A multiplanet enthusiast

 kw: book reviews, nonfiction, emigration, planets, mars

From time to time I use a very small prompt with an art generation program. The prompt for this image was "Planets everywhere." The site I used was Leonardo AI with the Lightning engine and its Dynamic style. The engine clearly doesn't understand either physics or astronomy. Actually, the way these programs work, the engine cannot be said to "understand" at all. But I like the surreal look of the image.

I produced this image about a year ago. It came to mind when I saw the cover art for Out of This World and Into the Next: A Physicist's Guide to Space Exploration by Adriana Marais. The author is based in South Africa, and she was one of the Mars One candidates for their plans to send people on a one-way trip. Mars One went out of business in 2019, but the banner has been taken up by Elon Musk and SpaceX.

The book is rather big; it's intended to be comprehensive, a sort of "History of humanity starting from the Big Bang." In keeping with the author's Germanic heritage, the Table of Contents is a thorough outline. Denying the trace of Teutonic detail-mindedness found in my own heritage, I'll touch a few items of interest and leave the rest to the interested reader.

The subjects of Part I, "Where do we come from?", remind me of a Bible verse. The three subjects (elaborated in 13 subchapters) are "The Universe", "Our biosphere", and "Are we alone?" Zechariah 12:1 tells that God "stretched out the heavens, laid the foundations of the earth, and formed the spirit of man within him." Each set of 4-5 items goes from broad to narrow. The tip of the arrow at the end of this Part introduces the Kardashev Scale. This is worth a bit of discussion.

Nikolai Kardashev proposed a three-point scale to describe the energy made available to a technological civilization:

1. A Type 1 civilization, labeled Planetary, has attained control of all the energy available on a single planet, including all the incident sunlight.
2. A Type 2 civilization, labeled Stellar, captures and uses all the energy output of one star, presumably its home star.
3. A Type 3 civilization, labeled Galactic, controls all the energy of its home galaxy.

Many, myself included, add Type 0, to represent the beginning of technology as evidenced by the first mastery of fire. This level was apparently achieved before Homo sapiens arrived on the scene, as much as two million years ago. How far have humans come towards becoming a full Type 1 civilization? Relevant facts:

• Worldwide energy use is about 625 exajoules per year (note: 80% is fossil fuel use). That's 174 billion megawatt-hours (MWh, = 1,000 kWh) per year.
• Solar energy influx is 1.53 million billion MWh per year. The ratio of these two numbers is about 1:8,800. In other words, our global energy use is 0.0114% of the energy available in sunlight.
• Internal heat flow of the earth, half radiogenic and half primordial, is 410 billion MWh per year. Thus total geothermal energy is about 2.4 times the global energy budget.

Thus we are about 1/90th of a percent along the way to being a Kardashev Type 1 civilization. Dr. Marais points out that photosynthesis converts just under 2% of sunlight into sugar, putting the biosphere's Kardashev level about ninety times farther along, compared with vaunted human technology.

Years ago I first examined the "carbon debt" indicated by the oxygen in the atmosphere and the oxidized iron in Earth's crust. Going back to the origin of oxygen-emitting photosynthesis (which came along half a billion to a billion years after other varieties of photosynthesis such that found in sulfur-digesting bacteria), we first find about a billion years of accumulation of "red beds" and other iron oxide deposits. Once the iron was all oxidized, oxygen began to accumulate in the atmosphere, as carbon-containing detritus was buried in the crust and later subducted beneath it by plate tectonics. I concluded that if we could find all the oil, natural gas, coal and tar in the crust, and also the graphite and diamond in the upper mantle, and burn it, that might use up most of the oxygen in the atmosphere. By contrast, 200+ years of fossil fuel consumption have "used up" about 0.01% of it. 

