Could there be another planet lurking in the dark, frigid outskirts of the solar system? This isn't as silly as it seems at first. No, I'm not talking Nibiru or any of that other nonsense (and it is nonsense), I'm talking about an actual planet, Earth-sized or so, that could be orbiting the Sun well beyond Pluto Neptune. Why would we think there might be one out there?
We see some stars in the midst of forming planets. The stars are surrounded by thick disks of material, and in some we can actually see gaps in the disk, dark rings like the gaps in Saturn's rings, that we think are due to forming planets gobbling up material in the ring. You'd think the disk would fade away with distance form the star, like our air gets thinner with altitude. But some disks appear to have sharp outer edges. This can ...
