Was Pluto Really Tombaugh’s Ninth Planet?
After Lowell’s death, the search for the ninth planet was taken up by Clyde Tombaugh. Tombaugh had a new piece of equipment Lowell did not have use of—a blink comparator. This device enabled Tombaugh to look at two photographic plates taken hours apart in a kind of animation, this way he could detect movement of an object—a planet—against the star fields.
On February 18, 1930, he detected movement between two plates. The object was dimmer than he expected, but it was obviously, he thought, the ninth planet.
The planet was officially named Pluto, and for years was listed as the ninth planet of the solar system. But before the accolades were done, there were problems. Lowell had calculated that the ninth planet, to cause the observed orbital disturbances of Neptune, had to be 6.6 times the mass of Earth. Other astronomers calculated it at twice Earth’s mass. Continuing observations indicated it was much less than that, too small, in fact, to account for Neptune’s orbital meanderings.
More recent observations, with the 200-inch at Palomar and the big instrument at Kitt Peak and even more recently with Hubble have shown that Pluto is even smaller than previously thought, only 1440 miles in diameter.
And there was another problem with this problematic ninth planet--its orbit. Pluto’s orbit was highly elongated, actually dipping inside Neptune’s orbit for 20 of its 249 year orbital period. And its orbit is inclined to the ecliptic 17 degrees. This means it sometimes is high above the orbit of Neptune, and sometimes well below it. It doesn’t act like a real planet.
As it moves away from Neptune, it travels to a strange region of the solar system some 50 AU from the sun. Known as the Kuiper (pronounced ki-per) Belt, Pluto is among thousands of similar and smaller objects orbiting the sun on the verge of interstellar space.
There is another problem with Pluto. It is not where it should be according to the Titius-Bode law, which states that planets form at certain specific distances from the sun. Here is a table showing the orbital distances of the eight planets and asteroids from the sun in AUs.
Ceres (the largest asteroid, now a Plutoid) 2.8
Note that Pluto’s orbit is much too close to Neptune and the sun according to the law. The ninth planet should be at about 77 AUs.
All these factors came to a head in 2006 to cause the International Astronomical Union (IAU) to downgrade Pluto from a planet to a dwarf planet, or Plutoid as such objects are being called. But more significantly, it is obvious that Pluto could not have been Tombaugh’s ninth planet. It was in the right place—at the time—but it isn’t big enough and its orbit is too bizarre.
So where is the ninth planet—or does it even exist?