The Ninth Planet: Where is Pluto, When and Why Was it Demoted?

The Ninth Planet: Where is Pluto, When and Why Was it Demoted?
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A Perturbed Uranus

When Uranus was discovered, astronomers plotting its orbit were surprised to find that it was not following the predicted path around the sun. There were anomalies in its path that shouldn’t be there, unless, there was another eighth planet beyond Uranus disturbing its orbit. The search was on for this new member of the solar system, and it was found in 1846. Neptune became the most distant planet in the solar system…for a time.

But soon, disturbances to its orbit were detected. Did this mean there was a ninth planet even beyond Neptune? For many astronomers, there had to be. It was the only explanation.

Astronomers and mathematicians worked feverishly to calculate the orbit and size of the ninth planet, and for years searched for it in the heavens. Percival Lowell searched the skies with the 40-inch refractor at Yerkes for decades without success. Actually, he did photograph what would become the ninth planet, but he thought it was too dim to be his target.

Lowell’s instincts would turn out to be correct decades later when what became the planet Pluto, was demoted from planetary to dwarf planet status.

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.

Mercury: 0.387

Venus: 0.723

Earth: 1

Mars: 1.524

Ceres (the largest asteroid, now a Plutoid) 2.8

Jupiter: 5.203

Saturn: 9.539

Uranus :19.19

Neptune: 30.06

Pluto: 39.53

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?

A Coruscating Place of Mystery

The bizarre region Pluto travels to on its far swing away from the sun is a place of rocks and ice and comets…and other Plutoids. In fact two other Plutoids, Makemake and Haumea have been discovered in the Kuiper Belt. They are smaller than Pluto, and have orbits that keep them in the Belt.

Kuiper belt and Pluto’s orbit

And in 2005, a planet was discovered on the outer edge of the belt, with an aphelion almost 1000 AU from the sun (its highly eccentric orbit brings to about 88 AU at perihelion). Named Sedna, it was larger than Pluto by about 27 percent, and was at the time dubbed the tenth planet. But the IAU’s redefinitions have since demoted it to a dwarf planet.

Do other dwarf planets lurk in this mysterious region? Do even larger real planets hide here? There is an enigma about the Belt that puzzles astronomers. They call it the Kuiper Cliff. Theoretical studies indicate the number of objects in the belt should increase as you move further away from Neptune.


They don’t. At about 50 AU, the number of objects drops off precipitously. One possible explanation—an Earth sized planet that swept away all the smaller bodies. Remember, the Titius-Bode Law says the ninth planet should be about 77 AUs from the sun. Is it hiding beyond the Cliff?

Could Tombaugh’s ninth planet be hiding in this strange region? Could it be the culprit that knocked Uranus on its side, then skulked back to its hidey-hole?

The Belt holds one more mystery for astronomers. It is a plethora of colors. Surface colors of cosmic bodies indicate their composition. In a region such as the Belt it would be expected most bodies would be of similar color. Not in this mysterious region. The bodies range from slightly blue to neutral to red to ‘ultra-red,’ a deep red coloration that could suggest organic material.

A Hypothetical Cloud of Comets and…?

Far beyond the Kuiper belt, Dutch astronomer Jan Oort proposed a huge region of comets existed consisting of ice and rocky bodies. This hypothetical region was named after him and is known as the Oort Cloud. Because it is hypothetical, and has not been detected, its distance and extent is in question. Estimates range from its beginning as close as 20,000 AU from the sun to beginning at about 50,000 AU. It is estimated to extend perhaps as much as 200,000 AU into interstellar space.

Oort proposed the Cloud to explain the existence of long-term comets—those that have periods of hundreds, even thousands of years. Short term comets, such as Halley’s, come from the Kuiper Belt. We have no explanation for the long-term events, unless there is an Oort Cloud.

But as noted, no telescope has seen it, no instrument has detected it, and the two Voyager spacecraft that escaped the Solar System detected no sign of it, so we aren’t sure it actually exists.

But if it doesn’t, where do the long-term comets hail from?

Summary of the Outer Mysteries

It is clear Pluto could not have been Tombaugh’s ninth planet. Does such a body exist, hidden behind the rocks, comets and dust of the Kuiper Belt? Did it at one time slip its boundaries and knock Uranus on its side?

Perhaps we will get some clue when the New Horizons spacecraft arrives at Pluto nine years from now.

And the Oort Cloud…is it there, or is it just a convenient artifice to explain the unexplainable?

The mysteries beyond Neptune are perhaps the greatest mysteries in the Solar System.


This post is part of the series: Mysteries of the Solar System

As our robot emissaries have moved out into the solar system, they have sloved many mysteries, but they have found many more. Our small piece of the universe is a mysterious land.

  1. Mysteries Of The Solar System–The Planets And Their Moons
  2. Mysteries of Our Solar System: What Lies Beyond Neptune?