A little thought is sufficient to realize that there are limits to the energy available from fossil fuels, and not just because the oxygen will run out (to be replaced by carbon dioxide). Rather, we must consider the medical fact that humans (and who knows how many other animals) begin to feel distressed when the amount of carbon dioxide in the atmosphere exceeds 1,000 ppm. So, we don't want to go anywhere near that! From the vantage point of 430 ppm today, up from 280 ppm prior to the Industrial Revolution, 1,000 ppm seems far, far off. It isn't. Global energy use is still increasing, fast. Think about that.

A final point. I have seen it proclaimed that human civilization today is approaching a Kardashev level of 1; estimates range up to 0.7. Hmph! We are far below that. Just focusing on Kardashev levels zero and 1, we can consider a logarithmic scale, so that a scale value of 0.7 would represent energy use of about half of total solar input, or 765 quadrillion kWh per year. That's about 4,400 times as much energy as we are presently using! We need a different scale for energy consumption less than 0.1 of the solar value (the logarithm of 0.1 is -1, and starting from 1 for "1 solar output", the result is a Kardashev level of 0). For the moment, I'll set this aside.

As for levels 2 and 3, I have a couple of thoughts. A spherical construction that encloses the entire Sun at a suitable distance (probably between Earth and Mars) could capture all of the Sun's energy. Such a construction is called a Dyson Sphere, after the physicist who popularized the concept. We would need to take apart at least all of Jupiter and possibly Saturn also to accomplish it. Communication is a bit of a problem; the travel time of light or radio across the sphere would take about twenty minutes, one-way. Communication in a Galactic civilization is another thing entirely; getting a signal across the Milky Way Galaxy would require about 100,000 years. Talking across dwarf galaxies such as the Magellanic Clouds would be quicker; only a few thousand years!

In Part II, "Who are we?", the focus at the culmination of the 15 chapters is Terraforming. The next-to-last topic is "The Anthropocene," a designation that some have proposed for a geologic age that began 250-300 years ago with the Industrial Revolution, and so far has resulted in the extinction of many species (though it is so far probably less than one percent). Can we indeed terraform Mars? Let us contrast this with our current activities, which could be considered "the anti-terraforming of Earth" (my coinage).

It is worthwhile to ask, does our experience of human nature give us any grounds for optimism that we can properly terraform another planet? It is wise at this point to jump to the conclusion in Part III (Where are we going?), its last section, "Transforming our world." That is the crux of the matter. Can we terraform Earth? This is a question asked by astrophysicist Neil deGrasse Tyson: Which will cost less, establishing one (or a few) million people in a new colony on Mars, or restoring the environment on Earth so that future global catastrophes will not wipe us off the planet? Clearly, he is in favor of the latter course, and I agree.

I hope Dr. Marais gets her chance to go to Mars. She wants it so much she can taste it. Her evocative essay "Off-World" almost gives us the taste for it. It is markedly utopian. She needs to add to her scientific perspective the lens of history. No utopian society has ever succeeded. The advice of a pioneer of space inhabitation, "Go with friends!", tells you only part of what you need to know. Because friends change. Those who once ran together grow apart. The current divorce rate of about 50% does not bode well for the ability of mere humans to establish the kind of society she describes.

I like the dream. I wish I could conclude it won't turn into a nightmare.

Preparing to write this, I ran the prompt "Planets everywhere" in OpenArt's DynaVision engine, with AI Enhancement turned on. This was the result. The physics is a little better, but this is still surreal. If you were to see this density of planets in the sky, it would be best to go far away. The chaotic gravitational regime would soon prove catastrophic. Still, it's a fun image.

===============

A couple of minor errata:

• On p. 188 Yuri Gagarin is said to have "experience[d] the weightlessness of being beyond Earth's gravitational field." Not even close. He was well within it, but orbiting such that the centripetal force and the gravitational force exactly canceled. He experienced microgravity. If he'd been paying attention, he might have noticed very slight tidal forces tending to reorient him if he tried to float free inside his capsule.
• On p. 195 we read that the wavelengths of visible light are "similar to the size of atoms". The wavelength range of visible light is 400 nm to 700 nm, or roughly half a micrometer. The diameter of a typical atom is 2-3 tenths of a nanometer, or some 2,000 times smaller. The author is making the point that light bounces off material stuff. Actually, X-rays of the sort used in medicine do have wavelengths similar to the size of atoms, and they pass right through most materials. So the physics is wrong anyway.

