For centuries, astronomers have speculated that the solar system contains undiscovered planets that orbit in the distant, dark reaches of the sun’s realm. From time to time, they have spotted the gravitational effects of unknown bodies, forcing them to look for the culprit. Both Neptune and Pluto came to light in this way.
Now, astronomers have a similar puzzle on their hands. For some time, they have been gathering evidence that a massive planet must be orbiting the sun at a distance of around 500 astronomical units, or 70 billion kilometers.
The evidence comes from the orbits of icy bodies in the Kuiper Belt beyond Neptune. These objects seem to cluster together in ways that can only be explained if they were being “herded” by some massive object.
This object — Planet Nine, as it is dubbed — must be between five and 10 times the mass of Earth, but ...
