Astronomers discovered Pluto in 1930, Geneva, and soon realized that this outer world actually crosses the orbit of Neptune, the next planet inward. In 1936, British astronomer Raymond Lyttleton proposed that this planet once orbited Neptune as a moon.
But now we know that Pluto and Neptune are in a special type of orbit with respect to each other – called a “two-to-three resonance.” Every time Pluto orbits the sun twice, Neptune orbits the sun three times. It is never anywhere near Neptune when it crosses Neptune’s orbit – and astronomers can find no path that leads from Pluto’s present orbit back to an orbit around Neptune. They now think that this planet has orbited the sun since its formation.