The first big scientific collaboration

kw: book reviews, nonfiction, science, astronomy, planets

During the early decades of the Eighteenth Century, several factors came together. The philosophical and scientific ideals of the European Enlightenment were loosening the nationalistic and political ties that "natural philosophers" felt toward their countries, even as political tensions mounted; the great empires of England, France, and Russia were settling down (though squabbling) and global transportation was now possible; advances in astronomical calculations had led to a great ability (with much laborious paperwork) to predict the positions of the planets and moons of the solar system; the achromatic telescope objective, invented in 1730, became affordable in useful sizes (up to about 50 cm diameter); and a hortatory essay by Edmond Halley, published in 1716, was being circulated.

That essay predicted the first of a pair of transits of Venus across the Sun on June 6, 1761, and called upon astronomers everywhere to observe it from as many places as possible around the globe, so as to obtain measurements that would permit more accurate determination of the scale of the solar system. Thus, the few years prior to 1761 saw a series of discussions, proposals, and requests by scientists for funding (from their various governments and other patrons), that led to astronomical expeditions from South Africa to Newfoundland, and from Siberia to India.

The events that led up to the heroic expeditions in 1761 and 1769 (for the second transit of the pair), the observations themselves, and the aftermath, are ably chronicled for us in Chasing Venus: The Race to Measure the Heavens, by Andrea Wulf. The men who "chased Venus" to all corners of the Earth were up against incredible odds. Not all succeeded. Some died. Some managed to transport half a ton or a ton of instruments to their remote targets, only to be clouded in and miss the entire transit.

In the First World, we think nothing of hopping in the car and driving several miles in a few minutes to buy groceries, or perhaps dozens of miles in an hour or less to try out a new restaurant. With a little more preparation, we travel by jet to "civilized" locales on six continents (getting to Antarctica is still a bit of an ordeal). Some years ago, I found that returning from my in-laws' home in Japan, to our own home in Oklahoma, took just under 24 hours, and we thought of that as a difficult trip.

During the heyday of the British Empire, a fast sailing ship could travel between 40 and 60 miles per day. If you were lucky enough to sail straight through without encountering any storms, you could cross the Atlantic in about two months. Land travel in areas without good roads (nearly anywhere outside a major city) was slower than that; 5-10 miles daily was typical. Getting from France to Tobolsk, in Siberia, took an astronomer named Chappe about a year in 1761; getting to California in 1769 took even longer, and he died there after making his observations.

I was quite interested to find that one pair of observers, who observed the 1761 transit from the Cape of Good Hope at the tip of Africa, was Charles Mason and Jeremiah Dixon. They observed the 1769 transit also, but from different locations. In between, they spent a few years in America surveying the boundary between Pennsylvania and Maryland, now called the Mason-Dixon line.

What makes a transit of Venus so special? It is one of the rarest of predictable phenomena. A pair of transits will occur eight years apart, minus about 3 days, but the next pair will occur more than a century later. The period from pair to pair alternates between 113.5 years and 129.5 years, for a total cycle of 243 years. A transit is also an excellent opportunity to take measurements to establish the solar parallax. This parameter is the difference in angle (properly, the half angle) between observations of the Sun from exactly opposite edges of the Earth; knowing the size of the Earth, one can then compute the Sun's distance.

Why aren't the transits more frequent? The orbit of Venus is at an angle of about 3.4° to that of Earth. Eight Earth years are just slightly longer than 13 Venus years. Thus, when the Earth crosses the plane of Venus's orbit, Venus usually isn't anywhere nearby. These crossings aren't exactly opposite one another because Earth's orbit is a little bit elliptical (The orbit of Venus is the most nearly circular of all the planets). So while conjunctions occur a couple times yearly, they are usually not transits.

