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Tombaugh was searching for "Planet X," a planet beyond [[Neptune]] that was believed to be influencing Neptune's orbit. Percival Lowell, founder of the observatory where Tombaugh worked, had calculated possible sky positions for Planet X based on observations of Neptune's orbit. However, Tombaugh did not make use of Lowell's calculations. Instead, he used the observatory's telescope to photograph the opposition point to the sun every night for a year, thus conducting a full sky survey of the Zodiac.

Tombaugh was searching for "Planet X," a planet beyond [[Neptune]] that was believed to be influencing Neptune's orbit. Percival Lowell, founder of the observatory where Tombaugh worked, had calculated possible sky positions for Planet X based on observations of Neptune's orbit. However, Tombaugh did not make use of Lowell's calculations. Instead, he used the observatory's telescope to photograph the opposition point to the sun every night for a year, thus conducting a full sky survey of the Zodiac.

When astronomers realized that Pluto is not large enough to influence Neptune's orbit, the search for Planet X was resumed. The idea of Planet X was debunked in 1989, when [[Voyager 2]] flew by [[Neptune]]. NASA scientists determined that Neptune was significantly heavier than previously supposed, invalidating the earlier orbital calculations by Lowell and others.

When astronomers realized that Pluto is not large enough to influence Neptune's orbit, the search for Planet X was resumed. The idea of Planet X was debunked in 1989, when [[Voyager 2]] flew by Neptune. NASA scientists determined that Neptune was significantly heavier than previously supposed, invalidating the earlier orbital calculations by Lowell and others.

== The dwarf planet controversy ==

== The dwarf planet controversy ==