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The Joy of High Tech

by

Rodford Edmiston

Being the occasionally interesting ramblings of a major-league technophile.

Distinctions

This started as a rant against bureaucrats who try to "compromise" in situations where anything over a specific, well-defined breakpoint means tragedy. For example, ichthyologists say "Take more than 1X tuna and the species will crash." Fishermen say "We must take at least 2X tuna or our industry will crash." So the bureaucrat says "We'll set the limit at 1.5X and everyone will be happy." Then is baffled when no-one is. (As well as wondering why he can't have tuna casserole any more.)

However, as I began writing I realized this was outside the scope of JOHT. Therefore, I changed it to a talk about clear - and not so clear - distinctions between astronomical bodies, which will probably be more entertaining to the readers.

There are certain breakpoints - certain clear distinctions - in the universe.

As an example, gas giant planets, brown dwarfs and stars all start out generally the same, as large bodies of hydrogen with traces of other elements. The difference between them depends on their starting mass, and often the ratio of the "other elements."

A star is a body which burns hydrogen - or, later in its life, helium and heavier elements - in sustained fusion. A brown dwarf is a body which fuses deuterium for a while, but never fuses plain hydrogen. There is a clear distinction, there; the mass required to provide enough squeeze and heat to fuse hydrogen is more than a brown dwarf has. While there is some leeway in how massive a body must be to have sustained hydrogen fusion - due to details of composition and environment - the bottom of this slightly fuzzy mass boundary is significantly higher than the minimum mass required to fuse deuterium. Deuterium - double-heavy hydrogen - is much easier to fuse than single hydrogen, but vastly scarcer. This isotope is soon consumed, and the brown dwarf is left without any power source aside from gravitational contraction. The breakpoint - the distinction between brown dwarves and stars - is the ability to fuse hydrogen.

A gas giant is distinct from a brown dwarf early in life due to being too small to fuse deuterium. Note that there will be some trace fusion activity in any body containing appropriate materials, due to sheer statistics, but these reactions are trivial. Note also that I specify that this distinction is for starting conditions. A gas giant close enough to a star will experience changes in composition, due to radiation effects - including simple heating - from that star.

The difference between gas giant planets and rocky planets is that most of the mass of the former is light elements in a gaseous, liquid or solid state (depending on depth in the gas giant) while the latter is mostly heavier elements which exist long-term in a solid state. Again, a clear distinction, though one clearer in our solar system than it may be in others.

Comets are (usually) solid bodies of light elements (which exist mostly as ices for the majority of their existence) and rocks, frozen together in a mass which is largely undifferentiated. As a comet approaches a star the most volatile elements begin to evaporate, with the less volatile ones following in order. If a comet does not fall into the star - or impact a planet – or escape the system, then on each pass it will lose more of the volatiles, until only rock and metal are left. This may actually be where many asteroids come from. It is definitely where several notable meteor showers come from.

Comets also tend to have highly elliptical orbits which are inclined - often steeply inclined - with respect to the plane of the ecliptic. That is, they don't orbit in the same plane as the planets and the asteroid belt. However, not all cometary orbits are outside the plane of the ecliptic, so this is not a determining factor in deciding whether something is a comet.

The distinction between a planet and a moon depends on what - if anything - the body orbits. Mercury orbits the Sun and is a planet. Our Moon - very similar in many ways, including size - orbits the Earth, and is a satellite.

The distinction between rocky planets and asteroids is largely a matter of size (though there are differences in composition and form) and the first distinction mentioned in this column where the dividing line is largely arbitrary. Indeed, immediately after the discovery of the largest several of them, the term for these bodies was minor planets or dwarf planets. Ceres, the largest, is 950 kilometers across, which is a good chunk of real estate by many standards. Other star systems could have belts with far larger bodies sharing orbits with swarms of smaller ones. So where is the distinction between asteroid and planet?

There is vague talk that a planet would be large enough to sweep its orbit clear of asteroids, but whether this would happen in a particular instance depends as much on several other factors as it does the mass of the body. One of these is the harmony of the spheres.

Gravitational resonances tend to shift orbiting bodies into mutually stable orbits, usually in the same plane. Which explains the Bode-Titus law most of us were taught in school, as well as why the majority of the solar system – well, from Neptune in - is so flat. In our solar system many smaller bodies are swept into a broad orbit between Mars and Jupiter. So even if Ceres were much larger it would probably still share its orbit with a plethora of smaller bodies. On the other hand, Ceres contains roughly a third of the mass in the asteroid belt. Just possibly, had things gone a bit differently, most of that material would have coalesced into a single body. Which still would have been small for a rocky planet.