In 1761 Europe was in the midst of the Seven Years' War. The astronomers sailed across the earth at the risk of their lives. Amazingly, none were lost to hostilities. In 1769, that war was over, but tensions remained. Getting Chappe to Baja California was a political nightmare for the French Academy…and for him. He had to travel under guard on a Spanish ship. Those who traveled to the frozen North, whether in Siberia or northern Scandinavia, had to go when travel was physically possible, in winter when the rivers were iced over. It is amazing that plump, well-fed astronomers did so, and generally prevailed. In the south, in 1769, Tahiti was the target of an expedition led by James Cook, who became the first European to visit Hawaii in 1778. The entire trip took three years. Southern observations in general were hampered by the lack of land in the southern Pacific Ocean.

It was necessary to observe the transit, and time it accurately, from locations as far-flung as possible, so that many partial parallaxes could be combined into a composite, theoretical equatorial parallax. This required measuring the longitude, and the exact moment of local (not statutory) noon, using tall case clocks and observations of the position of the Moon against reference stars, and doing hours and hours of calculations.

From this amazing body of work, the results in 1761 were a little disappointing. One of the biggest problems was that Venus has an atmosphere, which refracted sunlight, making it very tricky to determine exactly when a transit began or ended. One had to time the first moment of ingress, then the first moment light could be seen between the rear edge of Venus and the outer edge of the Sun a few minutes later; then the reverse sequence as the planet approached the other edge of the Sun and slid off the disk six hours later. Venus's atmosphere led to confusing phenomena such as the black drop effect, and to variations in the timing by ten seconds or more, sometimes much more, from observer to observer. As a result, the scatter of calculated values for the parallax ranged from 8.28 to 10.6 arc seconds. That led to a distance range of 77 to 99 million miles. Not very accurate.

1761 had been a practice run, many decided. They would do better in 1769. They did, with three times as many observers in about twice as many places. Although the weather was cloudier than it had been eight years before, larger numbers of better observations were performed. The "best value" as judged by the most competent astronomers was 8.78 arc seconds, which compares well with the value we accept today of 8.794. That, and a less accurate value for Earth's size, led to a distance about 1% too long, a tremendous achievement for the time.

The cosecant of 8.78 arc seconds is 23492.6; multiplying by the radius of Earth (3,960 miles) we get just over 93 million; the average distance to the Sun that we accept these days is 92.96 million miles. The distance calculated in 1769 was actually 93.73 million, based on a slightly larger value for the Earth's radius. So the angular measurement of the Solar parallax was actually in error by only 0.15%.

By 1772, most of the scientists had returned home, the results had been published, and life was settling back to normal. But a seed had been planted. Ms Wulf traces modern "big science" such as the Large Hadron Collider and the great observatories, both on Earth and in orbit, to this first global collaboration between scientists from a number of countries. One could say, though nations may war, among scientists peace prevails, as it must for civilization to continue.

Missing the transit

kw: astronomy, planets

Wouldn't you know it. I alerted friends via e-mail and FaceBook to yesterday's transit of Venus. The Sun was out here until just about ten minutes before it started, then it clouded up. Over the next several hours, I got a few calls from people all around the country telling me they were watching it in various ways (welding lenses, projection through binoculars, whatever). By then it was raining. I saw nothing. Bummer!

The next transit of Venus will be Dec 11, 2117. If I can hang on until I am 170 years old, perhaps I'll get a chance to see that one.

The Solar system begins to settle down

kw: book reviews, nonfiction, astronomy, astronomers, memoirs, planets

I grew up with nine planets, like nearly everyone alive today. I didn't learn any fancy mnemonic, but just learned the names right off: Mercury-Venus-Earth-Mars-Jupiter-Saturn-Uranus-Neptune-Pluto. From time to time over the years, a new estimate of the size of Pluto was announced. Once thought to be a very dark object the size of the Moon, it gradually shrank (and its albedo increased), finally settling down to a size of about 1,400 miles; the current figure from images by the Hubble Space Telescope is 1,430 miles or 2,302 km.

