We live in a golden age of golden ages. In the last half of the twentieth century, field after field after field of human endeavor burgeoned beyond anyone’s imagination.

But perhaps no field has seen its knowledge base increase as much as astronomy. Part of the reason for that, of course, is that the knowledge base was so very small in 1950. The mirror of the largest telescope in the world then was 200 inches across and only film was available to record images.

Today giant new telescopes have been built or planned. The largest, the Keck telescope in Hawaii, consists of twin observatories each with a 390-inch mirror. Computer-adjusted mirrors and digital recording of images make unprecedented clarity and precision possible.

Further, in 1950 humans were firmly attached to our home planet. I remember looking at a wall chart in a classroom when I was in the fourth grade that showed the earth’s atmosphere in cross section, with its layers such as the troposphere, ionosphere, and stratosphere delineated. Added were such benchmarks as Mt. Everest—not yet climbed—and the highest airplane and balloon flights then achieved. Also marked was “the highest point ever reached by man,” the altitude achieved by a V-2 rocket that had been aimed straight up. If I remember correctly it was 272 miles, only a little over 3 percent of the earth’s diameter.

Because of these technological limitations, even the largest and nearest planets and their moons were little more than blurry dots floating in a black sea. We didn’t even know how fast some of the planets rotated on their axes.

Boy, what a difference half a century makes.

Today you can hit a few buttons on your computer and see the latest images from the rovers that are happily tootling about the Martian landscape or from the Cassini spacecraft now exploring the Saturnian system. And the highest point ever reached by man is now well over eight billion miles away from earth—a million percent of our planet’s diameter—as the Voyager space vehicles that were launched in the 1970s to explore the planets sail majestically off in the direction of the nearest star system, Alpha Centauri.

Today every planet in the solar system that was known in 1950 has been explored close up, except Pluto. And a mission to that distant outpost of the solar system took off last week, scheduled to reach it in a decade.

But poor Pluto, discovered by Clyde Tombaugh in 1930 at the Lowell observatory in Flagstaff, Arizona, is now struggling mightily to maintain its status as a planet. It is a measure of just how far astronomy has come in the last few decades that after several thousand years, it is suddenly not clear what a planet is.

The word “planet” comes from the Greek for “wanderer.” The five that were known to the Greeks—Mercury, Venus, Mars, Jupiter, and Saturn—look like stars to the naked eye, but unlike stars they don’t stay in the same place in the sky. Instead they seem to wander around in complex ways as they, and the earth, orbit the sun.

In 1781, Sir William Herschel discovered Uranus orbiting beyond Saturn. It was clearly a new planet, with a diameter of about 31,000 miles. On January 1, 1801, the first day of the nineteenth century, Giuseppe Piazzi discovered Ceres orbiting between Mars and Jupiter, where it had been suspected for mathematical reasons that a planet was to be found. But Ceres is only about 480 miles in diameter. Soon more of what came to be called asteroids were found in the same area. Today they are generally agreed to be what is left of a failed planet, one that owing to Jupiter’s immense and disruptive gravitational field could not coalesce into a single body.

In 1846, after some very fancy mathematics indeed, Neptune was discovered beyond the orbit of Uranus. Again, it was clearly a planet, being nearly the same size as Uranus.

The number of planets then remained at eight until Clyde Tombaugh, at the ripe old age of 24 and lacking even a bachelor’s degree, discovered a faint dot of light that had shifted it position between the time photographs were taken on January 23 and January 29, 1930. Its location in the solar system turned out to be well beyond Neptune, and the Lowell Observatory, which had been searching for “Planet X” since 1905, promptly announced on March 13 that Planet X had been found. The number of “planets” in the popular mind rose to nine.

Pluto is so faint that little was known besides its orbit for decades, and its diameter was at first wildly overestimated. But it was the only thing known beyond Neptune, so what else could it be but a planet? Then in 1951 Gerard Kuiper (1905-1973), a Dutch-American who was perhaps the greatest planetary astronomer of the twentieth century, predicted the existence of what came to be known as the Kuiper Belt, an area of minor bodies beyond the orbit of Neptune, which he thought would prove to be the source of short-period comets.

As the ability to detect faint and distant objects increased by orders of magnitude in the years after Kuiper’s death, he was proved correct. Since 1992 numerous Kuiper Belt objects (KBOs) have been found, and at least ten of them are not far from Pluto’s size, which is now known to be about 1,400 miles in diameter, far smaller than Earth’s moon. A recently discovered KBO, called at the moment 2003 UB313, has been found to be considerably larger than Pluto, about 1,800 miles across, as reported in an article published yesterday in Nature.

So are these KBOs planets, with many, many more of them undoubtedly to come? Some people, including, for fairly obvious reasons of self-interest, the discoverers of 2003 UB313, have argued exactly that, saying that anything in orbit around the sun and large enough to have been shaped into a sphere by its own gravity is a planet. That, of course, would make Ceres and a few other asteroids planets too.

An alternative suggested by some is to continue to call Pluto a planet for historical reasons, but limit the number of planets to nine. Certainly the idea that the sun has nine planets has become deeply embedded in the popular culture, so booting Pluto out of the planetary club would be difficult. But it would essentially render the word planet scientifically meaningless.

Personally, I think Pluto must be demoted to being a KBO. Consider this: The eight undoubted planets all share a number of characteristics. (1) They have nearly circular orbits; (2) they orbit in or near the ecliptic (the plane defined by the earth’s orbit); (3) they gravitationally dominate the area of the solar system where they reside, having long since swept up (or flung away) smaller objects; (4) they are all over 3,000 miles in diameter.

Pluto has none of these characteristics. Its orbit is elliptical not circular (for 20 years of its 240-year orbit, it is actually closer to the sun than Neptune); its orbit lies at a 17-degree angle to the ecliptic; it orbits along with a cloud of other, similar objects; it is less than half the size of Mercury, the smallest of the planets. And Pluto has many characteristics in common with KBOs, including a specific gravity that is close to the specific gravity of comets and much less than that of the terrestrial planets (Mercury, Venus, Earth, and Mars).

I’m a great believer in the power of history to shape our understanding of the world. (What historian isn’t?) But history should not dominate science or semantics. And demoting Pluto from planetary status does not, I think, diminish Clyde Tombaugh’s great achievement. Using far more primitive equipment than what is available today, he found a KBO fully 62 years before anyone else did and 21 years before Gerard Kuiper even predicted their existence.

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