9a. Earth orbits Sun?
9b. The Planets
P--1 Planet Tabulations
P--8 Io and other
P--13 Pluto & Kuiper Belt
P--14 Comets and more
10. Kepler's Laws
(On the right: Telescope image
of Pluto and Charon)
After Percival Lowell established his Flagstaff observatory and published three books on Mars, its canals and supposed inhabitants, he turned his attention to the outer planets. Claims were made that just as the motion of Uranus deviated from predictions, that of Neptune did too. Lowell believed those claims, suspected there may exist one more unknown planet beyond Neptune, and started looking for it. Today, with the mass of Uranus better known, calculations and observations are found to fit each other well, removing the need for "Planet X."
Lowell died in 1916 without discovering any planet, and others continued to maintain his observatory (it still exists). In 1928 the observatory received drawings of Jupiter and Mars by a young amateur astronomer who had built his own telescopes, Clyde Tombaugh (born 1906); he was offered a job, and continued Lowell's search. He discovered a new trans-Neptunian planet in 1930: it was quite dim, but moved too slowly to be an asteroid.
He named it Pluto--the name of the Roman God of the Underworld (Hades to the Greeks), a name which also started with the initials of Percival Lowell.
But something was odd about Pluto: it seemed far too small to perturb a planet as massive as Neptune. For many years it was believed to be the size of the Earth, and later observations found it even smaller, not even the size of our own Moon. One reason it may have seemed larger turned out to be Charon, a big satellite, half the diameter of Pluto, discovered by John Christy in 1977.
Any more planets out there? Gerard Kuiper in 1951 argued that the rotating disk of dust and gas ("solar nebula") from which the planets condensed probably did not end abruptly with the relatively large mass of Neptune. Instead, it was possible that at the rarefied outer edge, material did not condense into single large planets but into many small ones.
He turned out to have been right, but it was only in 1992 that the first "Kuiper Belt" object was found--or rather, the first one outside Pluto, which today is considered part of the belt too. That was when David Jewitt and Jane Luu discovered "1992 QB1" with diameter of 200 or 250 kilometers (see "The Kuiper Belt" by Luu and Jewitt, Scientific American, May 1996, p. 47-52). The size of that dwarf planet was fairly typical for the belt, though many are bigger (see here) and at least one seems bigger then Pluto, too (see also here). Most orbit near the ecliptic, showing they came from the solar nebula--though an appreciable scatter of inclinations exist.
By 1996, over 30 such objects were observed, and as of November 2007, over 1000 (also see here). Many are locked in a 2:3 resonance the way Pluto is (those have been dubbed "Plutinos") and others display different resonances, all factors helping stability.
NASA launched in January 2006 a "New Horizons" spacecraft to fly by Pluto in July 2015 and to pass at least one Kuiper belt object at a later date.
That launch came before August 2006, when the International Astronomical Union "demoted" Pluto as "not being a planet" any more, upsetting members of the generations taught otherwise. Schoolteachers used to teach a mnemonic for the names of the planets in order of distance: "My Very Educated Mother Just Served Us Nine Pizzas." What will she serve us from now on--nutcakes? In fact, up in the sky nothing has changed. Pluto remains as always a planet--though not a major one, just part of a special family of dwarf planets.
Next in the solar system: #P-14 Comets and other small objects
Next Stop (following "The Planets"): #9c The Discovery of the Solar System, from Copernicus to Galileo
Timeline Glossary Back to the Master List
Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: stargaze("at" symbol)phy6.org .
Last updated: 27 February 2008