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Pluto: A Planet By Default
by Bruce Moomaw
Only a mission to Pluto will resolve the issue Cameron Park - June 28, 1999 - There was a huge furor earlier this year over whether the International Astronomical Union should allow Pluto to keep its official title as a planet, or officially redesignate it an asteroid because of its small size -- or perhaps even a giant comet, given the fact that it is composed largely of ices!

After an acrimonious dispute among the astronomical community, the IAU decided to keep things as they are. But was the IAU's decision correct, or was it an unwise surrender to an outcry from an uninformed public?

I think the decision was correct -- and here's why. The recent movement to relabel Pluto -- launched largely by prominent asteroid and comet expert Brian Marsden -- is simply because it has turned out over the past 20 years to be far smaller than had been previously thought, and far smaller than the next smallest planet Mercury.

In fact, it is only about 2300 km in diameter as opposed to Mercury's 4900 km, and only 1/25 as massive. Indeed, it's considerably smaller than our Moon and six other moons in the Solar System. But this in itself would not seem to be reason enough to change its long-time status with which most people have come to be familiar, for Pluto in turn is far larger than the largest asteroid, Ceres, which is only 940 km in diameter and only 1/12 as massive as Pluto.

Complicating matters further, Ceres is much bigger than all the other asteroids -- it's almost twice as wide and over 2 1/2 times more massive than the next biggest ones, Pallas and Vesta. Arguably one could set the number of planets at ten, including Ceres!

And to complicate matters still further, Dr. S. Alan Stern of the Southwest Research Institute set forth a new theory in 1991 which now appears to be probably correct.

At that time, most theories of the planets' formation stated that the initial cloud of dust and gas orbiting the ancient Sun had first condensed into trillions of clumps of material only a few dozen kilometers across -- and that the planets had then formed out of only a few chance accumulations of these clumps which then "snowballed" into much larger objects as their growing gravitational fields pulled in more surrounding material.

By this view, there had never been many intermediate-size objects hundreds of kilometers across orbiting the Sun (at least in the outer Solar System).

Stern points to several curious phenomena in the outer Solar System:

  • the fact that Uranus lies on its side, which is very hard to explain unless a very large object crashed into it during its early days;
  • Pluto's oversized moon Charon, which is hard to explain scientifically unless a large object had crashed into Pluto at some point and Charon had formed out of the splattered debris, as is now thought to be the case for our own Moon.
  • Neptune's largest moon Triton, which orbits the planet backwards and must therefore have originally been another small icy planet like Pluto that just happened to brush very close to the early Neptune and was captured by it.

Stern has proved to the satisfaction of many astronomers that any of these events -- let alone all three of them -- were fantastically unlikely unless there had been hundreds or even thousands of large objects, hundreds (and in some cases thousands) of kilometers in diameter, orbiting in the Solar System during its early days in addition to the swarms of smaller comets that were already known to have been there. There had after all been an intermediate stage in the size growth of the objects orbiting the Sun.

What happened to them? They met the same fate as the smaller comets: most of them collided to form the four outer planets, while most of the others, as their orbits were modified by the tugging of the giant planets, eventually flew past one of the planets closely enough to be catapulted away from the Sun altogether, or into the outermost depths of the Solar System.

Calculations indicate that the gravitational tugging of Neptune alone tended to clear most of the Sun-orbiting objects out of the region between 5 and 8 billion kilometers from the Sun --but even this region (the inner "Kuiper Belt") still contains hundreds of times more material than the Asteroid Belt!

The count of objects discovered there during the initial preliminary searches of the past decade indicates that it holds tens of thousands of objects between 100 and 500 km wide -- and probably dozens of objects up to 1000 km wide, as big as Ceres, although our initial telescope searches haven't discovered any of those yet. It may even hold several more objects as big as Pluto.

Further from the Sun -- all the way out to the suspected but incredibly distant "Oort Cloud" of comets trillions of kilometers from the Sun, a large part of the way to the nearest star -- are thought to hold far more objects, perhaps hundreds of them. It's possible that some of those objects are much bigger than Pluto, and some may be as big as the Earth.

So how many planets are there -- and how do we even define "planet", since it now seems virtually certain that there are many Sun-orbiting objects intermediate in size between Pluto and the biggest asteroids, and many others intermediate between Pluto and Mercury?

Stern himself thinks that we should call any object big enough to have been permanently rounded into a spherical shape by the force of its own gravity (so that it would regain that shape even if a collision took a large chunk out of it) a "planet".

But this would include every object more than 200 or 300 km wide, including over two dozen of the asteroids! It seems unlikely that most astronomers will be willing to accept such a big nomenclature change in the near future; certainly the public would be reluctant to do so.

The only other choice is to set some completely arbitrary size limit as the boundary between "planets" and smaller objects -- say, 2000 km diameter, which would allow Pluto to keep its traditional label as a planet while still keeping the Planet Club as exclusive as possible.

In either case, it makes the most sense to continue to call Pluto a planet, while accepting the fact that the term will always be far more ambiguous than we thought before 1990 -- and that, since the Oort Cloud is so incredibly far from the Sun that many of the objects in it will certainly never be discovered, we'll never really know how many planets the Sun has, no matter how we end up defining "planet".

Way out there at SpaceDaily

  • Possible Pluto Debris Spotted
  • Is Pluto An Ethane Fuzzball
  • Pluto a Planet By Default
  • High School Students Discover Kuiper 72
  • Kuipers Reflect In Two Colors
  • Views of Pluto
  • Space Science News - SpaceDaily Special Report

    Additional Pluto Links

  • Subaru Telescope
  • The Status of Pluto: A Clarification
  • The Status of Pluto: A Discussion
  • SEDS: The Kuiper Belt and the Oort Cloud
  • David Jewitt's "The Kuiper Belt": - co-discoverer of first Kuiper
  • S. Alan Stern's home page:
  • Additional Kuiper Papers by the Space Studies Dept at SWRI
  • Pluto at Space Science News

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