Has a tenth planet been discovered? Three astronomers, Mike Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale University), claim that the Kuiper Belt object with the catchy name of 2003 UB313 is larger than Pluto. If Pluto is the ninth planet, shouldn't this object be the tenth?
This is not the first claim of a tenth planet. These same three astronomers found an earlier object outside the orbit of Neptune with a diameter that is 80% of Pluto's. The problem is that Pluto itself is tiny when compared to any of the other planets. Is Pluto a planet? Not by any reasonable definition.
To some extent the term planet is arbitrary, because the classical planets, Mercury, Venus, Mars, Jupiter, and Saturn, are simply the star-like objects visible to the eye that move in a regular motion in the sky. This is a definition of appearance rather than of physics. The size and nature of these planets was unknown to the ancient astronomers. Not until the invention of the telescope and the development of the theory of gravity could astronomers measure the masses of these bodies.
The number of planets increased to seven in 1781 when William Herschel discovered Uranus from observations made on March 13 and March 19 of that year. While the planet had been observed by many observers before this, with the earliest made by John Flamsteed in 1690, Herschel was the first to recognize that Uranus was in our Solar System. Its large size on the sky immediately showed astronomers that Uranus is much larger than Earth, so it is clearly a planet.
During this era, other smaller bodies were observed. Ceres, found in 1801, was the first. The next asteroid was found in 1802, and numerous more asteroids were catalogued over the following decades. But the asteroids are clearly much smaller in size than the planets. The solar system therefore divided up in a tidy package: a few large planets and numerous tiny asteroids.
The discovery of Neptune colored the discovery of Pluto, because Neptune was discovered through prediction in one of the most striking achievements of theoretical astrophysics. Shortly after the discovery of Uranus and the realization that the astronomical community already had numerous observations of the planet stretching back to Flamsteed, theorists began to compare the orbit expected from Newton's theory of gravity to the observed motion. This theory included the gravitational pull on Uranus of Saturn and Jupiter. Theorists found that there were slight discrepancies, and with the passage of time and the collection of more observations of Uranus, the discrepancies multiplied. This suggested to a number of astronomers that another planet was still to be found. Two astronomers, John Couch Adams and Urbain Leverrier, independently predicted where in the sky this unseen planet should be. The first expected a planet 55 times Earth's mass, and the other 36 times Earth's mass. The test came in 1846 when Johann Franz Encke and Heinrich Louis d'Arrest at the Berlin Observatory, at the urging of Leverrier, looked in the predicted location and found Neptune less than one degree away. The mass estimate wasn't so good; the actual value is 17 times Earth's mass.
This triumph conditioned astronomers in a bad way: having found gold in Uranus's motion, they attempted to find more. From the discovery of Neptune through the 1920s numerous astronomers calculated from the slight deviation of Uranus's orbit from the predicted orbit the characteristics of additional planets. In addition to these predictions, several astronomers argued for the existence of one or two additional planets beyond Neptune from the grouping of comets: some comets have aphelions at Jupiter, other at Neptune, and two groups have aphelions at 100 and 300 AU, suggesting to astronomers that planets reside at these positions. But unlike with the predictions for Neptune, the predictions for these additional planets were scattered in their predictions of the planets' positions and masses.
At the beginning of the 20th century predictions were made for the position of an unknown planet by two prominent astronomers: William Henry Pickering and Percival Lowell. Pickering used a diagram technique to predict the position of a planet “O.” Lowell used the same mathematical techniques as Adams and Leverrier to predict two possible positions of a planet “X.” Both calculations are based on the deviations of Uranus's orbit from the expected orbit, given the gravitational pull of Saturn, Jupiter, and Neptune, and both were in agreement over one of the possible positions for the planet. Lowell predicted the planet's mass as 7 times Earth's mass, and Pickering predicted 2 times Earth's mass.
I've seen Lowell described as a gifted amateur astronomer. He must be the first amateur with the resources to build a world-class observatory—the Lowell Observatory in Flagstaff, Arizona—and to staff it with professional astronomers. Percival Lowell is a member of the Lowell family of Massechussets. This heritage gave Lowell not only great wealth, but also great connections, because his brother, A. Lawrence Lowell, was the president of Harvard University. Percival Lowell was an amateur, but he was in a position to have his ideas heard and acted upon.
