Episode 29: Was the Asteroid Belt a Planet? Part 1
Recap: A discussion on the history of the asteroid belt's discovery and four solid ways of showing that the asteroid belt could not have been a single planet that broke up or exploded. Stay tuned for next episode on the "Exploded Planet Hypothesis."
Puzzler: One supposition of exploding planet people is that Mars was a moon of this hypothetical Planet V. An observation they point to to support this is that half of Mars is heavily cratered, which was the side facing Planet V when it exploded 65 million years ago. What is one of the many lines of actual scientific understanding about Mars that this is not correct?
Solution to Episode 27's Fact or Fake?:
Item 1: From a dark sky site, there are around 10,000 stars visible to the unaided eye. FAKE - About 6000 stars are naked-eye visible, but from any given location at any given time you'll see around half.
Item 2: There is documentation as far back as the 1800s with people claiming ownership rights to the moon, though not necessarily to try to sell it to other people for profit. FACT
Item 3: After Galileo discovered the four largest moons of Jupiter, he gave all the rights to their surface to his local funding source, the Medici family. FAKE - But he did name them the Medician Stars, though we now know them as the Galilean Satellites.
Q&A: Donovan W. from Mobile Alabama, USA, AKA "Ravenhull" on the SGU message boards who asks: "Is there any evidence that Earth's magnetic poles having been significantly offset from the physical axis? That is, the physical northern magnetic point having been at a point below the Artic [sic] Circle or such? Is it possible for the magnetic poles to have any stable position that far off of the planet's rotational axis, with me assuming the cores sharing the same axis."
The answer is, "None that I could find." A concept that you generally learn in introductory electricity and magnetism or general physics classes that freaks out AP test supervisors is called the "Right Hand Rule" or "RHR" for short. This is where you can use various fingers on your right hand to figure out if a charge is moving one way, which way the magnetic field will move and which way the force will point.
In this case, the RHR shows that if Earth's core's spin is coupled with the surface, at least in direction, that the resulting magnetic field will be perpendicular to it. It's not exactly perpendicular, as discussed in Episode 25 on the magnetic pole shift, because of turbulence and other factors. But basic physics indicates that it should be fairly close.
And, in my searching, I could not find any evidence that there had ever been a strong dipole field that was significantly far away from the spin axis of Earth, such as a north magnetic pole at the equator. If anyone happens to know of any research that indicates otherwise, please let me know.
However, that's not the case with the ice giants, Uranus and Neptune. Uranus' magnetic field is tilted by 58.6° and shifted by a third of the planet's radius, while Neptune's is tilted at 46.9°, and offset from the center of the planet by half of Neptune's radius. The ice giants' magnetic fields are really weird. People who actually like E&M have looked at the problem and think it may be characteristic of how material flows in the planet's interiors. It could be generated in a thin spherical shell of electrically conducting liquid ammonia, methane, and water oceans, as opposed to the metal in the core of Earth.
Neptune's field is also unusual in that it has very strong quadrupole moments, as opposed to just the dipole or "bar magnet" that we use to simplify Earth's and the other planets. This means that it's really complicated, but people who like E&M and fluid dynamics and other things like it are working to understand what's going on.
- Sources for Audio Clips and Quotes
- Coast to Coast AM, Hour 3, from August 22, 2006
- Dreamland from Feburary 22, 2012
- Additional Resources
- Asteroids 1 Ceres, 2 Pallas, 4 Vesta, 10 Hygiea
- DAWN mission at Vesta (and then on to Ceres!)
- Titius-Bode Law
- Magnetic Monopoles and Maxwell's Second Law (Gauss's Law for Magnetism)
- Nice Model for early solar system dynamics
- Additional Resources for the Q&A
- Upcomming Meetups/Conferences
- Relevant Posts on my "Exposing PseudoAstronomy" Blog
This episode is about the asteroid belt and the general, non-conspiracy misunderstanding that it could have been a planet. Just the general misconception and some of the history.
Looking back through the vast library of episodes I now have out, I'm drawn to Episode 13, "The True Story of Planet X," where I talked about the discovery of Uranus and Neptune and then Pluto.
