Is Pluto now considered to be a comet rather than a planet?

Pluto's orbit intersects with Neptune which has led some astronomers to previously conclude that Pluto is a lost moon of Neptune. Unfortunately for that theory, Neptune and Pluto are in a gravitational 3:2 resonance with each other. Twice Pluto's year ( 247.7 years x 2 =495.4 years) is the same as three times Neptune's year ( 163.72 x 3 = 491.18 years).

Curiously, however, Neptune's moon Triton is in a retrograde orbit around Neptune which some astronomers say is the result of a collision between Triton and Pluto that ejected Pluto into its current orbit, and then spun around Triton after the gravitational slingshot was finished. Be forewarned, however, that over millions of years, Pluto's orbit becomes chaotic which means that if you try to forecast where it came from a billion years ago, the answer you get depends on the initial conditions you provide the calculations in a way that defeats your ability to predict accurately. This means that Pluto could have been an escaped moon of Neptune even though our predictions of its exact location may never be able to prove this conclusively.

Since 1978, Pluto is known to have a satellite called Charon which orbits Pluto every 6.4 days. Pluto itself is a 2300 kilometers in diameter body with a density from 0.7 to 1.0 gm/cc or about that of water. Thus, it probably consists of essentially solid ice with a possible crust of solid methane. Ice is not a very good substance from which to build mountains, so it is expected that over billions of years, Pluto has become an almost smooth, featureless round ball. Charon, on the other hand, is a 1186 kilometer in diameter satellite, making this a double planet system with the satellite almost as big as the 'planet'. If Pluto was wrenched from Neptune, then Charon must have been captured long after the event. If there are objects as large as Charon for Pluto to have captured, perhaps Pluto itself is just such a body too, and it may have been captured into the family of the solar system independently of any hypothetical interaction with Neptune. Perhaps Pluto was captured into an elliptical orbit, and then a chance encounter with Neptune and Neptune's satellite Triton regularized Pluto's orbit to the one we now see.

There seem, as yet, to be many possibilities for where Pluto came from. Dr. Brian Marsden at Harvard has advocated that Pluto be reclassified as a cometary nucleus, one of the largest ones known in the solar system, but a member of a family which spans icy bodies almost as big as Pluto and its moon. The nucleus of Halley's Comet is about 16 km across. A recently-discovered object called Chiron in 1977 orbits in a highly elliptical path just inside Saturn's orbit to nearly the orbit of Uranus every 50 years, and has a size estimated as 100 - 300 kilometers. By 1990, it was beginning to show a tail like a true comet, as it makes its way towards its 1995 perihelion at 8.4 times the Earth-Sun distance.

Curiously, Pluto was discovered by Clyde Thombaugh in 1930 through its gravitational influence on Neptune. Yet there is no way that such a small body as Pluto is now known to be could influence Neptune in this way. Thombaugh's discovery was simply a lucky outcome of a careful photographic survey of the ecliptic plane for a new planet!.

For more information about this 'debate' have a look at the article in Sky and Telescope, August 1994 where proponents of both sides of the debate have a go at each other.