Monday, July 14, 2008
Late Night Science: The Once and Future Prince of Dwarves
by Jason S. Bardi
Approaching 1:00 a.m. on Monday morning, I am wondering if I will get my first blog post finished ('twas due 45 minutes ago). I have that desperation that anyone who is up past midnight has, and because of this, coupled with the fact that the story I am telling concerns a discovery also made past midnight, I am dedicating this column to all the artists, writers, scientists, and other crazy creative people who stay up late troubling over great things.
This is the story of Planet Ceres, the Prince of Dwarves. You may have heard Ceres' name mentioned about a month ago, when the brouhaha over Pluto's status erupted again. I am not going to say much about Pluto other than to point out the obvious: its drama is a human drama and has very little to do with the planet itself. Pluto is an inanimate object. It hurtles out there, beyond the orbit of Neptune. It lies so far away from fickle human categorization that even if it had consciousness, a personality ("that Pluto's a hellofa guy"), it would be safely immune from hurt feelings. Pluto has no personality. It is a rock. How could it care what happens here on Earth?
We humans care a great deal. The big news, about a month ago, was that Pluto was reclassified again. Once a planet, it had been classified a dwarf planet two years ago. This story got an overwhelming media response. Then in June it was reclassified as a "plutoid." This time around, the story also garnered a fair bit of media attention -- if for no other reason than many reporters seem amused that the chosen name was plutoid. "Such a (not) graceful term," declared MIT's Knight Science Journalism Tracker, which also summarized the major coverage of the event in the wires and dailies. See here.
ABC's Ned Potter summed it up well in his blog when he said: "The goal of science is to understand and categorize the universe, and new categories are needed as our understanding of the universe changes…"
This is not the story of Pluto. Instead, this is the story of another rock caught in the renaming game -- one much closer to earth and with an interesting history of its own.
What you may have heard about Ceres is what the IAU said in its official Pluto release. "The dwarf planet Ceres is not a plutoid," the release says, "as it is located in the asteroid belt between Mars and Jupiter." (Plutoids are objects further than Neptune).
What you may not have read about Ceres is how it was discovered there, between the orbits of Mars and Jupiter, more than 200 years ago. By the way, this story is an excerpt (including extensive unused material) from my forthcoming book The Fifth Postulate (Wiley, 2008). Pre-order your copy on Amazon today!
With no further ado, here is a story I like to call:
The Once and Future Prince of Dwarves
One of the greatest discoveries of the nineteenth century was also perhaps its earliest. It was just after midnight on New Year’s Day 1801, and an Italian monk and astronomer named Giuseppe Piazzi was up late searching the skies from his rooftop observatory in Palermo, Sicily. He saw something that night that would change his life forever. Peering through his telescope, he spotted an object, watched it for several nights, carefully recorded its location as it traversed the sky, and eventually lost track of it.
Piazzi wasn’t sure what he was looking at. Was it a comet? His letters indicated that he thought it could be—perhaps a comet without a tail. He hoped, though, that it was something else, something "better than a comet," as he wrote. What exactly qualifies as better-than-comet status? Just what Piazzi was intending to find: a new planet.
Finding a new planet was, in fact, one of the great scientific quests of the day. It was a completely new and exciting endeavor, and Piazzi and others were not just looking for any new planet. They were looking for the “missing” planet. The ancients knew of only five planets other than Earth -- Mercury, Venus, Mars, Jupiter, and Saturn. But the scientific world had been turned on its head in 1781 when William Herschel at his private observatory in Bath, England, spotted something he thought could be a star or a comet. It turned out to be the new planet Uranus.
This raised the possibility that there might be even more undiscovered planets in the solar system. J. D. Titus and J. E. Bode gave this possibility a firm theoretical basis by forwarding an idea that there logically must be a planet between Jupiter and Mars. The basis of their claim was their observation that the six known planets followed an orderly, even spacing, thus suggesting that all the planets should be so spaced. But the gap between Mars and Jupiter was about twice what would be predicted by their theory. The emptiness was deafening.
By the turn of the eighteenth century, astronomers had been looking for a planet between Mars and Jupiter for several years, and the search was heating up. Baron Franz Xaver von Zach, the court astronomer at Gotha, began a formal, systematic search involving a team of astronomers. Piazzi, in his observatory in Sicily, was one of a group of twenty-four astronomers in Europe who made up this team. With such a large force of astronomers gearing up to look for the missing planet, Piazzi must have been surprised when he hit pay dirt quickly that New Year’s Day, spotting his object in orbit around the sun between Mars and Jupiter, exactly where the missing planet should have been.