I had said that, until 1781, the solar system was known to consist of Earth, Venus, Mercury, Sun, Mars, Jupiter, and Saturn, along with the moon, some other moons, and some unexplainable and unpredictable comets. That was it, and it wasn't until William Herschel observed a ball-like object that he described as non-star-like and moving among the fixed background stars.
Herschel's discovery was of Uranus, and it set the stage for an intermediate discovery in 1801 that I left out of that episode for streamline-edness-sake. It was on New Year's Day, January 1, 1801, that the Italian monk Giuseppe Piazzi noticed a faint dot of light that also appeared to move across the sky, a little faster than Herschel's planet but more regularly than a comet.
He observed it for 41 days, became ill, and the object was lost. Pierre-Simon Laplace, Dean of the French astrophysical establishment said that from the limited observations, an orbit could not be calculated.
The young whipper snapper Carl Friedrich Gauss in Germany, who was 24 years young at the time, said - and I paraphrase here - that Laplace's conclusion was nonsense, that he could compute an orbit from only three observations, and did so, successfully predicting where the object would next be found in the sky, on December 31, 1801.
But I'm skipping a bit of other history here, something that many of you may heard of called the Titius-Bode Law, originally proposed by Johann Daniel Titius in 1766, and refined in 1772 by Johann Elert Bode.
The Law is an empirical law that is a basic geometric progression that roughly gives you the distance between the known planets (at the time) and the Sun, in units of Earth-Sun distances. It does this reasonably well with errors averaging around 2% except for Mars at just over 5% off, and Saturn at just under 5% off.
When Uranus was discovered in 1781, it was found that the Titius-Bode Law held for it, too, being off by only 2.08%. The problem is that it predicted there SHOULD be a planet between Mars and Jupiter, but none had been found.
If you'll permit me an aside in my small, non-linear rambling through history, some of you who know about this Law may be jumping ahead and waiting for me to say that it's just an empirical relationship and there's no reason why anyone should have used it back then, much less today to argue for a missing planet in the asteroid belt.
While that may be true - there is absolutely NO physical reason for it to work, and it doesn't work - there are many empirical laws in physics and other fields that DO work and we don't really know why. A good historical example brings us once again to Kepler, who I seem to bring up in almost every episode. He's just that important. Or, you can disprove most astronomical pseudoscience today with stuff he knew 400 years ago.
Either way, Kepler's three Laws of Planetary Motion were completely empirical. He did not know why they worked, or why they were, they just ... were. It wasn't until Newton came along years later and gave them a theoretical framework that we then knew why they worked.
Similarly, in electrodynamics, every physics student has to memorize Maxwell's four equations or laws. The second one is commonly said in physics parlance as "div-B equals zero," which means basically that magnetic monopoles cannot exist. This is an empirical law, and if magnetic monopoles ever are found, then this will have to be changed - the Law will be shown to be wrong.
My point in this aside is that one shouldn't really diss the scientists of the day for looking to the Titius-Bode Law for predictive power, and when Ceres was discovered and confirmed, for astronomers of the day jumping to the idea that it's the missing planet predicted by that Law.
It remained classified as a planet for a half century, even though several other asteroids were discovered during that time. Once more and more asteroids were discovered, though, and it was found that they all had a somewhat similar orbit, the group of objects were called asteroids instead.
And that's about all the history that I'm going to go into in this episode. The point of all that is that Ceres WAS considered to be a planet, and there is a historic reason for why it was considered to be a planet. And there are still people today who want to use the Titius-Bode Law, which fails miserably to predict Neptune and Pluto, to argue that there used to be a planet there.
Why It's Not an Exploded Planet: Mass
One way to look into the question of whether the asteroid belt ever could have been a planet is to look at the mass involved. A concept that's often difficult to keep in mind - I know I forget it all the time, is that mass does not scale linearly with diameter. Mass goes as the cube of the diameter.
This is important because you can look at the biggest asteroid - Ceres, and think, "Okay, this sucker is about 1000 km in diameter." You can then look at the second-biggest asteroid, Vesta, which is currently being orbited by the Dawn mission, and think, "Vesta is around 500 km across." The third-biggest asteroid is Pallas, and that's also around 500 km across. And finally, the fourth-biggest is Hygiea, at a highly ellipsoidal 530x400x370 km.