In recent years, the discovery of large objects (but smaller than Pluto) in the Kuiper Belt and beyond precipitated a discussion of Pluto's status: Is it a planet or not? (The Kuiper Belt is like the asteroid belt, but composed of icy bodies that are farther from the Sun than Neptune.)

As of mid-2006, Pluto is definitely not a planet; that is, it is defined as a "dwarf planet", a designation that always merits the quote marks, because semantically, "dwarf planet" means "planet of a smaller size" rather than "not a planet at all".

This image from the Wikimedia Commons shows the status of the "dwarf planets" found beyond Neptune as of 2008. Six of these were found by Mike Brown, who was also instrumental in the decision to demote Pluto, though he disagrees with "dwarf planet" at least as much as I do. His memoir How I Killed Pluto and Why It Had It Coming is a delightful romp through the life of a young astronomer who set out to "find a planet", and was known for just about a year as the first person to find a planet since the 1930s. Bigger than Pluto, and 2.5 times farther from the Sun, the body he provisionally called Xena was called the Tenth Planet for those heady months, until the IAU decided to define "planet" for the first time. It took a few tries to get it right.

The current definition, settled on as the book was being written, includes these factors:

• A planet circles the Sun, not any lesser body in the Solar System.
• A planet is large enough to have gravitationally settled into a sphere.
• A planet gravitationally dominates its orbit, sweeping it clear of other bodies.

There is a bit of a quibble about Trojan asteroids in Jupiter's orbit, and similar bodies in the L4 and L5 regions of other planets, but the fact remains that Pluto shares its orbital region with several bodies of similar size, so it is demoted from planet status based on the third factor.

The book tells two stories. One is the search for planets beyond Neptune, and the other is the courtship and marriage of the author to his wife Diane, and the early years of their daughter's life. Both stories reveal a human side of an astronomer's life that is seldom seen. Astronomy is by its nature a night job, though this has abated somewhat now that professional telescopes are computer driven and gather their images automatically. But if an astronomer is not staying up all night tending a telescope, the midnight oil still gets burned: studying the images, using the information they convey to locate earlier images that may reveal more facts about the object of interest, and tons of book work eat up huge amounts of time. A night's observations can require months to digest and analyze, and long hours are the norm if a scientist doesn't want to get scooped by someone else willing to work longer hours.

Xena eventually got renamed Eris, and a satellite around it was named Dysnomia. Eris is the goddess of discord, and the body is fittingly named for the squabbles that erupted once its discovery was announced. Since about 2008 the larger Pluto-like bodies have been called Plutoids, and a few hundred other Kuiper Belt objects (KBO's) are called Plutinos.

The author conjurs a fantastic image of an alien approaching the Solar System, and noticing first the four Giant planets, led by Jupiter. Then four smaller bodies are seen from closer in, that lie between Jupiter and the Sun (Earth is the third, but the alien doesn't know that). Finally, two bands composed of multitudes of bodies are discerned, one inwards of Jupiter, the asteroid belt, and one outwards of Neptune, the Kuiper belt (plus an outer region called the scattered disk). We then can realize that the eight planets, four big and four small, dominate the System; a few rounded bodies are the largest members of the two belts, and it remains to be seen whether the Kuiper belt plus scattered disk will yield up an even larger body, or several. Eris may have a big brother or two waiting to be found.

Solar System astronomy is healthy and exciting.

P.S. What it takes to find a new Plutoid.

1. A telescope with at least a meter aperture (Mike Brown used a 48-inch Schmidt at Palomar, first with film, later with digital sensors).
   
2. A wide-angle camera with 50-100 megapixels of sensor. This combination can record bodies of Magnitude 18-24 (Xena/Eris was M17 when found).
3. A half year to a year of observing time, mostly the half-month each month when the moon is the darkest and out of the way.
4. Fast computer power to compare the millions of points on one day's images with the next.
5. Gallons of midnight oil, so to speak, because the computer produces "possibles", which an expert human has to verify into a shorter list of "probables", and make extra observations to nail them down.
6. Time reserved on larger instruments, including the Hubble, if possible, for that nailing down process.
7. An accommodating spouse; you'll be traveling a lot.