Lowell published his positions for planet X in 1915, although in 1905 he was already confident enough in his calculations to set an observing program in motion at the Lowell Observatory. This early work did not find a planet X, although two of the photographic plates made by Lowell in 1905 contained the image of Pluto. Not until long after Lowell's death on November 12, 1916 was a program launched with sufficient resources to find Pluto. In this project, carried out at the Lowell Observatory using funds provided by Lowell's estate, the sky was systematically photographed in an attempt to find planet X. The project was named the Gemini project, after the constellation expected to contain planet X. It was led by Clyde Tombaugh. This project began on April 1, 1929. On January 21 and 29 of 1930, two plates were taken that showed the motion of Pluto, which was near the position predicted by both Lowell and Pickering. Pluto's discovery was announced as the discovery of the planet predicted by Lowell on March 13 by Lowell's widow, Constance. That date happens to be Lowell's birthday and the 149th anniversary of the discovery of Uranus. So Pluto is a planet because that is what the Lowell Observatory astronomers sought.
It became clear almost immediately, however, that Pluto did not match Lowell's prediction. Pluto's orbit is highly elliptical, which is contrary to Lowell's prediction. More important, the size of Pluto on the sky was predicted as 1", but telescopes of that era saw Pluto as a point. Upper limits on the diameter ranged from 0.4" to 0.1". Under the assumption that Pluto's density equals Earth's, these values gave a mass for Pluto of 34% of Earth's for a diameter of 0."3 and of 1% of Earth's for a diameter of 0."1. A planet with the larger mass is much too small to influence Uranus's orbit in a measurable way.
On the theory side, E.W. Brown showed shortly after Pluto's discovery that the measured deviations of Uranus from the theoretical orbit are not consistent with the gravitational effects of a ninth planet. This means that the deviations are of no value in searching for a new planet, and the deviations could not have been produced by Pluto. In effect, Lowell's calculations are pure nonsense; he, Pickering, and others were seeing evidence of a planet in the systematic errors in measuring and calculating Uranus's orbit.
We now know, from the orbit of Pluto's moon Charon, that Pluto has only 0.25% of Earth's mass. Its mass is near the geometric mean of Ceres and Mercury, with it being 16 times the mass of Ceres and Mercury being 23 times the mass of Pluto. Pluto certainly does not behave like the other eight planets, for while the eight planets are in fairly circular orbits, with Mars having the most eccentric orbit, Pluto is in a highly-eccentric orbit, with the perihelion inside the orbit of Neptune.
But what distinguishes Pluto from the other planets is the company that it keeps. It is one of the very large group of objects that constitute the Kuiper Belt; in fact, Pluto was the first Kuiper belt object to be found. Like Pluto, the other objects in the Kuiper Belt are in very eccentric orbits outside the orbit of Neptune. Like many other Kuiper Belt objects, Pluto's orbit is resonant with Neptune's orbit. The Kuiper Belt objects are numerous and generally much smaller than Pluto, but several, perhaps as many 10, of these bodies are expected to be as large as Pluto. With the recent discovery of 2003 UB313, one of these ten planetoids has been found. But because of the history behind its discovery, Pluto is still considered a planet.
While Pluto has the characteristics of a Kuiper Belt object, the remaining eight planets fall into three much different groups: the terrestrial planets of Mercury, Venus, Earth, and Mars; the giant gaseous planets of Jupiter and Saturn, and the giant frozen planets of Uranus and Neptune. Each of these groups is defined by the physics governing the planets in the group, physics that is not shared by the members of the Kuiper Belt.
For these reasons, we should return to an eight-planet Solar System, for Pluto is nothing more than one of the largest Kuiper Belt objects, just as Ceres is the largest of the asteroids. The discoveries of other large Kuiper Belt objects should not lead to more planets, but to fewer.
1 The masses for the Kuiper Belt objects 2003 UB313 and Sedna are estimates based on the density of Pluto. The masses for the asteroids Ceres, Pallas, and Vesta are based on the density of Earth. The radii of 2003 UB313 and of Sedna are estimates given by Brown, Trujillo, and Rabinowitz based on brightness. The masses of the planets and the radii of all other objects are directly measured.