Another candidate for distinguishing between planets and asteroids is that planets are large enough to have undergone gravitational differentiation. That is, during formation, while they are still fluid from the heat of gravitational collapse, the denser materials settled to the core, leaving only traces in the much lighter crust. Several moons are large enough to have undergone this process, but we already have a distinction between planets and moons. Ceres is thought to possess a rocky main body covered in ice, a lesser form of differentiation. Whether Ceres has undergone the more extreme version of this process - which would have segregated most of its denser elements deep inside - is still being studied. The fact that - alone of the asteroids - Ceres is very close to spherical leads some to believe it has. If true, that characteristic is out the window for making the distinction between a planet and a major asteroid.

Note that during the conference on the status of Pluto a few years ago, there was considerable discussion about not only whether it was a planet, but also in regard to declaring Ceres - discovered more than a century earlier - a planet. However, that would have thrown the door wide to accepting many other known - and eventually far more that are currently unknown - bodies as planets.

As we move into the outer reaches of the solar system, past the gas giants, we are in a very different realm. Those bodies here which have not become comets and closely approached the Sun are remnants of the original material which formed the planets and their satellites. They are solid, cold objects, composed mostly of methane, frozen water and ammonia, with rocks mixed in. Many may have thin atmospheres of hydrogen and other, deep cryogenic substances. They are too small to retain more than a trace at any moment, but there is enough heat even here to slowly boil the materials out of their substance, continuously replenishing that trace. Like comets, they will eventually be mostly rock and metal, though that could take many billions of years.

The Kuiper (rhymes with viper) belt is like the asteroid belt, but much further out from the Sun. It extends from just beyond the orbit of Neptune - about thirty Astronomical Units - to about fifty-five AU. This "belt" is a torus of bodies, much larger and containing far more mass than the asteroid belt. The Kuiper belt was long thought to be the source of periodic comets with orbits of less than two hundred years. However, it now appears that the belt is dynamically stable, with bodies moving in all three dimensions within the torus but rarely leaving it.

The Scattered Disk overlaps the Kuiper belt, and contains similar bodies. However, they are far less orderly. The inner border is at 30–35 AU, and the orbits involved can extend well beyond 100 AU. Scattered Disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40° and perihelia greater than 30 astronomical units. A disturbance from inside or outside the solar system could send these objects dropping towards the Sun in a highly elliptical orbit.

The Oort (Rhymes with, uh... Oh, look it up!) Cloud is still hypothetical. If it exists, it is a vast formation of primordial material extending as much as a light year from the Sun. It would be composed of an inner cloud - the Hills Cloud, a disk-shaped volume like a stretched-out Scattered Disk - and an outer, spherical cloud. There are no known direct observations of any bodies which are indisputably from either of these regions. However, long-period comets have to come from somewhere.

The distinction between bodies in the scattered disk and the Oort cloud is primarily one of temperature, which in turn depends on distance. Objects in the outer portion of the scattered disk are essentially indistinguishable in composition from those in the inner portion of the Hills Cloud.

The conditions in the outer cloud make the Kuiper Belt seem balmy. Orbits are vast, orbital velocities a slow creep, the Sun's grip so feeble that bodies are easily lost. Indeed, for our solar system, the Sun is simply the nearest star as far as these bodies are concerned. While there are many objects here, and much mass, the volume occupied is so great that the average distance between objects is enormous even on an interplanetary scale.

How big are these bodies? How many are there? Unknown. If they exist, they are almost certainly identical in makeup to virgin comets. Given developments in both terrestrial and space telescopes, we may find out in a few years. Most likely, the wait will be longer.

A few decades ago a plan was put forward to launch a gigatonne fusion warhead several AU out of the plane of the ecliptic. Once far enough away from the Sun it would be set off light an fantastic flashbulb. Since the timing of the explosion would be exactly know, when an object reflected the flash it would not only be revealed but when it showed would tell observers exactly how far it was from the explosion. So far, there are too many legal complications for serious plans to proceed.

So here we end, off in an odd corner where we simply don't have enough information to make distinctions.

Distinctions can be important, if only to help us put things in order. Scientists are often criticized for being obsessed with labeling and pigeon-holing. However, this categorization process aids scientists in understanding things. Knowing what the breakpoints are, and why they exist, is vital to figuring out the larger rules.

Another criticism against scientists is that they are always changing things. Again, this reveals a misunderstanding about what science is and how it works. Scientists are supposed to examine their assumptions, and test the rules they develop. The exception tests the rule, in the laboratory or in nature. If the rule can't pass the test presented by an exception, it needs to be changed or replaced.