P.P.S. Magnitudes. Take the time to read This Wikipedia article. Briefly, bright stars have small numbers as their Magnitude; "First magnitude" refers to bright stars. The dimmest stars visible to a young person in a dark sky have Magnitude 6 or 6.5. Magnitude is the negative logarithm of brightness, to the base of the fifth root of 100 (about 2.5), so a magnitude difference of 1 is a brightness ratio of 2.5:1 and a magnitude difference of 5 is a brightness ratio of 100. With a 10-inch diameter (250mm) telescope, the dimmest stars visible are magnitude 15. The brightest Plutinos have magnitudes of about 17 or 18, so they are very faint indeed. You'd need a telescope the size of a minibus to see one of them visually.

Planets, planets everywhere

kw: speculation, planets

I read an article in which the author used a chart like this one to argue that many small extrasolar planets are yet to be found. While the conclusion is right, some of the reasoning is flawed.
This chart shows a frequency diagram of the 432 exoplanets so far catalogued, plotted by their line-of-sight masses (mass times the sine of inclination). The "1" bin includes all those with a mass of one Jupiter or less. The strong inflection in the shape of the distribution is said to indicate that the smaller masses are underrepresented. And they are, as we will see.

The appropriate analysis of wide-ranging data is not linear but either logarithmic (power law) or lognormal. The same data, collected into bins with a range of 2:1 in mass, show a more useful picture:
Here, "1" is again "one Jupiter", but the mass range is 0.71-1.41 rather than 0-1. The parenthesis points out that one Earth mass is 0.0031 Jupiter mass; one Mj is 318 Me. Here the sharp inflection to the left of 0.5 Mj also indicates that the mass distribution is artificially cut off at low masses. But now we have the ammunition needed to estimate just how much.

The only stellar system for which we have a reasonably complete catalog of planetary masses is our own. This chart of the distribution of planetary masses is based, not on Mj but on Me, the mass of Earth. Looking closely, we see four pairs:

• Mercury and Mars have very small masses, 0.055 and 0.11 Me.
• Venus and Earth are the second pair: 0.815 and 1 (of course).
• Uranus and Neptune are next, at 14.5 and 17.1 Me, respectively.
• Saturn and Jupiter are the heaviest: 95 and 318 Me.
  

These fit a lognormal distribution with a logmean (base 10) of 0.57 and a logSD of 1.36. The confidence interval about both these parameters is rather large, but I'll use them to produce a fit from which we might learn something useful.

Here a Lognormal distribution with our system's parameters and the exoplanet data are cast in the same coordinates, re-figured so that unit mass is one Earth. The lightest exoplanet on this list (which includes only those found by astrometric means, rather than eclipses) is just under 2 Me. Making the quite unfounded assumption that the larger exoplanets are fairly sampled, I fitted a distribution with the logmean and logSD of the Solar System. This chart tells us that if our own Solar System is at all representative, a complete census of the stars already searched would yield about 4,000 planets, with a statistical peak near 4 Me (3.7).

Is this reasonable? For it to be anywhere close to true, we must also assume that the preponderance of "hot Jupiters" so far found is also due to selection bias, because they are so much easier to find than any other kind. I sure look forward to the first data from the Terrestrial Planet Finder mission (canceled last week by the President, but sure to be reinstated by a future administration).

The little planet that could

kw: book reviews, nonfiction, astronomy, planets

I miss Pluto. I learned the names of the nine planets when I was very young, and a few years later, with my father's help, built a 3" (76mm) diameter reflecting telescope from a kit. I still have it, and still use it. It is really too small to do much with the planets, but I've seen them all but Pluto—at 15th Magnitude, it is beyond the reach of any visual instrument smaller than 12.5" (320mm).

