15-1 A search for a planet between Mars and Jupiter led to the discovery of asteroids

After William Herschel’s discovery of Uranus in 1781 (which we described in Section 14-1), many astronomers began to wonder if there were other, as yet undiscovered, planets. If these planets were too dim to be seen by the naked eye, they might still be visible through telescopes. These planets would presumably be found close to the plane of the ecliptic, because all the other planets orbit the Sun in or near that plane (see Section 7-1). But how far from the Sun might these additional planets be found?

The Hunt for the “Missing Planet”

Astronomers in the late eighteenth century had a simple rule of thumb, called the Titius-Bode law, which relates the sizes of planetary orbits. The law states that from one planet to the next, the semimajor axis of the orbit increases by a factor between approximately 1.4 and 2. For example, the semimajor axis of Mercury’s orbit is 0.39 AU. Venus’s orbit has a semimajor axis of 0.72 AU, which is larger by a factor of (0.72)/(0.39) = 1.85. (Unlike Newton’s laws, the Titius-Bode “law” is not a fundamental law of nature. It is probably just a reflection of how our solar system happened to form.)

The glaring exception is Jupiter (semimajor axis 5.20 AU), which is more than 3 times farther from the Sun than Mars (semimajor axis 1.52 AU). Table 7-1 depicts this large space between the orbits of Mars and Jupiter. If the rule of thumb is correct, astronomers reasoned, there should be a “missing planet” with an orbit about 1.4 to 2 times larger than that of Mars. This planet should therefore have a semimajor axis between 2 and 3 AU.

Six German astronomers, who jokingly called themselves the “Celestial Police,” organized an international group to begin a careful search for this missing planet. Before their search began, however, surprising news reached them from Giuseppe Piazzi, a Sicilian astronomer. Piazzi had been carefully mapping faint stars in the constellation of Taurus when on January 1, 1801, the first night of the nineteenth century, he noticed a dim, previously uncharted star. This star’s position shifted slightly over the next several nights. Suspecting that he might have found the “missing planet,” Piazzi excitedly wrote to Johann Bode, the director of the Berlin Observatory and a member of the Celestial Police.

Unfortunately, Piazzi’s letter did not reach Bode until late March. By that time, Earth had moved around its orbit so that Piazzi’s object appeared too near the Sun to be visible in the night sky. With no way of knowing where to look after it emerged from the Sun’s glare, astronomers feared that Piazzi’s object might have been lost.

Upon hearing of this dilemma, the brilliant young German mathematician Karl Friedrich Gauss took up the challenge. He developed a general method of computing an object’s orbit from only three separate measurements of its position on the celestial sphere. (With slight modifications, this same method is used by astronomers today.) In November 1801, Gauss predicted that Piazzi’s object would be found in the predawn sky in the constellation of Virgo. And indeed Piazzi’s object was sighted again on December 31, 1801, only a short distance from the position Gauss had calculated. Piazzi named the object Ceres (pronounced SEE-reez), after the patron goddess of Sicily in Roman mythology.

Ceres orbits the Sun once every 4.6 years at an average distance of 2.77 AU, just where astronomers had expected to find the missing planet. But Ceres is very small; its equatorial diameter is a mere 974.6 km (Figure 15-1). (The pole-to-pole diameter is even smaller, just 909.4 km.) Hence, Ceres reflects only a little sunlight, which is why it cannot be seen with the naked eye even at opposition; it can be seen with binoculars, but it looks like just another faint star.

Figure 15-1: R I V U X G
Ceres Compared with Earth and Moon A drawing of Ceres, the largest of the asteroids and the first one discovered, is shown here to the same scale as Earth and the Moon. Ceres is too small to be considered a planet, and it cannot be regarded as a moon because it does not orbit any other body. To denote their status, asteroids are also called minor planets.

(NASA)

On March 28, 1802, Heinrich Olbers discovered another faint, starlike object that moved against the background stars. He called it Pallas, after the Greek goddess of wisdom. Like Ceres, Pallas orbits the Sun every 4.6 years at an average distance of 2.77 AU, but its orbit is more steeply inclined from the plane of the ecliptic and is somewhat more eccentric. Pallas is even smaller and dimmer than Ceres, with an estimated diameter of only 522 km. Obviously, Pallas was also not the missing planet. Ceres and Pallas are asteroids. Asteroids are small objects with various mixtures of rock, ice, and metal that orbit the Sun between Mars and Jupiter. (Like all asteroids, Ceres and Pallas are classified as small solar system bodies, or minor planets; these are all objects other than planets and comets that are in direct orbit around the Sun.)

