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Planet-sized problems

The debate over the definition of planethood has been simmering for years – and it bubbled up again this week, thanks to new research into free-floating planemos, or planetary-mass objects. It turns out that the debate could well be settled this summer.

Jon Lomberg / JonLomberg.com
Artwork shows a planemo, or planetary-
mass object, surrounded by a disk of gas
and dust that could form satellites.


For most of the last 75 years or so, the conventional wisdom has been that there were nine and only nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. However, several recent discoveries have complicated that picture. Astronomers now know that Pluto is actually part of a wide belt of icy mini-worlds known as the Kuiper Belt – and that at least one other Kuiper Belt object just might be bigger.

Meanwhile, almost 200 seeming planets have been detected around stars beyond our sun, and other curious objects such as brown dwarfs and free-floaters have been spotted as well.

Considering that there could be scores of objects like Pluto on our solar system's rim, should we demote Pluto from the ranks of the major planets? Or should other mini-worlds such as Xena, Sedna and even good old Ceres get a promotion instead? What classification system makes sense, not only for our little neck of the celestial woods, but for other yet-to-be-explored planetary systems as well?

As additional not-your-typical-planets are discovered, it's important to know exactly what you're talking about. That's why the planethood debate is important – not just for astronomy geeks, but also for future generations of students and explorers. Heck, astrophysicist Neil deGrasse Tyson says he received "hate mail from third-graders" after he dissed Pluto's planetary status.

Brian Marsden, the director of the International Astronomical Union's Minor Planet Center at the Smithsonian Astrophysical Observatory, has been involved in the debate for as long as anybody. "It still goes on," he told me today.

A working group associated with the International Astronomical Union has been chewing over the definition of planethood for several years, but the IAU hasn't yet resolved the issue.

There's general agreement on the upper limit for a planet: If the mass of a celestial body is 13 times the mass of Jupiter, then internal thermonuclear fusion starts up, and the body is classified as a star or a brown dwarf – that is, a failed star. It's the minimum bar for planethood that's trickier, particularly because smaller objects are generally measured in terms of diameter rather than mass.

"Three possibilities were being discussed," Marsden said:

  • On one hand, you have the "Plutocrats" who say anything Pluto's size (about 2,000 kilometers wide) or larger should be dubbed a planet.
  • On the other hand, you have those who favor setting the bar higher – say, 4,000 kilometers wide – and lumping Pluto in with the other Kuiper Belt objects.
  • On the, um, third hand, you have the planemo proponents, who would favor setting the bar much lower – as low as 300 kilometers wide. Anything above that would be considered a planemo.

"We're talking about the body itself, not what it's doing," Marsden explained. "Certainly the moon is perfectly good planemo, as are some of the moons of Jupiter."

Even the smallest planemos would tend to be spherical, conforming to our classical image of a planet. But clearly, not all planemos are planets. Marsden said planets might be defined as planemos that orbit a "fusor" – that is, any object that is generating energy through fusion.

That definition would add Xena, Ceres and other mini-worlds to the traditional planet list. However, some astronomers would toss out those candidates – and Pluto, by the way – on technical grounds, by adding a rule that excludes planemos that belong to belts such as the main asteroid belt or the Kuiper Belt.

If all this has left you feeling muddled, you're not alone. The International Astronomical Union's effort to come up with a sensible definition of planethood is somewhat muddled as well.

"Because the committee was divided on this matter, yet another committee was formed," Marsden said. This group, which includes non-scientists as well as astronomers, is to meet sometime in the next few weeks and come up with a recommendation for the IAU's general assembly in