A little over three years ago, the International Astronomical Union (IAU) voted to approve a contentious proposal to refer to Pluto and a number of other small orbs as "dwarf planets", but not "real planets". Though the vote seemed a landslide, many were voting "Yes" just to put an end to the bickering. The minority in that vote is, however, vocal, dedicated, and not inclined to give up. And now one of their number, Alan Boyle, has written The Case for Pluto: How a Little Planet Made a Big Difference.

One big difference was that, during the time Pluto was still known as the Ninth Planet, the New Horizon mission was proposed, funded (plenty of back-and-forth there, though), and launched. Shortly before the launch five years ago, this image was taken with the Hubble Space Telescope. The astronomers were aiming for a good, up-to-date image of Pluto and Charon; the two littler moons were a bonus! This is a NASA image. Related images can be found here.

In his book, Dr. Boyle recounts the history, not just of Pluto, but of the larger non-naked-eye planets Uranus and Neptune also.

By the way, Uranus can be seen with the naked eye when it is in opposition, only from a very dark locale; you have to be young and know exactly where to look. It was never identified before telescopic discovery. It was apparently seen and charted a few times, but nobody connected the observations of an obscure 6th magnitude "star" that seemed to be gone the next time you looked for it. But it is an easy object with binoculars or a small telescope. Now, Neptune, well, that's hard with a 3-incher.

Another aside: why do I say you have to be young to see Uranus? After the age of 30 or 35, the eye's iris won't open bigger than 5 or 6mm diameter. It takes a full 7mm opening to see 6th magnitude objects.

Back to the book: the author also recounts in some detail how important the designation of Pluto as a planet has been to getting funding approved and finally appropriated for the New Horizons mission. This mission is seen as doubly important now that Pluto is recognized as the first-discovered of a new class of object, a member of the Kuiper Belt of icy-rocky bodies. A major class of these Trans-Neptunian Objects (TNO's) are being called Plutinos. This Wikipedia article is a comprehensive presentation of the TNO's and their kin.

Strictly speaking, Pluto is a Cis-Neptunian object; its orbit crosses inside that of Neptune. This, to me, is an important consideration regarding the definition adopted by the IAU which has two parts:

1. A planet is round because its self-gravity is great enough to overcome the strength of the materials that compose it.
2. A planet has dynamically cleared the area of its orbit of other bodies.

That second part is what got both Pluto and the asteroid Ceres excluded from being "Planets". Ceres dwells among the asteroids, which seem to cross its orbit with abandon, and Pluto not only dwells among many Plutinos (which share its 3:2 resonance with Neptune), but also in the vicinity of Neptune itself. Here's the kicker; Neptune hasn't cleared out the Plutinos, many of which cross its orbit (the Cis- ones), so is it also not a planet????

Furthermore, dynamical computer simulations of the early Solar system indicate that Jupiter did the clearing of the entire inner system, particularly Mercury's orbital vicinity, while gradually being pushed outward to its present orbit. We don't see that Mercury actually did the clearing job; so is it also not a planet????

Finally, bodies as large as Mars, or maybe larger, could lie at quite great distances, say 1000 AU (~150 billion km) or more. We'll see how detectable they might be in a moment. Almost by definition, the slow pace of things out there has prevented any orbit-clearing. How big must a body be to be a "real planet" at such distances?

Dr. Boyle makes a strong case for a different kind of look at the situation. The Solar System has three classes of spherical objects, other than the Sun:

• Four rocky planets, which are all Sunward of the main asteroid belt.
• Four gaseous giant planets, which are all beyond the main asteroid belt.
• An unknown number of rocky/icy orbs (and you can include the larger moons of Earth, Jupiter and the other giants here), including Ceres, Vesta and Pluto, and now Eris (slightly larger than Pluto), Sedna, Makemake and others discovered very recently. By the IAU resolution, these are called "dwarf planets".
  

Other than the rocky four and the giant four, we now know that there are many, many of the self-gravitating spheres, and that poor, denigrated Pluto represents the majority consituency! At this point, no matter what "kind" of planet you designate Pluto, it is of critical importance as the first member of a very exciting class of object. We have as much to learn from them as we do from the larger bodies; indeed, concerning the early history of the Solar System, we have the most to learn from the more distant bodies.

