It's official. Pluto is not a red-blooded planet. As decreed in August by a vote of the General Assembly of the International Astronomical Union, Pluto is now a dwarf.
At first, the IAU seemed ready to defend Pluto. On August 16, the union's seven-member Planet Definition Committee released a draft Planet Definition Resolution, which stated that round objects in orbit around the sun are planets. Pluto is a round object in orbit around the sun. Therefore, Pluto is a planet. This definition would have given everyone the right to utter "Pluto" and "Jupiter" in the same breath, even though Jupiter is 250,000 times larger. The draft resolution would also have opened the door to granting planet status to at least three objects that had, until recently, been considered unworthy.
Plutophiles had about a week to rejoice before the astronomers returned from their deliberation with a change of heart. According to the final IAU definition, a planet must still be round but must also dominate the mass of its orbital zone. In other words, a full-fledged planet must have no competitors in its zone.. Poor Pluto is crowded by thousands of other icy bodies in the outer solar system, some bigger than Pluto itself, so it fails the test. To soothe Pluto's boosters, the IAU elected to call it a dwarf planet, without entirely quantifying what that is.
