Here's one more reason why leaving Pluto off the planetary list just doesn't make sense. Caltech astronomer John Johnson and his colleagues say they've found two planetary systems where pairs of Jupiter-scale planets are so close together that their orbits almost certainly cross. So much for "clearing the neighborhood of their orbits." The reason why they haven't smashed together is probably because they're locked in resonant orbits (2-to-1 and 4-to-3) that keep them out of each other's way. Other exoplanets do a similar kind of orbital dance. And so do Pluto and Neptune. Their orbits are in a 2-to-3 resonance, ensuring that Pluto keeps its comfortable status as a dwarf planet. (Which is a type of planet, right?)
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