Piazzi continued to track the object as long as he could, but he became ill on February 11, presumably from too many sleepless January nights exposed to the elements. In any case, the object passed behind the sun, and he lost sight of it. Piazzi had already sent letters that January to the directors of the observatories in Paris, Berlin, and Milan, telling them that he had discovered a small comet with no tail—or something—and giving them the coordinates. By then, Piazzi had decided to presumptively name his object. He called it Cerere Ferdinandea, combining the names of a Roman goddess of the harvest with his own Sicilian king. We know this object by its more common name: Ceres. In one of the letters, he was bold enough to suggest that it might be a planet.
By that summer, word had gotten around. The discovery was announced with excitement in a long article in a leading German astronomy journal edited by Baron von Zach. This was “a long supposed, now probably discovered, new major planet of our solar system between Mars and Jupiter,” wrote von Zach.
For the discovery to be meaningful, however, this mysterious new planet had to be located again. The only problem was, nobody knew exactly where to find Ceres. It had disappeared, and this started one of the great hunts in the history of astronomy. Astronomers, professional and amateur alike, began scanning the skies with telescopes to locate the mysterious new rock -- all without success. Ceres was still there, of course, rounding its way around the sun. But where?
This was a wonderful problem in applied mathematics. Mathematics alone could (and eventually would) locate the planet if it could be used to compute Ceres’s orbit. Then, given the orbit of Ceres, astronomers of the day could locate the missing planet by simply searching along the path where they knew it should be. But how could mathematicians determine the orbit?
Perhaps the easiest way, in theory, would be to chart enough of it—making careful observations of its exact location in the sky night after night as it made its journey around the sun. If you were able to observe enough positions of Ceres on enough nights, and if you were able to record the observations, you would have enough of a plot of its actual orbit to be able to fill in the gaps.
What if all you had was enough data to fill one tiny gap? How could you determine the orbit of Ceres if there was only a limited amount of available orbital data? This was exactly the situation in which mathematicians and astronomers found themselves in 1801. Piazzi had carefully recorded the position of Ceres on several nights over a period of forty-one days from his observatory in Sicily.
Many astronomers of the day reasoned that it would still be possible to calculate the entire orbit from these observations. They could estimate its orbit, and from that they could determine where Ceres should be by calculating how far it would have traveled since Piazzi last saw it. One would then have to simply point a telescope to the location farther along this path where it should be.
With that in mind, an astronomer named Johann Karl Burckhardt used the tools of "celestial mechanics" as they were known and published a preliminary orbit of Ceres in July. In September von Zach published the complete set of observations made by Piazzi. They were both hopeful that this would be enough. It was not.
The difficulty was that prior to the discovery of Ceres, celestial mechanics had only been successfully applied to predicting the orbits of comets. Comets are much brighter than planetary bodies and may be visible for a longer period of time on their approach to the sun. They are easier to handle, in other words. Ceres is not bright enough to be visible to the naked eye, so it had to be spotted with a telescope. This greatly limited the area of the sky that could be systematically searched.
Computing the orbit of Ceres was exceedingly difficult because recording observations taken over just a few weeks only gives you a few closely spaced points on the overall path of the object around the sun. Such a small piece of the orbit confounded any attempt to extrapolate the entire orbit because small errors in the observations could have massive effects on the overall calculation.
A month after Baron von Zach published Piazzi's full set of observations, he reported the sad news that several astronomers had been looking for Ceres in the previous two months without success. Burckhardt’s efforts to plot the orbit had failed. Such was the desperate state of affairs when the Ceres data reached Gauss sometime in the fall of 1801.
Gauss was not an astronomer in the regular early-nineteenth-century sense. He had neither telescope nor observatory. But, being adept at pushing pencils around on a desk, he was perfectly suited to tackle this problem. In 1801, when Ceres swept across Gauss’s radar, he pushed everything else aside, picked up his pencil, and went to work.
“Could I ever have found a more seasonable opportunity to test the practical value of my conceptions, that now employing them for the determination of the orbit of the planet Ceres, which after the lapse of a year must be looked for in a region off the heavens very remote from that in which it was last seen?” Gauss wrote.
