Center Stage

Natural History, Feb, 2001 by Richard Panek

Repositioning the Sun at the hub of our celestial neighborhood was only the first step.

CELESTIAL EVENTS

It almost goes without saying that today we wouldn't think of the solar system as "solar" if there had been no Copernicus to put the Sun at its center. But more than 450 years after the beginning of the Copernican revolution, it can be difficult to remember that "solar" is only half the story.

When we look at the night sky now, we take for granted that the little lights that wander--the planets--bear a greater resemblance to Earth than they do to all the little lights that don't wander--the fixed stars. For thousands of years, however, the reverse seemed true. Earth was Earth, and the little lights were little lights, whether they were on the move or not. The conceptual divide was between Earth and just about everywhere else--between "here" and "there." If a further distinction existed out "there," dividing planets from stars, it fell, vaguely, between "far" and "farther."

The Copernican system alone didn't necessarily change that. To be sure, it rearranged the heavens, dispatching the Sun to the center of the universe and sending Earth, the planets, and the stars spinning in orbit around it. But for an earthly observer, the basic distinction between here and there continued to hold.

Only with the advent of the telescope in the early seventeenth century did astronomers begin to identify similarities between Earth and the other planets, such as their shared global shape and the presence of moons. And only with the invention of dedicated precision instruments throughout the rest of the century did astronomers begin to appreciate the relative distances to celestial objects. As Rice University historian of science Albert Van Helden once wrote of this era's technology, "Every new discovery brought the planets closer to the earth, and every improvement of the telescope showed the stars to be even farther away."

Early in the seventeenth century--at the end of the reign of the geocentric universe--most astronomers estimated the distance to the fixed stars to be 20,000 Earth radii. By the turn of the eighteenth century, that estimate stood at 20,000,000,000 Earth radii--a millionfold increase. On such a scale, the planets suddenly didn't seem so far after all. However improbably, those wandering lights in the night sky were relatively near, leaving the stars alone to occupy the impossibly vast stretches of far and farther.

Nothing captured this new celestial hierarchy better than the coining of the term "solar system" which apparently dates from the early 1700s. In the three centuries since then, our ears, sensitized by the Copernican revolution, have come to hear an emphasis on the word "solar." But the understanding that Earth belongs to a system was no less profound.

By the end of the eighteenth century, philosopher Immanuel Kant and mathematician Pierre-Simon de Laplace had independently proposed that the Sun and planets shared a common birth, condensing out of a mass of gas. Despite some untenable details of their hypothesis, the German astrophysicist Carl Friedrich von Weizsacker retained much of their basic thinking in the more sophisticated model of the nebular evolutionary process that he worked out in the 1940s.

Indeed, our conception of the solar system continues to evolve to this day. Just within the past decade, astronomers have discovered Kuiper Belt objects--small, icy bodies at the outer reaches of the solar system that may number in the billions. As University of Arizona researcher Jeffrey Larsen reported at an astronomy meeting this past August, "Our solar system seems to be populated with many objects that even a few years ago were not seen because they are extremely faint. We still do not know yet how far out they go."

Or do we? In late October, just two months after Larsen spoke, three researchers announced that although they had conducted six extensive surveys, they hadn't detected any Kuiper Belt objects at a distance of more than fifty-five astronomical units (AU) from the Sun, approximately at the outer range of Pluto's orbit. (One AU equals the distance between Earth and the Sun, or about 93 million miles.) Does the solar system really peter out at Pluto? Or will better technology and instrumentation reveal fainter objects? And how would either scenario alter current theories of solar system formation?

While astronomers ponder these questions, the rest of us can enjoy an especially spectacular planetary display in February by our solar system neighbors. On the first day of the month, Jupiter and Saturn will line up strikingly with the first-quarter Moon in the southern sky. And on February 20 and 21, Venus will reach a magnificent -5.4 magnitude, the brightest it will be until the end of 2002. Without magnification the planets can never be anything more to earthly observers than lights that wander. Even so, they can also serve as nightly reminders of how much our understanding of the universe has changed, near and far.