While reading, I got interested in how one determines the parameters, the search space, of a distant object. With a little figuration, I got this much:

• Velocity in a circular orbit is proportional to the inverse square root of the orbit's radius. This radius for Jupiter is 5.2 times that for Earth. 1/5.2 = 0.192; √0.192 = 0.4385; that times Ve (29.8 km/s) is 13.1 km/s, the measured orbital velocity of Jupiter.
• Subtract a body's velocity from that of Earth and divide the result by the distance from Earth to the body. For bodies beyond Neptune, you can pretty much ignore the body's velocity, it is less than 18% of Earth's velocity. You can also come pretty close by taking the distance from the Sun; one AU out of thirty or more is pretty small. So for Neptune, 29.8 / 4.5 billion = 6.6x10^-9
• For such distant bodies, multiply by the number of seconds in an hour or a day, so you can get hourly or daily parallax. For Neptune, the daily parallax is 0.00057.
• Take the arcsine of that: 0.033° in this instance, or 1.97 arcminutes or 118 arcsec.

For the "back edge" of the scattered disk, the tattered end of the Kuiper belt, R is about 150AU or 22.5 billion km. V = 2.4 km/s so let's ignore it. The daily Earth motion is 29.8x86,400 = 2.57 million km. Divide that by 150 AU to get 0.000114. Arcsin of that is 0.0066° or 0.39' or 24". That is quite large, so sets of images (we don't use glass plates any more) taken a day apart, looking for a "blink" of 24 arcseconds or greater can detect any Kuiper Belt Object.

How about brightness? If you are just looking for Pluto-size and larger, start with Pluto's Magnitude of 15 at a distance of 40 AU. Brightness decreases as the fourth power of distance (square once for the inverse-square law for sunlight, and square again for the apparent area of the object). At 150 AU we get (40/150)⁴ = 0.005. Turn that into magnitudes, and it adds 5.7, making a "back-edge" Pluto-sized KBO a 21^st Magnitude object.

Modern telescopes can record objects as dim as Mv=30 or so. If we're generous to add ten magnitudes to our distant KBO, we figure thus: ten magnitudes is 1/10,000, and the fourth root of that is 10. So the most distant "Pluto" we might see would be 1,500 AU away. In addition, back at the "back edge", ten magnitudes figures out to a diameter ratio of 100, so a body as small as 15-20 km, anywhere in the KB, ought to be findable with current equipment! One way to make a body more detectable is to drive the telescope at its expected angular speed, just a whisker different from the siderial rotation of the sky. A body moving at that speed will be a point. This is most used for asteroid detection, but I suspect it is also being used for KBO's.

Now, many astronomers expect that there are larger bodies 'way out there. How far away could we see a Jupiter? At 40 AU Jupiter's magnitude would be 7. 31-7=24 magnitudes, which works out to brightness factor of 4 billion, whose fourth root is 251. 251x40 = 10,040, which puts such a body in the nearer parts of the Oort cloud.

At present, it takes exceptional equipment to reliably record a 31-Magnitude object. It will take the next generation of space telescopes to find the more distant ones. But equipment we have already is sufficient for finding thousands of dwarf planets beyond Neptune. The more of these we find, the more important Pluto becomes. Also, we gather more potential targets for New Horizons. We need to be looking now for KBO's beyond Pluto which will be well situated for observation by the spacecraft after it flies by Pluto and Charon, and heads into the rest of the Kuiper Belt.

It is currently moving at 18 km/s, in a region near Saturn where orbital velocities are less than half that. It'll still be moving above 10 km/s when it passes Pluto, but even at that rate, it will be in the 100-AU-wide Kuiper Belt for about fifty years.

So the way I see it, echoing the author, Pluto has been promoted. No longer the Ninth Planet, it is the First TNO, just as Ceres is now promoted to First TMO (Trans-Mars Object). Pluto, it is nice to have you back!