With no explanation for the gravitational forces controlling the gas giants, astronomers continued to search for the tenth planet, dubbed, Planet X. A researcher named Robert Harrington calculated that a planet three times the distance from the sun as Neptune had to exist. It needed to possess a 32-degree angle orbit from the system's plane. While studying Halley's Comet, Joseph Brady, an astronomer with the Lawrence Livermore National Laboratory, also hypothesized that a Jupiter-sized planet was most likely in retrograde orbit beyond Neptune.
During a brief period in 1983, the Infrared Astronomical Satellite (IRAS), showed an “unknown object" the size of Jupiter near the edge of the solar system. However, after further analysis, it was found to merely be a distant galaxy.
The quest for a tenth planet in the solar system was essentially abandoned in 1993 following a flyby of Neptune by Voyager 2 in 1989. Data collected from the flight revealed that Neptune's mass was considerably smaller than previously thought. When the Jet Propulsion Laboratory analyzed this information in regards to the orbital situations of Uranus and the other gas giants and found there were no discrepancies in the gravitational forces. This meant that the existence of Planet X or a tenth planet was unnecessary and highly unlikely. This was again substantiated, according to astrophysicist Neil deGrasse Tyson, by information made in the original calculations of Neptune's orbit. Documents show that the research was tainted by an astronomer making poor adjustments to his telescope's gear box.
In 2006, Pluto was removed from the status of planet, returning the planet count in the solar system to eight.
Above left: Voyager 2. (Supplied by NASA at Wikimedia Commons; Public Domain; http://upload.wikimedia.org/wikipedia/commons/d/d2/Voyager.jpg)