(P-13) Pluto and the Kuiper Belt

(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.

[In the Greek, "ch" is the letter chi, pronounced as in German, Scotch and Russian (like "kh"), though US astronomers pronounce it as in "Christie." However, John Christie claims he chose the name because the initials fit those of his wife Charlene, and he pronounces it as in her name.]

Later two additional small moons (Nix and Hydra) were also observed, and more recently two more, bringing the total to 5. Pluto's orbit is odd, too: it intersects the one of Neptune, and its period is in resonance with that of Neptune--three orbits of Neptune equal two of Pluto. Gravity makes such resonances stable, and the resonance also keeps the two from colliding.

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.

(Comment by Chuck Britton: Pluto wasn't the first planet to get demoted. Ceres was the first asteroid to be discovered and it neatly filled in the empty space between Mars and Jupiter. So, Ceres was called a 'Planet', until enough asteroids were discovered to make astronomers question the status of Ceres)

The "New Horizons" Mission to Pluto

Closest Approach to Pluto

After Voyager 2 passed Neptune in August 1989 it was tempting to explore the solar system even further, and a mission "New Horizons" was planned in 2001 to fly by the planet Pluto. The mission was informally called the "Pluto Express" because great velocity needed to reach the planet within a few years.

"New Horizons" was launched from Cape Canaveral on 19 January 2006, and it further increased its speed in February 2007 by a close encounter with Jupiter . Its closest approach to Pluto took place on July 15, 2015, and it also took images of Charon.

During the encounter itself all electric power on the spacecraft was devoted to acquiring images, delaying their transmission to Earth which required more than 4 hours to cover the distance.

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 .

Updated: 3 April 2014 ; More recently 16 October 2016

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(P-13) Pluto and the Kuiper Belt

The "New Horizons" Mission to Pluto

Above is background material for archival reference only.