Defining Planets (Part VIII)
Published on Feb 24, 2009 at 7:41 pm.
7 Comments.
Filed under planets.
All month long, I have been posting about the definition of the term planet, and I still have not come up with a definitive definition. That suggests that either I am really slow, or else I am trying to be thorough and that this is not an easy thing to define. I rather like to think the latter.
So far, I have discussed size, mass, structure, composition, and orbit as possible things that may be useful in our definition. Each one has failings. So, now I am going to discuss some other possible characteristics.
As I said early on in this series, the term planet basically means wanderer. To the ancients, most of the heavenly objects seemed to stay put. Seven bodies moved. These were the Sun, the Moon, and five wandering stars. All seven bodies were considered planets. There was long debate on whether or not the Sun and Moon should be classified with the five wandering stars, but as long as the geocentric model of the heavens held sway, there was no real compelling reason to bump them from the list. But, with the heliocentric model, the Sun is the center of the Solar System, and the Earth and the five wanderers orbit the Sun, making it seem more sensible to lump Earth with the five wanderers, to make six planets. The Moon was still an oddity, but with moons being eventually found orbiting Jupiter and Saturn, that made it seem that moons could be a different sort of body — one orbiting a planet. The idea that planets are anything that orbit the Sun held for a long time, but eventually it became unsatisfying once the shear number of tiny objects orbiting the Sun was realized.
But, if all of the physical criteria that we’ve tried to use have been in some way lacking, perhaps we should go towards more historical criteria. So, we might wonder if there is any difference in how planets are discovered versus the other bodies of the Solar System. Well, the classic wanderers of Mercury, Venus, Mars, Jupiter, Saturn, together with the Sun and the Moon, were known to the ancients. We don’t have any records of how they were discovered. They have simply been observed since before people were writing down their discoveries. Uranus is no help here. William Herschel discovered Uranus purely by accident. In fact, he didn’t even realize what he had found at first. Today, many comets and asteroids are discovered by accident. Astronomers, either professional or amateur, are looking for something else and an anomalous body winds up in the field of view. Somewhat more are discovered during targeted searches: astronomers compare images taken of the same part of the sky some time apart to look for anything that moves.
Neptune, though, is the different one. Neptune was discovered after analysis of a small perturbation in Uranus’ orbit. The French astronomer LeVerrier calculated where the perturbing body should be, and sure enough it was there when they looked! Then, Pluto was found by accident while looking for something that may have been perturbing Neptune’s orbit. As it turns out, Neptune’s orbit did not really have a perturbation, so there was nothing there to actually perturb it. But, since they were looking for something, they found tiny little Pluto, which happened to be in the vicinity, but far too small to have had any effect on Neptune. Had astronomers not been looking in that area, Pluto might not have been the first Kuiper Belt body found, and we might not be having all this discussion about what constitutes a planet. And, of course, as I said in earlier postings, the first asteroids discovered were found during a search for a non-existent missing planet between Mars and Jupiter.
So, there is no set way that planets have been found in the Solar System. And, of course, there are bodies just like the planets of the Solar System orbiting other stars. Those should be planets, too. However, these extrasolar planets are found in even different manners than are the planets of the Solar System. So, the manner of discovery doesn’t really make something a planet or not.
Perhaps, we should just go back to the basics and say that planets are things that appear to wander around the sky. But, there are uncountable numbers of such things. We need some way to limit them. Of course, to the ancients, that meant simply the ones that can be seen. If we limit ourselves, though, to just the naked eye objects moving in the sky, they we have very few planets. But, even that is not unambiguous. Uranus and some of the largest asteroids are technically just above the naked eye threshold for visibility, only they are so dim that they get lost in the many stars in the sky. Worse, they get lost in light pollution. The skies are so bad today in much of the world that even in what most people consider dark skies these bodies would not be visible. So, do we limit the planets to binocular visible bodies? What size binoculars? The only way to sensibly use this definition is to go back to just the classical planets. But, if we did that, what are Uranus and Neptune? And, of course, what are the bodies like Jupiter and Saturn that orbit other stars? Obviously, such a definition is not likely to be accepted by very many people.
So, that means that we need to consider yet more possible planetary criteria. And, that will be in my next posting.
-Astroprof
Astronomy Link List on February 25, 2009 at 5:38 am: 1
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Laurel Kornfeld on February 26, 2009 at 1:07 pm: 2
I vote for the latter option in your first paragraph. You’re definitely trying to be thorough and consider all aspects of a very complex issue–something that the IAU notably did not do, something no one is capable of doing in a few rushed hours of discussion.
Sili on February 26, 2009 at 2:35 pm: 3
If I were the conspiracytheorist type, I’d suggest that New Horizons is really a big bomb, sent out there to blow Pluto out of the sky and solve the problem (for now).
Boxorox on February 27, 2009 at 7:47 am: 4
Yes, plausible criteria are vital for establishing a valid definition for the descriptor planet. As I mentioned in a previous reply, the problem here resembles armchair arguments between mountaineers who swap claims of what constitutes mountains versus hills. There is no clear delineation. But perhaps a better outline for this discussion can be found in the standardization geologists employ to define the various soils: silt, dust, sand, pebbles, gravel, boulders and such. A point is reached in the convention where we have to draw a line and set somewhat arbitrary, but rigid, tolerances for the characteristics which separate one type of body from another. These criteria will concern size, mass, sphericity, orbital eccentricity, orbital inclination to the ecliptic. I would also add that a subjective, but hopefully quantifiable qualification can be found acceptable in that a planet should be expected to be a planet throughout the lifespan of its star. As you noted in a previous post, some Kuiper Belt objects may be relegated to non-planet status simply because they are not spherical enough due to a major collision in the past that prevented isostacy and hydrostatic equillibrium to let it resume its previous planetary spherical shape. Perhaps if a body cannot retain planetary standards because it cannot stand up to environmental conditions, it isn’t really a planet? After all, the Earth probably endured a huge collision with a Mars-sized object in the Early Heavy Bombardment which resulted in the creation of the Moon. Earth was probably a planet before this event, and it most surely was a planet afterward. Or can we even establish a definition of planet that would actually excluse Earth (and others) from being planets? Sure, we could, but no one would be comfortable with that and we would have to invest a whole new class to cover the mid-sized terrestrial bodies.
Lilly on March 2, 2009 at 11:17 am: 5
I just got a novice level telescope for Chirstmas and I want to know where to look in the sky for planets. I live in the outskirts of Pittsburgh. Are there any websites listing where planets are right now or maps?
Astroprof on March 2, 2009 at 12:01 pm: 6
A couple of good web sites are out there. I rather like Sky and Telescope’s site which has a customizable sky map. If you are new to astronomy, I’d also suggest ether Sky and Telescope or Astronomy Magazines as good guides. Both have sky maps in them.
Link list – 25th February 2009 | Astronomy Link List on April 6, 2009 at 10:06 am: 7
[…] Defining Planets (Part VIII) Astroprof’s Page Perhaps using a historical context is also incorrect when defining what a planet is. Here the Astroprof explains more on part 8 of his series. […]