If you add all these up, putting them together, you get something around 2500 km across, which is over half as big as our Moon (the Moon being about 3500 km across). Throw in all the other stuff, and you're sure to get an object that's at least as big as the Moon.
Seems reasonable until you remember that mass goes as volume, which goes as diameter cubed. So Ceres, with a diameter of about 1000 km, or around 27% the diameter of the moon, only has a VOLUME of 2% of our moon. Vesta at half Ceres' DIAMETER is going to have a volume that's 1/8 Ceres', or only around 0.25% our Moon. Same with the other two largest ones, and then you keep getting smaller and smaller.
So in reality, if you add up all of the asteroids, even accounting for the trillions of tiny ones that we haven't discovered yet, the total MASS and VOLUME is going to be just a few hundredths of our moon. Under most reasonable definitions of a planet, this is not what one would consider to be a planet even if all of this stuff were in one object.
Why It's Not an Exploded Planet: Differentiated Asteroids
A second way to show that the asteroid belt could not have been a planet is that at least the largest two asteroids themselves are differentiated. This means sort of what it sounds like -- differentiated is when an object has separated somewhat into different layers based on density. You can think of a vinaigrette dressing that's been left sitting too long as having differentiated. Earth is differentiated because it has a core, mantle, and crust. So is the Moon.
So are Ceres and Vesta. Both of these asteroids have differentiated, meaning that when they formed, they formed by themselves and over a few million years the heavy elements sunk to the core while lighter ones rose to the top. This could not be the case if these are chunks of an exploded planet, but I'll talk more about that in the next episode.
Why It's Not an Exploded Planet: Orbital Dynamics
A third way to look at why the asteroid belt is not the result of at least a RECENT broken up larger object is the dynamics of the asteroids in the belt as a whole. One person - Barbara Hand Clow, whose website is titled "Journeys Through Nine Dimensions" and has a book out called Catastrophobia and is a 2012 person - claims that the asteroid belt was from a planet that exploded 11,500 years ago. [Clip from Dreamland on February 22, 2012, starting at 13:17]
Tom Van Flandern, who I will talk about much more extensively in the next episode, as well as have clips of him, believes that the asteroid belt was two planets, one that exploded 250 million years ago, another 65 million years ago, and then one of its moons exploded 3.2 million years ago ... the other moon being Mars. Anyway, the asteroid belt's dynamics tell a different story.
As I mentioned in Episode 23 on Zecharia Sitchen, the asteroid belt is dynamically stable over long periods of time. It is an evolved system that is not going to look significantly different a billion years from now and didn't look significantly different a billion years ago. In fact, there are families of asteroids that travel together, and based on the basic orbital dynamics and how spread out they are, we can calculate when each family broke up from a single parent asteroid. Many of these date to hundreds of millions or billions of years ago. As in, not the entire belt 11,500 years ago, or 3.2 million years ago, or any other number these folks like to throw out.
To claim it just means that either they are not aware of the physical evidence that says otherwise, or they need to be able to show that the basic physics used to reach these conclusions, along with thousands of other non-related conclusions because it's all the same basic physics, is wrong. Or why somehow the asteroid belt is an exception somehow for some reason. Like maybe Carl Sagan's invisible dragon flew threw it.
Why It's Not an Exploded Planet: Planet Formation
A fourth and final way (at least for this episode) to show why the asteroid belt is not the remnant of a broken up planet is the basic argument that some of you are probably familiar with - Jupiter. Most models of planet formation have Jupiter forming reasonably close to where it is today, and very early in the solar system's history. In a nutshell, the gravity of Jupiter would disrupt the formation of a planet close by, so you could think of the asteroid belt, in a way, as the planet that never formed ... even though there's not nearly enough mass for it to be made into a planet, anyway.
And for those of you who know about the Nice model - to stave off that question - the Nice model also has Jupiter forming roughly where it is today, and the outer gas and ice giant planets just formed closer than they are today. The argument for a planet not being able to form there due to resonances with Jupiter still holds.
Conclusions: And that's about it. That's why the asteroid belt was not a planet. But that doesn't mean we can't have a little more fun with this topic. To whet your appetite, or, depending on how you feel about such things, to warn you away from the next episode on April 8, here's a clip of some of what will be discussed: [Coast to Coast AM clip from August 22, 2006, Hour 3, starting at 06:06]
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