https://www.washingtonpost.com/news/speaking-of-science/wp/2016/01/20/new-evidence-suggests-a-ninth-planet-lurking-at-the-edge-of-the-solar-system/A fascinating possibility-- Planet X strikes back! Or maybe not. Percival Lowell first hypothesized the existence of a "Planet X" for the same reason Brown and Batygin are predicting a "Planet Nine", which are anomalies in the orbits of trans-Neptunian objects. Pluto was originally thought to be Planet X, and, I think, similar in size to this hypothesized Planet Nine. However Pluto was found to be too small to have an effect on the orbits of Neptune and Uranus (and the perceived "anomalies" eventually turned out to be the result of erroneous estimates of those planet's masses), and Planet X fell off the map, except for those of conspiracy theorists and New Agers.
Now it is back, except this time it is lurking somewhere between the Kuiper Belt and the Oort Cloud (I didn't know there was a gap between them). It's quite incredible if it actually exists-- think about it, we've been finding thousands of planets outside of the Solar System for the past few years now, but managed to miss the a big planet in our own system. Would it even be in the Solar System, though? The heliopause, the point at which solar wind stops, extends 100 AU (1 AU = 150 million km) out from the Sun. This planet's orbit is hypothesized to be 200 AU at its closest to the Sun and 600 AU at its furthest. By comparison, Uranus orbits at around 20 AU from the Sun. So it may not even technically be in our Solar System. It also means it will be a long time before any probe we send gets there--
Voyager 1 only crossed the heliopause back in 2012 and it had been flying for 35 years. I suppose we'll have to stick to trying to take pictures of it for the time being.
