According to Colwell

While petitions to protest the IAU’s definition of “planet” and “dwarf planet” swirl around the internet, it is worth remembering the Pluto has always been a misfit as a planet. The standard explanation of planet formation in our solar system neatly describes the four terrestrial planets (Mercury, Venus, Earth, Mars) and the four giant planets (Jupiter, Saturn, Uranus, Neptune). When I teach freshman astronomy from a popular astronomy textbook, Pluto is handled with the other icy objects of the outer solar system, not with eight other planets. Today, attending a graduate-level lecture on the formation of the solar system, we reviewed the concept of the minimum mass solar nebula wherein a lower limit to the mass of the disk from which the planets formed is estimated. In this exercise the masses of the planets are augmented by enough Hydrogen and Helium to make their composition the same as the Sun. We looked at a figure from a thirty-year-old paper that presented this calculation - for eight planets. Pluto was not included because it is obviously not in the same category as the other eight planets. It will make for a cleaner presentation for me in the classroom not to have to explain why Pluto doesn’t fit the patterns of the other planets. It deserves to be the leader of its own class, “dwarf planets”, rather than the misfit in the planet family.

This new picture of Saturn’s rings from the Cassini spacecraft show the same phenomenon that I observe with the stellar occultation measurements described elsewhere. Notice how the brightness of the B ring varies between the bottom and top of the picture. Fingerlike clumps of particles are believed to be the cause of this asymmetry, and I have posted a prediction for our next stellar occultation measurement based on one model of this clumps, called “self-gravity wakes”.

The IAU did what I originally thought they would do and demoted Pluto from a “planet” to a “dwarf planet”. I agree with the decision. The reasoning is pretty simple. If we want the word “planet” to be reserved for a small, unique group of large objects orbiting a star, then there is no easy way to have Pluto be a “planet” the way Jupiter and the Earth are planets. Pluto is one of the largest (but not the largest) members of a vast swarm of objects orbiting the Sun in what is called the Kuiper Belt. The new definition excludes objects that are not the dominant objects in their orbital zones. This has some physical basis for it, as planets originally grow by gravitationally gobbling up (or kicking away) everything in their gravitational zone of influence. If you can’t manage to unite your neighborhood, then the best you can be is a dwarf planet. But the resolution is pretty vaguely worded about just how “clear” the “neighborhood of the orbit” has to be. Jupiter, the king of the planets, shares its orbit with thousands of Trojan asteroids that are in a special 1:1 resonance with Jupiter. So what rules out Pluto: the dozen or more “Plutino” Kuiper Belt Objects that share its 3:2 resonance with Neptune; the fact that its orbit crosses that of Neptune; other KBOs whose orbits cross that of Pluto?

Now if only we can find a better term than “small solar system bodies” for all the stuff smaller than dwarf planets.

Perhaps foolishly emboldened by the success of our model of self-gravity wakes (aligned clumps of particles) in Saturn’s rings in predicting our last observation of Saturn’s A ring, I have applied our model of Saturn’s B ring to our next observation and made a prediction about what we’ll see there. The B ring is the largest planetary ring in the solar system, and it is the big bright one in the middle in pictures of Saturn’s rings. Here’s a nice example. The B ring is particularly intriguing because, unlike the A ring where there are a lot of structures that we have some explanation for, the B ring is dominated by grooves and valleys and peaks in particle packing that are unexplained. The central part of the B ring seems to be totally opaque, so it is hiding its secrets from the stellar occultation measurements I study (where we observe the flickering of light of a star through the rings). But the inner and outer parts of the B ring allow a little light through, and we were able to piece our first observations from Cassini together to identify the signature of the same parallel clumps of particles that are prominent in the A ring. The B ring work has not yet been published, so it is with some trepidation that I present these predictions of our next measurement of the B ring.

