Tiger stripes

What is heating up the south pole of Enceladus?

Planets X

What with all the approximately Pluto-sized and larger objects being discovered beyond Neptune, it's interesting to revisit the strange history of Planet X, the large, possibly giant trans-Neptunian planet that Pluto was originally supposed to be.

Today it's almost certain that Planet X does not exist, and the anomalies in planetary motion that it was proposed to explain were simply errors. On the other hand, other people have proposed, based on theories of solar system formation, that there could be respectably-sized icy planets lurking even further out in the dark.