The End of the Old Solar System, the Beginning of the New

Today marks not one but two milestones in planetary exploration. It is the 25th anniversary of Voyager 2’s flight past Neptune, the most distant planet ever seen up close. It is also the exact day that the New Horizons spacecraft is crossing Neptune’s orbit on its way to Pluto, the mysterious world that marks the boundary between the solar system we know and the one we don’t.

The known solar system has planets that come in three well-studied varieties: rocky (like Earth), gas giant (like

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