It was an elusive planet that for 200 years appeared to explain Uranus'wobbly orbit. And there was the sister sun theorised to be near our solar system that caused asteroids to swerve towards earth.
There is just one problem: neither "Planet X" nor "Nemesis" ever existed, researchers now say. Or probably not.
"The outer solar system probably does not contain a large gas giant planet [Planet X], or a small, companion star [Nemesis]," said University of Pennsylvania astronomer Kevin Luhman, who directed the study using Nasa's Wide-field Infrared Survey Explorer (WISE) telescope. The results were published in The Astrophysical Journal.
Most theories had estimated Planet X to be up to four times the size of Jupiter - the biggest planet in our solar system. They suggested it would be found some 1,486 billion kilometres from the sun, or about 10,000 times farther than earth's orbit.
But the images gathered by the telescope did not detect any object larger than Jupiter.
Luhman does not rule out the possibility that a planet is lurking somewhere in the asteroid belt. It would be hard to find if it were closely aligned with a bright star that blinds the telescope or was much smaller than theorised.
But after this latest survey, Luhman said the odds of finding one were very unlikely: "That is like a one in a hundred chance."
Scientists first imagined the existence of Planet X in 1781 when they discovered Uranus, a gas giant that astonished astronomers with its orbital variations, apparently incompatible with Newton's laws of gravity.
Observers concluded that these irregularities could be explained by the existence of an unknown planet that was exerting its own gravitational force.
The existence of Nemesis, a sun-like star nearby, was first posited in the 1980s. The star, by occasionally coming closer to the sun, interfered with the orbit of comets and asteroids leading them to occasionally hit earth.
Such collisions are blamed for the five mass extinctions over the last 540 million years - the most recent being the dinosaur extinction 65 million years ago.
But the WISE telescope didn't find anything. The hunt may have turned up empty, but the study did uncover 3,525 stars and brown dwarfs - celestial objects whose mass puts them between a star and a large planet - within 500 light years of the sun.