WHAT'S A NICE PLANET LIKE THIS DOING HERE?

Updated Sunday 15 May, 2011 12:18 PM

WHAT'S A NICE PLANET LIKE THIS DOING HERE?

by Thomas Wm. HAMILTON

What, if anything has happened to Pluto? Did the IAU (International Astronomical Union) really abolish a planet? A lot of hysteria has been spilled over the IAU action, but I think that much of the concern comes from not thinking the matter through, both on the part of the IAU and of its critics.

The IAU has redefined Pluto as a dwarf planet, and thrown into that category a couple of relatively newly discovered objects such as Eris (formerly nicknamed Xena), and the long known Ceres. Eris is actually slightly larger than Pluto, but Pluto has three moons while Eris seems to have just one.

What about the other (nondwarf) planets? It was recognized even before the space program began exploring them that we have two very different types of objects called planets. Jupiter, Saturn, Uranus and Neptune turn out to be basically giant balls of gas. They are sometimes called gas giant planets or jovian planets. Other characteristics besides low density include lots of moons, rings, and a fast rotation.

By contrast we have Mercury, Venus, Earth and Mars, which are small, rocky, rotate slowly, and have few if any moons. These are often called the terrestrial planets.

A few astronomers have argued that Uranus may actually be something of a transitional form, because it is a bit denser and smaller than the other gas giants. However, it has the stigmata of rings and lots of moons, so I say leave it where it is.

Now of course we also have close to 300 known planets going around other stars. Given the difficulties of discovery, it is no surprise that nearly all of them fall solidly in the jovian camp in terms of size and mass. In fact, a majority are actually heavier than Jupiter. Exactly one has been found, orbitting a pulsar, that is in the range of the mass of Pluto. The same pulsar has three other known planets. Some argue they are remnants of a planetary system destroyed when a star collapsed to form the pulsar.

Regardless, we have been referring to all four objects around the pulsar as planets. When necessary, one can modify the word planet with an adjective--small, large, dense, lightweight, hot, cold, jovian, terrestrial, plutonoid, dwarf, etc.

Some have argued the IAU action was hasty, ill considered, and basically non-represetantive of astronomical feelings on the matter. The complaint of hasty has some merit, as parliamentary maneuvering had the IAU drop a committee draft proposal that had been worked on for more than a year in favor of something cooked up literally overnight.

The accusation of ill-considered derives from the definition arrived at. This definition explicitly is only for the Solar System, leaving the non-solar planets drifting uncategorized in space. That includes the one around a pulsar that would be a dwarf planet were it around the Sun. The definition is also ridiculed for saying a planet must have cleared minor junk away from its orbit. Of course, Mars, Jupiter and Neptune share orbital space with asteroids in Lagrangian points, the so-called Trojan asteroids. And Pluto crosses Neptune's orbit. The last time I checked, 879 asteroids were known that come close to Earth, suggesting our own planet is far from having cleared its space.

The charge of a non-representative decision comes from the fact that the vote was taken on the last day of an international convention, when well over half the attendees had already left.

So the politics of planetary definitions is as rife with controversy and shenanigans as any other politics or conventions. How to handle it?

First we should admit that the word planet comes to us from ancient times, when the Sun and Moon were counted as planets because they moved in the sky, and the Earth was not a planet. We do not have to lock ourselves into a definition derived from thousands of years ago that may have pleased Ptolemy, but is not in accord with current knowledge.

Current knowledge tells us that there are profound and basic differences between the giant planets and the terrestrial, at least as many as between either of those two and the newly defined dwarf planets. About the only thing they have in common is that they all go around the Sun and are large enough to have a basically spherical shape.

Perhaps some day we will find true transitional objects, but for now we can make clear cut distinctions. The word planet by itself refers to a large number of objects, which fall into any one of three classes. Under 3500 km diameter, and usually found with a density of 1 to 3 times that of water are the dwarf planets or plutonoids.

With a density of 3 to 5.5 (or more) times that of water and diameters of 5000 to 15,000 km we have the terrestrial planets.

With a density of 0.7 (or less) to 1.8 times that of water, rotating rapidly, and with diameters of 25,000 to 150,000 (or more) km are the giant planets, which presumably includes most of the non-solar discoveries.

Where does this leave Titan, Ganymede and the other large satellites? We could call them captured terrestrials. Triton would be a captured dwarf. Yes, I know theory says they weren't captured (except maybe Triton and our Moon), so the phrase should be captive terrestrials or captive dwarf. The inner solar system would then have five planets, one of them a captive dwarf. Jupiter would hold four captives in thrall, two terrestrial (Ganymede and Callisto) and two dwarf (Io and Europa), along with the nearly sixty other moonlets it has.

Our Moon and Miranda (moon of Uranus) are believed to be recombined from collisions that shattered a pre-existing object. In our Moon's case the collision was with Earth, and the bulk of the intruder wound up as part of Earth, our Moon representing just a small surviving fraction. So recombinant objects are another category, our Moon as a recombinant captive dwarf planet, and Miranda a recombinant moonlet.

Are we creating too many categories? I'm not certain. As we learn of more and more objects, classification becomes more complex. I have not even mentioned the rogue category (planets not in orbit around anything), or whether planets accompanying a brown dwarf deserve a separate class. Things were so much simpler when we could just say we want Class M planets.

Please Comment In The Discussion Forum

Hit Counter