It was an elusive planet that for 200 years appeared to explain Uranus's wobbly orbit. And there was the sister sun theorised to be near our solar system that caused asteroids to swerve towards Earth.
But 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")," concludes 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 about 1486 billion kilometres from the sun, or about 10,000 times farther than the Earth's orbit.
But the images gathered by the telescope did not detect any object larger than Jupiter.
Luhman doesn't rule out the possibility that a planet is lurking somewhere in the asteroid belt.
It would be hard to find if it was closely aligned with a bright star that blinds the telescope or was much smaller than had been theorised.
But after this latest survey, Luhman said the odds of finding one were very unlikely.
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.
Attempts to track this mysterious Planet X led to the discovery of Neptune in 1846. But the estimated mass of Neptune couldn't explain the deviations of Uranus's orbit.
Finally, in the 1990s, researchers determined that they had slightly overestimated the mass of Neptune, which meant the planet could be the reason for Uranus's orbital behaviour.
Yet Planet X believers were still not convinced.
The existence of Nemesis, a sun-like star nearby, was posited in the 1980s.
By occasionally coming closer to the sun, the supposed star would interfere with the orbit of comets and asteroids, leading them to occasionally hit the Earth.
