Still and yet, more planet definition stuff (regarding one of the criteria being "[A planet] is large enough that it has cleared the orbit through which it moves"):
Update: A telling comment from Hal Weaver at Johns Hopkins’ Applied Physics Laboratory. apropos of dynamically cleared orbits:
“Regarding the resolution itself, I’m with Andy Cheng in concluding that the situation is still somewhat muddled. What exactly is meant by a planet ‘clearing its neighborhood?’ Since many ‘plutinos’ … (including Pluto) …cross Neptune’s orbit, I’d say Neptune’s neighborhood still needs some clearing! … It just seems a bit risky to me to base a definition on a theoretical construct (’dynamically cleared regions’) that’s only approximate at best and may change significantly as our understanding of planet formation evolves over time.
“I further note that there have been particularly large swings in the theories of outer solar system dynamical evolution during the past decade. What was ‘conventional wisdom’ five years ago has been replaced with the latest fad, and I don’t expect that situation to change any time soon.”
And some good news for those depressed about losing a planet:
And so we come to Mu Arae, a G-type dwarf star much like our Sun and catalogued as HD 160691. A new study by Krzysztof Gozdziewski, Andrzej Maciejewski, and Cezary Migaszewski re-examines this already intriguing planetary system to discover yet another planet, the fourth to be found there. These radial velocity measurements were made by the Anglo-Australian Planet Search project and build on the earlier detections of three worlds, two being Jupiter-style companions with orbits projected at 630 days and 2500 days respectively. A third planet has been characterized as a ‘hot Neptune’ in a 9-day orbit.
(Ooh baby - hot Neptune on Jupiter action!)
The new analysis finds an interesting fourth planet of about 0.5 Jupiter masses in a 307-day orbit. That fourth world makes Mu Arae the second known four-planet system, the other being 55 Cancri, and in both cases the planetary orbits are nearly circular, an architecture not so different from our own Solar System....