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.

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 Prague this August.

“We may even be allowed to vote on it,” Marsden said, which holds the promise of finally producing an official ruling on planemos and planets.

To Marsden’s way of thinking, giving Pluto “special status” as one of the nine and only nine planets would be exactly the wrong thing to do. That would be like “waving a red rag in front of a bull,” he said. He’d be fine with reducing the solar system’s planetary list to eight, and perhaps giving Pluto a place of honor among Kuiper Belt objects as a consolation prize.

But another way of resolving the Pluto problem might be to celebrate our planetary diversity rather than getting hung up on the historical “nine planets.”

“In the solar system, we’ve got four groups of planets – that is, planemos orbiting the sun,” Marsden said. Under the scheme he described, there would be midsize rocky planets (Mercury, Venus, Earth and Mars); rocky dwarfs (planemo-sized asteroids within the orbit of Jupiter); gas giants (Jupiter, Saturn, Uranus and Neptune); and icy dwarfs (Pluto and the other planemos in the Kuiper Belt and beyond).

That kind of classification system would carry over quite easily as more powerful telescopes – and who knows, maybe even on-the-spot expeditions – chart planemos beyond our own solar system.

You don’t have to be a professional astronomer to have an opinion on the matter. In fact, the astronomers want to come up with a system that makes sense to those Pluto-crazy third-graders as well as planetary scientists. That’s exactly why the IAU is officially bringing non-scientists into the debate. Here are some of your own thoughts, e-mailed in response to this week’s planemo story:

Larry: “Pluto is the cutoff point. If it is as large as Pluto then it is a planet if it is not then it is not.

Brad: “A planet should be defined by its size and shape – asteroids and free-floating rocks are rarely spherical. If it has enough mass to shape itself into a sphere (the most efficient expression of energy), then it should be called a planet.”

Dale Sikora, Billings, Mont.: “The word ‘planet’ refers to a gas giant or a rocky satellite orbiting a star.  Aside from planets, we also have comets, asteroids, Oort Cloud objects, Kuiper Belt objects, and now planemos. The Kuiper Belt objects (and theoretically asteroids) cause us the problem in making a definition because some may be the size of Pluto — a planet.  So are other Kuiper Belt objects planets as well?  Some Kuiper Belt objects are obviously comets or other nondescript balls of ice and dust, but all objects in the orbital plane of a star, which are large enough to have a significant moon or large enough to clear their own orbital path, should be considered planets.” 

Ron: [A planet is] “any object orbiting a star, with a minimum mass of 1.0 X 10²² kilograms (Pluto is 1.27 X 10²² kg) and a maximum mass of J13 (or a mass 13 times greater than Jupiter, which is the minimum mass for a brown dwarf to support deuterium fusion). Any object of this mass range, free floating in the space between stars, would be a rogue planet.Any object of this mass range orbiting a planet (as defined above) would be a moon. If both objects are of planet mass and orbiting a star, then the larger is a planet and the smaller is a moon.

Brian: “planet (n.) - A stellar object which is massive enough to form a spherical body under its own gravity, but not massive enough to ignite nuclear fusion, and has either no natural satellites or has natural satellites which are less massive than itself. If Pluto is still to be considered a planet, then any Kuiper Belt object with the same diameter or larger would have to be considered a planet regardless of its orbital eccentricity and inclination.”

Amy: “I am an astrophysics major at New Mexico Tech University and if I were to be the one to define what a planet is, my answer would be: A planet is any object of relatively circular shape (unlike the shapes of asteroids) whose orbit around the sun of the solar system to which it belongs is on relatively the same plane as the rest of the objects of that same solar system. Its orbit would not be so elliptical that it is brought within the orbits of inner planets (as are comets). Said object would be capable of having satellites that orbit it primarily as it orbits the sun.  By this definition, Pluto would be a planet because it revolves around the sun on the same relative plane as all other planets (though slightly angled), is circular in shape, and it has three known satellites so far discovered. Though its orbit intersects that of Neptune, it never has a tail and never comes with the orbits of the other planets.”

Chuck Miller: “I would define a planet as a body of matter orbiting a star and not orbiting anything else.  A moon would be a body of matter orbiting a planet, and being larger than ring material.  Comets would not be considered planets. Asteroids or meteors are bodies of matter the size of Ceres or smaller. Any body of matter larger than Ceres which does not orbit a star is an extrasolar planet.”

T.W.: “A definition of a planet: A mass that is orbiting a star that possesses an atmosphere, that is further sub-categorized by composition of atmosphere.  Lacking an atmosphere, it should be defined as an asteroid.”

That definition, by the way, might demote Mercury as well as Pluto. Also, it may be tricky to determine whether a distant body has an atmosphere. Pluto, for example, is thought to have a thin atmosphere during portions of its orbit.

Mardi Coleman, Dallas: “To me, a planet is any object with enough mass and matter to have a spherical shape (unlike an asteroid) and its own gravity, but not enough mass to start thermonuclear fusion, and orbits a star (not another planet) in its own, unique orbit (not shared with other bodies like the Kuiper Belt objects), regardless of the orbital path (circular vs. elliptical). A planet and a moon could be the same size. As long as they meet the above definition, the difference would be what they orbit, not their size or mass. For example, if a planet the size of Jupiter had an object the size of Earth orbiting it, the smaller body would be the larger body's moon. Sort of a ‘moonet.’ And yes, I consider both Pluto and the unnamed 10th planet to be planets.

Robb: “A planet of late seems to be anything that orbits a star and has enough mass to pretty much clear out its orbit.  But then again, what would you call the moons around our gas giants?  Some are bigger than our rocky planets, and would be planets themselves if they were in orbit around the sun.  Hey, let the astronomers with all their smarts decide.  God knows they paid enough for their education to have earned that right.”

Feel free to register your opinion by clicking through to our Live Vote, and/or adding your comments below.