Because the B ring is so large, I’ve broken it into two parts, and left out the central opaque region where we can make no prediction (except that it will also be opaque). The diamonds show my prediction for the next measurement on September 9, 2006. The values shown in these graphs are optical depths, which is a measure of how opaque or transparent something appears. The curious thing about Saturn’s rings that I’m seeing in these studies is the transparency of the rings depends on the angle you look at them. So I’m predicting that the new measurement will show the B ring to have a relatively low optical depth (be more transparent). The other curves show some of the previous observations to give an idea of the range of possible values. The prediction curve is not continuous because we did not have enough good data to fit the model to it all the way across the ring. Here are the predictions. Check back the week of September 11 to see how I did.

Apparently the IAU general assembly did not take to the definition offered up by their sub-committee for determining what is and is not a planet in our solar system. That’s good news in my opinion. I and others complained about the new “Pluton” category as arbitrary and also confusingly overlapping “dwarf-planet”. A revised proposal reads:

A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic-equilibrium (nearly round) shape, and (b) is the dominant object in its local population zone, and (c) is in orbit around the Sun.

Point (b) is almost exactly what was proposed by King Aardvark on his blog and a comment here. Way to go King! This new definition makes Pluto a dwarf-planet, like the largest asteroids, and that suits me fine. It is a member of the Kuiper Belt, and more than a dozen other objects are in the same orbital resonance that Pluto has with Neptune. So, 8 planets, and a whole bunch of dwarf-planets. Unless they change the wording again before Thursday. If I had a vote, I’d support this definition.

A new study of the after-effects of a collision between two clusters of galaxies provides additional evidence for the existence of dark matter. Dark matter is so dark - how dark is it? - it’s so dark that it doesn’t interact with (emit, reflect, scatter, absorb) light at all. We can’t see it, but we know it’s there because the mass of the dark matter does exert a gravitational force on all the good old regular matter that we can see. If you throw a tennis ball you can predict the path it will follow to very high precision because we know the mass of the Earth. For a cooler example, Cassini mission planners are exploring the possibility of flying within 25 km (about 15 miles) of the surface of Saturn’s moon Enceladus in a couple of years. Bear in mind that both Cassini and Enceladus are hurtling around Saturn at several miles per second and that Cassini is about 1 hour and 20 light minutes from Earth meaning there is no remote control steering of this spacecraft. If it were coming too close to Enceladus there is nothing we could do about it. Yet the only concern with this close encounter is the possibility of small grains of ice produced by Enceladus’s cool geysers smashing into some sensitive pieces of Cassini. The navigation team at JPL is fully confident in their ability to have Cassini pass so close to the surface of Enceladus because we understand very precisely how gravity makes things move.

Which brings us back to dark matter. Observers have noticed for decades that the stars and gas around distant galaxies were moving faster than they should based on how much mass could be seen and inferred to exist within the galaxy. The conclusion of this (and many other observations) is that there is additional dark matter within the galaxy whose only effect is on the trajectories of everything else. The new study, led by Doug Clowe at the University of Arizona, analyzed the distribution of matter in the Bullet Cluster of galaxies where a collision between two clusters some 100 million years ago or so separated the gas from the stars. Clowe’s group was able to show that the extra matter was still with the stars in the galaxies and not with the gas. Since cool gas has been an alternative explanation for the dark matter observations, this new observation significantly strengthens the case for dark matter because if the extra mass were gas it would have been stripped away from the galaxies along with the visible gas. Instead it appears to have stayed with the galaxies (based on their stellar motions) just as expected for dark matter.

So what is this stuff, anyway? There are some exotic candidates, but perhaps the most intriguing thing is that the majority of the universe is not ordinary matter or even dark matter, but the cleverly named dark energy, which is the name for the unknown thing that seems to be pushing the universe apart (faster than it would be going from the initial Big Bang and inflationary expansion). If you don’t already feel insignificant enough when you look at the night sky, all that you and the Hubble Space Telescope can see accounts for only 4-5% of the universe. 22% or so is dark matter and the vast majority is dark energy, and as far as understanding either one, we’re pretty much in the dark.