 1994. R. Albrecht (ESA/ESTEC) and NASA. It's not over yet. In the past week a small but growing group of scientists made their first formal attack against the International Astronomical Union's August 24th resolution that left the solar system with eight planets and downgraded Pluto to a new class of objects known as "dwarf planets." On Thursday two heavy hitters in the planetary-science community — Mark Sykes, director of the Planetary Science Institute,...