Is there another planet beyond Pluto? Prior to 1781 that question could have been asked in regard to Saturn. In that year, Sir William Herschel discovered Uranus, after detecting what he believed to be a comet. Calculations to determine the orbit of Uranus were made, and the planet was found to conform to the "law" of planetary distances suggested by Johann Elert Bode (1747-1826).
However, a problem later arose. After sixty years, it was noticed Uranus was not following its predicted orbit, evidence that suggested another planet, the gravity of which was perturbing Uranus, must exist beyond it. Calculations for the position of this planet were made by Jean Urbain Le Verrier (1811-1879) and John Couch Adams and, in 1846, Neptune was discovered by Johann Galle (1812-1910) and Heinrich d'Arrest (1822-1875). Neptune's gravitational pull accounted for most of the differences between the predicted and observed positions of Uranus, but there was still a discrepancy.
The search continued for yet another planet. Percival Lowell (1855-1916) expended a great deal of energy looking, but came up empty-handed. However, Lowell's calculations laid the groundwork for the discovery of Pluto, which was finally found by Clyde Tombaugh (1906-) in 1930. However, Pluto turned out to be such a small, low-mass object that it could not possibly account for the perturbations. Some astronomers argue that another planet, with a mass of three to five times that of the earth, might be out there.
If there is a Planet X, it will be very difficult to find. Calculations show it would have a highly inclined (tipped) orbit, and would take 1,000 years to complete a trip around the Sun. At that distance the amount of sunlight it would reflect would be very small, making it a very dim object. Worse yet, one calculation places it within the constellation of Scorpius, which has a dense concentration of stars. Finding a faint planet there would be comparable to identifying a particular grain of sand on a beach.
To make a bad situation worse, there is no agreement on where in the sky to look; some have suggested the constellations Gemini and Cancer. It has also been suggested that the gravitational tug of a Planet X could perturb material in the Oort cloud. This cloud, suggested by astronomer Jan Oort, is one source of comets. Planet X, if it exists, could deflect some of this material, causing it to fall into the inner solar system and become new comets.
Most astronomers argue that there is no Planet X. Tombaugh's search for Pluto was very extensive; he found Pluto and nothing else, because there is nothing else, the argument goes. As far as the remaining perturbations, perhaps they are just errors in the imperfect calculations made in the nineteenth century.
- Planetary Atmospheres - Origin And Evolution, General Principles, The Terrestrial Planets, Atmospheric Circulation Patterns, The Giant Planets
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