By November 1801, he began making notes on the problem, and by the end of the month he had solved it, working out a solution to a system of seventeen equations that predicted where Ceres would be. He communicated his calculated orbit to von Zach, and in December von Zach published the predicted orbit, writing, “Great hope for help and facilitation is accorded to us by the recently shared investigation and calculation of Dr. Gauss in Brunswick.”
By then, astronomers were already searching the skies for Ceres, looking where Gauss predicted it would be. Winter weather being what it was, they were not immediately successful. On December 7, however, von Zach located the planet almost exactly where Gauss predicted it would be, as Gauss himself wrote, on “the first clear night, when the planet was sought for as directed by the numbers deduced from it, restored the fugitive to observation.”
The secret to Gauss’s success was a technique he invented called the method of least squares, which is a way of minimizing the error of observations. The problem, as Gauss saw it, was to determine the orbit by finding a curve that corresponded with the observed data and then correct the curve to find the best fit. The method of least squares helps by approximating an orbit based on the few observations and then improving, or “fitting,” the orbit to the data. The fitting works by minimizing the differences between the observed and computed points in the orbit multiplied by themselves (thus “least squares”). It was a simple but remarkable discovery.
The truly remarkable thing about Gauss, though, is that when he invented the method of least squares, he didn’t think much of it. He didn’t even bother to publish it because it was so obvious to him that he was convinced some other mathematician had surely thought of it already! In fact, other mathematicians did develop the method of least squares independent of Gauss, including the French mathematician Adrien-Marie Legendre, who was also the first to publish the method and was subsequently recognized as its inventor.
Gauss was overjoyed about the rediscovery of Ceres, and his enthusiasm was echoed by dozens of his contemporaries, many of whom had been trying to do just what he had done. In his overly humble way, he was careful not to promote himself too aggressively, and he credited much older mathematicians like Isaac Newton for working out the theoretical foundations upon which he laid his mathematical prediction.
Nevertheless, the calculation launched Gauss to fame and elevated him to a stature on a par with the top astronomers in Europe. When Alexander von Humboldt returned to Europe from the United States in 1804, he went immediately to Paris, cosmopolitan capital that it was. There he was impressed to find a name of a young German mathematician mentioned over and over as one of the great geniuses of the day.
Gauss’s rediscovery of Ceres had a profound effect on his personal and professional life. He soon gained membership in numerous scientific societies, and by the time he died, he would be a member of all the major societies in Europe. He began to spend more and more time on astronomical observations, carefully following planets and comets and watching eclipses. On many nights during the next half century, he could be found late at night observing the stars through his telescope and taking measurements with his sextant and recording all. He named his firstborn son Joseph, after Piazzi.
Ceres, on the other hand, did not fare so well.
Astronomers were astonished by the discovery of a similar object they called Pallas a few months later. Two years after that another, named Juno, was spotted. All three were in orbit around the sun at approximately the same distance. Yet another object like them, called Vesta, was discovered soon thereafter.
These discoveries had a tremendous effect on popular astronomy. Some would watch the skies for years afterward, hoping to make a similar discovery. All of this was in vain. It was decades before more of these objects were discovered. They were not really planets at all, but object we would have called asteroids, a word that the astronomer William Herschel coined because objects like it appeared like stars in his telescope.
In the second half of the nineteenth century, astronomers began spotting them in droves—hundreds more by the end of the century. A single photograph taken in 1900 revealed five entirely new ones. By the early twentieth century, there were so many known asteroids that they were regarded as dull curiosities or worse—annoyances. They stood in the way of observations of distant solar systems and were themselves uninteresting.
Later in the twentieth century this changed again. Asteroids are now the focus of important geological questions because they are representatives of the early solar system. Ceres is a special example of this. It was formed within ten million years or so after the birth of the solar system. It is a throwback to an earlier time in the solar system—a primordial planetoid 600 miles in diameter. It is a remnant of the cloud of matter that collapsed and formed our solar system nearly five billion years ago.
