Sorry, Conspiracy Theorists: Planet X Doesn't Mark the Spot
Well, that’s that then. There are no other big planets in our solar system, no other giant planets orbiting the Sun in the distant, cold reaches well past Neptune.
This news comes from a paper analyzing observations by WISE, the Wide-field Infrared Survey Explorer, a scrappy little mission that spent 13 months mapping the entire sky in infrared wavelengths. This is where warm objects are bright, things like dinky stars, asteroids, galactic dust, and more.
WISE was very sensitive, and as able to see objects that were pretty faint. For example, it found tens of thousands of previously undiscovered asteroids, some of which get pretty near the Earth. These glow in the infrared, heated by the Sun.
What it didn’t discover, though, was another giant planet in our solar system. And it’s pretty definitive: it would’ve seen a planet the size of Saturn out to a distance of 1.5 trillion kilometers, over a tenth of a light year! A planet the size of Jupiter would’ve been seen out to twice that far.