TCS Daily

What's In A Name?

By Sallie Baliunas - August 8, 2005 12:00 AM

"We are, thereby, thanks to you, definitely in possession of a new world"
-- French astronomer Le Verrier to German astronomer Galle, who first recognized the new planet Neptune on September 25, 1846

Three new worlds (one with a satellite) just popped into notice in the far solar system.

Let's revisit the realm beyond Pluto by starting with a question:

True or false? The ninth and furthest planet from the Sun in our solar system is Pluto.

Planets and smaller bodies are faint and difficult to see at large distances from the Sun and Earth. The five bright planets, Mercury to Saturn, visible by eye, were noted in antiquity as they traversed the fixed background of constellations. Ancient Greek skywatchers termed the bright objects wanderers, from which the word planet originated.

The telescope, first used for astronomy in the early 17th Century, opened the range of the eye to dimmer objects. As a result, along with the Earth, the six planets orbiting the sun were joined by Uranus after Sir William Herschel reported in 1781 a lumbering object that displayed a small disk under telescope magnification and thus marking it as a non-stellar object. Post-discovery work on the object's position and determination of its orbit led to its identification as a planet that orbits the sun with an 84-year period.

Sir William Herschel's finding of Uranus took extraordinary observational acumen as he swept the skies. Meanwhile, the eighth planet, Neptune, was found through theoretical prediction, a new tool for detecting planets.


In the first decades of the 19th Century, mathematicians comparing the calculated and observed positions of Uranus found systematic deviations. The fault could have arisen from errors in the earlier observations of positions used to calculate the orbit of Uranus or the formulation of Newton's law of gravity governing the motions of celestial bodies.

John Couch Adams (1819 - 1892) in England and Urbain Jean Joseph Le Verrier (1811 - 1877) in France separately considered another reason for the deviations and calculated a modified orbit of Uranus. Their modifications assumed that the orbit of Uranus would deviate from expected if there were present a more distant, large and previously unknown planet acting in accord with Newton's gravitational law and adding to the perturbations of Uranus. In 1845 and 1846 both Adams and Le Verrier made predictions of the position of the hypothetical planet, and then worked to persuade telescope observers to look for it.

Adams approached George Biddell Airy (1801 - 1892), the Astronomer Royal, to search for the planet. Airy paused in mid-1846 to consider the dedication of telescope resources to such a venture:

        It was so novel a thing to undertake observations in reliance upon merely 
        theoretical deductions; and that while much labour was certain, success 
        appeared very doubtful.

Although that novelty of the utility of theory sounds odd to our modern ideas of scientific research, Airy had considered one of the other explanations for the deviations in Uranus' orbit, namely, that Newton's law of gravitation did not hold over large distances. Newton's law had been tested on nearby celestial bodies like the Moon, Jupiter's satellites and comets at times of close passage to the earth. Nonetheless, Airy consented to the search.

Le Verrier sent his predictions to astronomers at the Royal Observatory in Berlin, who first reported on Sept. 25 the new planet. It turns out that Airy's colleagues had recorded the planet in their observations of August, but had not finished analyzing their notes. Once the 165-year orbit of Neptune was determined and could be estimated back through time, more than a dozen earlier indications of the planet were found in observers' notes, for instance, those of Galileo, who marked the motion of what must have seemed like a star in his small telescope from one night to the next in 1613. The earlier observers did not realize the star-like object was a planet, so credit for discovery is shared by the mathematicians Adams and Le Verrier, with Johann Gottfried Galle (1812 - 1910), the German astronomer who peered through the telescope at Le Verrier's request, as the first to recognize Neptune.

The Ninth Planet and Beyond

Clyde Tombaugh discovered Pluto at Lowell Observatory in 1930. Pluto, orbiting the Sun once every 250 years, became the furthest known and ninth planet in the solar system.

At July's end, though, three distant solar system objects were announced, 2003UB313, 2003EL61 and 2005FY9 (those are their catalogue entries; the International Astronomical Union will issue official names according to its guidelines.) Their orbital periods are 557 years, 285 years and 308 years, respectively; a small satellite orbits 2003EL61.

Telescopes have progressed in ability to gather information over swaths of the sky in relatively short periods. Coupled with modern instruments keenly sensitive to faint objects, extremely efficient surveys of the mysterious space beyond Pluto are turning up ice-covered rocky bodies that appear to be primordial solar system material. Around 600 of the objects have been discovered since 1992, and most are smaller than 600 miles in diameter. Researchers estimate 70,000 objects larger than approximately 60 miles across.

These objects swarm in the Edgeworth Kuiper Belt, shaped like a flattened donut and orbiting the Sun at a distance of approximately 30 to 50 A.U., where an A.U., or astronomical unit, is the Earth-Sun distance of approximately 93 million miles.

The Kuiper Belt begins just beyond the orbit of Neptune; that makes Pluto a Kuiper Belt Object (KBO) or trans-Neptune Object, and the brightest of them. Although Pluto shares properties with KBOs, the International Astronomical Union reaffirmed in 1999 that Pluto is still called a planet, absent scientific reasons to reclassify it.

But 2003UB313 has intensified the debate.

One reason is that most KBOs previously discovered were no more than half the diameter of Pluto. Even the large KBOs like Quaoar (reported in 2002) and Sedna (announced in 2004) didn't dwarf Pluto.

If Pluto is a planet owing to its diameter, then so might be 2003UB313, which is at least the size of Pluto.

So, True or False?

Pluto remains a planet, but another, more distant Kuiper Belt Object, 2003UB313 orbiting the Sun once every 557 years, is at least as large and might be called the 10th and most distant known planet in our solar system. Science now, more than 2,500 years later, may be able to sharpen the scientific definition of planet, a cultural inheritance from ancient Greece.

Further Reading

Fred L. Whipple 1981 Orbiting the Sun: Planets and Satellites of the Solar System Harvard University Press 338pp.


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