Perhaps Ceres was inappropriately named. In ancient myth, Ceres was the Roman goddess of plants, but there is nothing organic about her namesake. It is a rock and nothing more, with no atmosphere and no life. Ceres is smaller than the Earth and much smaller than Jupiter and the other giant planets, and smaller even than the moons scattered throughout the solar system. Ceres and the rest of the objects in the asteroid belt never really had a chance to pull together because of the gravity of the solar system. Blame Jupiter, the biggest of the planets and gravitationally speaking the most influential. Ceres and the other thousands of asteroids were caught between the gravitational pull of it and the sun.
Ceres is similar to the icy moons of the outer planets. Its diameter is about a quarter that of Earth’s moon, and it circles the sun in 4.6 years. What is most interesting about Ceres is that it is the planetary equivalent of a wooly mammoth frozen in the ice. It may still contain some of this primordial ice. Studying it may give insight into the formation of our own planets and our neighbor planets.
Appreciation of Ceres grew again in the last quarter of the twentieth century because of this possibility. The asteroid belt is something like an astronomical archive, and two of its most ancient and valuable tomes are Ceres and Vesta. These two asteroids have remained intact since the dawn of the solar system 4.5 billion years ago, escaping severe damage by collisions with other protoplanets. NASA now has a mission called the DAWN spacecraft that will reach Ceres around 2015. See the NASA Fact Sheet.
DAWN uses a flashy technology called an ion drive that relies on charging ions of the element xenon and then firing them out of the back of the engine. This provides very little thrust, and on Earth such an engine would be ineffective against the friction of our atmosphere. But in space, the ion engine can slowly accelerate the satellite for months. DAWN has solar sails, ion thrusters fueled by 400 kilograms of xenon, a satellite dish, cameras, and a payload of scientific equipment, such as infrared and gamma-ray spectrometers, that will map the surface, search for water, and determine the chemical and mineral compositions of the asteroids. Scientists hope to understand some of the processes that were taking place in the earliest days of the solar system, such as the role of water.
Now back to Pluto. Even before DAWN was launched in 2007, the status of Ceres was under consideration when the International Astronomical Union (IAU) set up a committee that would decide on the criteria that defines a planet. Possible definitions rely on the size, orbit, and uniqueness of the orbit. Under these criteria, Ceres was not set to be reconsidered as a planet. The real deliberations were over whether or not Pluto should be given planet status.
Pluto had been declared a planet when it was discovered some seventy-five years ago, but at that time nobody had defined what a planet was. Size alone will not cut it. Judging by size, the Earth’s moon is bigger than Pluto. So are the two largest moons of Saturn and Jupiter. So is Neptune’s moon Triton. In its report, the IAU defined a planet as anything spherical, revolving the sun, and larger than 2,000 kilometers across. Under this definition Pluto would be a planet, but then so would a nearby object discovered in 2003. Some advocated on Pluto’s behalf. Others dismissed it outright. Some suggested waiting. But waiting was the last thing on the agenda, because the confusion had created something of a backlog of small objects that could not be named until astronomers knew what naming convention to use.
Unfortunately for Pluto, planets are not named by organizations like the IAU on the basis of popular culture or history. The stage was set for a showdown at the Twenty-sixth General Assembly of the International Astronomical Union in August 2006. The matter was simply referred to the entire IAU membership, and the IAU declared Pluto and Ceres dwarf planets.
What is next for Ceres? Right now nothing. Ashley Yeager from Science News reports that if objects similar to Ceres are detected, they will be named Ceroids.
For now, the last word belongs to the International Astronomical Union release says, "Current scientific knowledge lends credence to the belief that Ceres is the only object of its kind. Therefore, a separate category of Ceres-like dwarf planets will not be proposed at this time."
Among the asteroids, Ceres is the only dwarf planet, which makes it the king of the asteroids. And among the dwarves, it is unique -- a prince. There seems to be no other object like it in the solar system, and I look forward to 2015, when the Prince of Dwarves begins to yield secrets of itself and perhaps the earliest days of the solar system.
- G. Waldo Dunnington, Gauss: Titan of Science (Washington, DC: Mathematical Association of America, 2004).
- C. T. Russell et al., “DAWN: A Journey to the Beginning of the Solar System,” www.ssc.igpp.ucla.edu/dawn/pdf/ACMConferencePaper, last visited May, 2007.
- Donald A. Teets and Karen Whitehead, “The Discovery of Ceres: How Gauss Became Famous,” Mathematics Magazine 72 (April 1999);
- W. K. Bühler, Gauss: A Biographical Study (Berlin: Springer-Verlag, 1981)
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