A new question then faced the astronomers of the early nineteenth century: Did a missing planet even exist? Some speculated that perhaps there had once been such a full-sized planet, but it had somehow broken apart or exploded to produce the asteroids. The search was on to discover this population. Two more such asteroids were discovered in the next few years, Juno in 1804 and Vesta in 1807. As telescope technology improved, hundreds of more asteroids were discovered. Going beyond telescopes in 2007—two hundred years after Vesta’s discovery—NASA’s spacecraft Dawn was launched on a mission to visit Vesta and Ceres (more in Section 15-3).

An Abundance of Asteroids

The next major breakthrough came in 1891, when the German astronomer Max Wolf began using photographic techniques to search for asteroids. Before this, asteroids had to be painstakingly discovered by scrutinizing the skies for faint, uncharted, starlike objects whose positions move slightly from one night to the next. With photography, an astronomer simply aims a camera-equipped telescope at the stars and takes a long exposure. If an asteroid happens to be in the field of view, its orbital motion during the long exposure leaves a distinctive trail on the photographic plate (Figure 15-2). Using this technique, Wolf alone discovered 228 asteroids.

Figure 15-2: R I V U X G
The Trail of an Asteroid Telescopes used to photograph the stars are motorized to follow the apparent motion of the celestial sphere. Because asteroids orbit the Sun, their positions change with respect to the stars, and they leave blurred trails on time exposure images of the stars. This bluish asteroid trail was recorded by accident by the Hubble Space Telescope. The shorter streaks (mostly yellow) are imaging artifacts.

(R. Evans and K. Stapelfeldt, Jet Propulsion Laboratory; and NASA)

Today, more than a million asteroids have been discovered, of which more than 100,000 have been studied well enough so that their orbits are known. About 5000 more are discovered every month, some by amateur astronomers. After the discoverer reports a new find to the Minor Planet Center of the Smithsonian Astrophysical Observatory, the new asteroid is given a provisional designation. For example, asteroid 1980 JE was the fifth (“E”) to be discovered during the second half of May (the tenth half-month of the year, “J”) 1980. The same scheme for provisional designations is also used for small objects found in the outer solar system beyond Neptune, or trans-Neptunian objects (see Section 14-10).

If the asteroid is located again on at least four succeeding oppositions—a process that can take decades—the asteroid is assigned an official sequential number (Ceres is 1, Pallas is 2, and so forth), and the discoverer is given the privilege of suggesting a name for the asteroid. The suggested name must then be approved by the International Astronomical Union. For example, the asteroid 1980 JE was officially named 3834 Zappafrank (after the American musician Frank Zappa) in 1994.

Ceres (formally called “1 Ceres”) is unquestionably the largest asteroid at 975 km; it has one-third the mass of all the asteroids combined (including Ceres). In recognition of its size, Ceres is also called a dwarf planet—large enough to pull itself into a sphere, but unable to gravitationally clear out its surroundings (see Section 14-9).

Besides Ceres, only two asteroids—Pallas and Vesta—have diameters greater than 500 km. A general feature of asteroids is that smaller ones are more numerous. Thus, we find other asteroids have diameters between 200 and 300 km, and 200 more are bigger than 100 km across. Infrared observations indicate that there are between 700,000 and 1.7 million asteroids greater than 1 km in size. And while a million asteroids might sound like a lot, the vast majority of asteroids are smaller than 1 km across.

Interestingly, the dwarf planets Ceres and Pluto have a common story line. Like Pluto, Ceres was considered a planet at first and retained that designation for about 50 years. Just as Pluto recently went from being the smallest planet to being one of the largest trans-Neptunian objects, Ceres had been reclassified and went from being the smallest planet to the largest asteroid. Ceres and Pluto were simply the first objects discovered in these new categories.

The Asteroid Belt

Like Ceres, Pallas, Vesta, and Juno, most asteroids orbit the Sun at distances between 2 and 3.5 AU. This region of our solar system between the orbits of Mars and Jupiter is called the asteroid belt (Figure 15-3).

Figure 15-3: The Asteroid Belt Most asteroids, including the large asteroids Ceres, Pallas, and Juno, have roughly circular orbits that lie within the asteroid belt. By contrast, some asteroids, such as Apollo and Icarus, have eccentric orbits that carry them inside the orbit of Earth.

What of the nineteenth-century notion of a missing planet? This idea has long since been discarded, because the combined matter of all the asteroids (including an estimate for those not yet officially known) would produce an object barely 1500 km in diameter. This combined matter is considerably smaller than anything that could be considered a missing planet. We now understand that asteroids are actually debris left over from the formation of the solar system.