PlutoScientists calculated that Neptune’s gravity was not enough to cause all of the irregularities of Uranus’ orbit.

They also found that something else was perturbing Neptune’s orbit.

Percival Lowell calculated the probable location of this third body and searched for it in the decade prior to his death in 1916. He never found “planet X”.

In 1930 Clyde Tombaugh found the planet only 6° away from Lowell’s predicted location. He announced his discovery on March 13, 1930. (March 13 was Lowell’s birthday)

Neil deGrasse Tyson with Clyde Tombaugh’s Wife

These are the actual photos Clyde Tombaugh used to locate Pluto.

Actually Pluto is too small to cause the perturbations calculated for Uranus and Neptune. Also, these calculations were later found to be in error, so the discovery of Pluto was nothing more than “dumb luck”.

Pluto in bulk:

Semi-major axis: 39.5 A.U.

Eccentricity: 0.25 (very large)

Pluto’s orbit is inclined 17.2° to the ecliptic, more than any other planet. Pluto’s orbital period is 248.6 years (1.5 times Neptune’s).

Pluto’s orbital path carries it inside the orbit of Neptune at perihelion. So, at times, Pluto is the eighth planet from the Sun rather than the ninth, as it was from 1979 to 1999.

Pluto has three known moons, Charon was the first one discovered. The other two, Nix and Hydra have been discovered very recently.

Charon’s orbit is inclined 118° to Pluto’s orbital plane, so Charon orbits at a right angle to Pluto’s orbit around the Sun.

Pluto’s rotation angle is tilted at 120°, so the moon still orbits over the equator of Pluto.

Charon’s orbital period was used to find the mass of Pluto. Charon’s orbit was lined up such that there were a series of Charon-Pluto eclipses from 1985 to 1991. (This only happens once every 124 years.)

These eclipses helped astronomers to find the mass, radii and other information about Pluto and Charon.

Pluto’s mass is 0.0025 Earth masses. Remember, Earth is one of the smaller planets; so, in size, Pluto is more like a moon than a planet.

Pluto is smaller than seven solar system moons: Earth’s Moon plus the moons Io, Europa, Ganymede, Callisto, Titan, and Triton.

Pluto’s diameter is 2300 km. (one-fifth that of Earth)

Charon’s diameter is 1300 km. This is over half the diameter of Pluto.

Pluto and Charon are tidally locked. The same side of Charon always faces Pluto, and the same side of Pluto always faces Charon.

Charon’s orbital period is 6.4 Earth days.

Charon’s mass is about one-sixth that of Pluto. Were Pluto still a planet, that would be the largest satellite to planet ratio in the solar system. Now that “honor” belongs the the Earth and Luna.

Pluto is similar in mass and radius (and; therefore, probably in composition) to the Neptunian moon Triton.

Pluto has a high albedo, 0.6, which aided in its detection.

Frozen CH4 is a major surface constituent. Pluto is the only planet cold enough to have solid CH4.

Pluto is so different from the Jovian planets and is so much like the moons of these planets, it was thought to be an escaped moon of one of the gas giants.

This was a good explanation, until Charon was found. It is unlikely that two “lost” moons would come together to form the Pluto-Charon system.

There may have been lots of ice chunks like Pluto orbiting the Sun at one time. Many of them eventually captured to form Jovian moons while another formed Charon. But, all of this is just speculation.

Pluto was removed as a planet when the definition of a planet was officially defined. A planet must: (1) orbit the Sun, (2) be large enough to be formed into a sphere by gravity, and:

(3) it must have enough gravity to clear its orbit of other large objects. Pluto fails to meet this last qualification.

Many trans-Neptunian objects have been found at orbits beyond Neptune’s. One, Eris, is actually larger than Pluto.

The discovery of Eris was the last straw in the reassignment of Pluto to dwarf planet status. (One asteroid, Ceres, is also considered a dwarf planet.)

New Horizons launched in 2006, scheduled to arrive at Pluto in July, 2015.