A Planet is Found ... Wait a minute, something is odd ...

Sir William Herschel

William Herschel 's (1738-1822) discovery of the planet Uranus and the fact that its orbital size fit Bode's Law so well made the lure of the gap between Mars and Jupiter irresistible. Certainly such a planet could not be very bright or it would have been known long before. It must be no brighter than one of the faintest stars, if visible to the unaided eye at all. An organization was set up in Germany by Franz Xaver von Zach, director of an observatory in Gotha, Germany, to search for the unknown planet. ¹ "A society of astronomers was formed; the zodiac was divided into 24 zones, and each zone was confided to a member of the society". ²

The Sicilian astronomer Giuseppe Piazzi (1746-1826) was one of the members, but hadn't yet received his notification in the mail. He was engaged at Palermo in forming his extensive catalogue of stars, not searching for a new planet, when on January 1, 1801, he observed a faint star of seventh magnitude where none had been a few days before. ³ Piazzi was a careful observer, repeating each observation at least twice. He was familiar with comets moving against the background stars, but this object was star-like, not fuzzy. He first called it a starlike comet, without nebulosity; but soon it was realized that it must be a new planet, with a period of revolution of about four years. ⁴ He followed the moving object for six weeks, until February 11, then got sick. He sent his observations to other observers hoping they would keep track of the object, but mail was slow because Italy was full of warring armies, and soon after he tried to find it again it was lost in the evening twilight. Piazzi had hoped to publish his observations along with an orbit computed from them, but there was no good method of finding an orbit from such a short span of observations, and nothing was found at the computed position. However, a brilliant young mathematician named Carl Friedrich Gauss (1777-1855) invented a new technique for determining an orbit from just a few observations and tried it out on the lost object. The position predicted by Gauss was right on and the object was recovered by Franz von Zach on December 31, 1801, Wilhelm Olbers found it the next day. Piazzi named it Ceres after the ancient Roman grain goddess, guardian goddess of Sicily. ^{5, 6}

The simple picture is shattered

Cerium Cerium is an iron gray, rare-earth metal about as soft as tin. It was discovered in 1803 and named for the asteroid Ceres. Cerium dioxide is used to polish optical surfaces such as telescope mirrors and lenses. Palladium Palladium is a precious, gray-white metal in the Platinum group of metals. It was discovered in 1803 and named for the asteroid Pallas.

Ceres certainly was no major planet. But then no major planet was expected in this location. Its mean distance from the sun is 2.77 times that of earth, Bode's Law predicts 2.8, so it did seem to be the missing planet. The missing planet soon became more mysterious. While searching for Ceres, Wilhelm Olbers (1758-1840) discovered another moving star of seventh magnitude on March 28, 1802. This object was named Pallas and has an orbit nearly the same size as Ceres but strongly inclined to the ecliptic. Olbers (of Olbers Paradox) was a physician with a busy practice at Bremen, Germany. He was also an amateur astronomer, but highly respected among astronomers. He observed comets and computed their orbits using a method he devised and published at the urging of his friends. During the entire nineteenth century his method was used by successive generations of astronomers young and old. ⁷

On September 2, 1804, Carl Ludig Harding, also of Bremen, discovered the third minor planet, Juno. ⁸

After the discovery of Ceres "astronomers congradulated themselves upon the harmony of the system being restored. They had long wanted a planet to fill up the great void between Mars and Jupiter, in order to make the system complete in their own eyes; but the successive discoveries of Pallas and Juno again introduced confusion, and presented a difficulty which they were unable to solve, till Dr. Olbers suggested the idea that these small anomalous bodies were merely the fragments of a larger planet, which had been exploded by some mighty convulsion." ⁹

Olbers had conceived this idea after he discovered Pallas and thought there should be more fragments and their orbits should all pass through two opposite points in the sky (one where the planet had exploded, and the other opposite in the sky). He examined three times each year all the faint stars in these two regions, and on March 29, 1807 discovered Vesta. ¹⁰ (Juno had been discovered near the opposite point).

These small objects have been known by a number of names: star-form planets, telescopic planets, planetoids, minor planets, and asteroids. The term asteroid was given by William Herschel in the early nineteenth century to indicate that, although planets they appear like stars. ¹¹

Helium An unknown line in the solar spectrum was noted by the French astronomer Pierre Janssen during an 1868 eclipse. The same year J. N. Lockyer measured it and concluded the new spectral line was due to an element unknown on Earth. He and the chemist Frankland named the new element Helium. The chemist William Ramsay discovered Helium on Earth in 1895.

The founder of the science periodical Nature, J. Norman Lockyer, in his 1877 book comments on the origin of the asteroids:

"To account for the existence of the asteroids, it has been suggested that they may be fragments of a larger planet destroyed by contact with some other celestial body. One fact seems, above all, to indicate an intimate relation between all the minor planets: it is, that if their orbits are figured under the form of material rings, these rings will be found so entangled that it would be possible, by means of one among them, taken at hazard, to lift up all the rest." ¹² [from Lockyer's 1877 book, but probably written in February 1873 when 126 asteroids were known]

Lockyer also mentions that Pallas and perhaps other asteroids, from their hazy appearance, might have atmospheres. More farfetched was his statement that, because of the low gravity "On such planets giants may exist; and those enormous animals which here require the buoyant power of water to counteract their weight, may there inhabit the land". ¹³

Recent values for the first four asteroids are summarized in the following table: ¹⁴

Asteroid	1 Ceres	2 Pallas	3 Juno	4 Vesta
Diameter	960 x 932 km 597 x 579 miles	570 x 525 x 482 km 354 x 326 x 300 miles	240 km 149 miles	530 km 329 miles
Mass	870 x 1018 kg	318 x 1018 kg	20 x 1018 kg	300 x 1018 kg
Rotation Period	9.075 hrs	7.811 hrs	7.210 hrs	5.342 hrs
Orbital Period	4.60 yrs	4.61 yrs	4.36 yrs	3.63 yrs
Semimajor Axis	2.766 AU	2.776 AU	2.669 AU	2.361 AU
Orbital Eccentricity	0.0779	0.2309	0.2579	0.0903
Orbital Inclination	10.58 deg	34.85 deg	12.97 deg	7.13 deg
Spectral Class	C	U	S	U

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References

¹ Edward A. Fath, The Elements of Astronomy (New York: McGraw-Hill, 1928), p. 166.

² J. Norman Lockyer, Elements of Astronomy (New York: D. Appleton and Company, 1877), p. 152.

³ Henry Norris Russell, Raymond Smith Dugan, and John Quincy Stewart, Astronomy (Boston: Ginn and Company, 1945), p. 347.

⁴ A. Pannekoek, A History of Astronomy (New York: Interscience Publishers, 1961), p. 352.

⁵ Myron Lecar, Man and Cosmos Nine Guggenheim Lectures on the Solar System, ed. James Cornell and E. Nelson Hayes (New York: W. W. Norton and Company, 1975), 136,137

⁶ "The Solar System: OTHER CONSTITUENTS OF THE SOLAR SYSTEM: Asteroids: HISTORICAL SURVEY OF MAJOR ASTEROID DISCOVERIES" Britannica Online.
http://www.eb.com:180/bol/topic?eu=118805&sctn=2#s_top
[Accessed 07 January 1999].

⁷ Pannekoek, p. 352,353.

⁸ Mark Littmann, Planets Beyond (New York: John Wiley & Sons, 1988), p. 21.

⁹ Elijah H. Burritt, The Geography of the Heavens (New York: Huntington and Savage, 1850), p. 236.

¹⁰ Ibid., p. 234.

¹¹ Russell, p. 347.

¹² Lockyer, p. 155.

¹³ Ibid., p. 153.

¹⁴ Asteroid Fact Sheet: Information on Selected Asteroids
http://nssdc.gsfc.nasa.gov/planetary/factsheet/asteroidfact.html
[Accessed 28 May 1999].

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Illustrations:
"Sir William Herschel" scanned from Elements of Descriptive Astronomy, a Text-Book, Herbert A. Howe, Silver Burdett and Company, New York, 1897