During the meal at the second seder, the topic of discussion turned to astronomy. When the subject of the planets came up (specifically regarding the decision to "demote" Pluto), my fourteen year old niece, who tends toward the fundamentalist side, announced that the Torah says that there are only seven planets. When I mentioned to her that there were, in fact, more than seven planets, she simply repeated her assertion with a "that's-what-the-Torah-says-and-that's-that" tone.
I didn't press the point any further that evening, but as I thought about it the next day, I realized that my niece was both right and wrong. The problem, very simply, is that she and I were not using the same definition of the word "planet."
Allow me to give some historical background.
When the ancients studied the skies at night, they made some observations about the positions of the stars. They noticed that as the sky rotates overhead during the night (remember, they didn't know that it was the earth that was rotating), the stars in the sky moved, but kept their positions relative to each other. In addition, they kept their positions during the year as well -- a particular star could be counted on to disappear below the horizon at a certain time of the year and reappear at the same position (relative to the other stars) at a later point during the year.
However, there were seven heavenly bodies, they noted, that did not keep their positions. These planets "wandered" among the other stars -- sometimes moving forward against the other stars, sometimes regressing backwards (and sometimes displaying both behaviors at different times). These seven wanderers became the "planets," from the Greek word for wanderer. The seven bodies that displayed these properties were the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn.
Of course, as time went on, we gained a better understanding of the heavens than the ancients had. Copernicus showed that most of the "planets" actually revolved around the sun, not around the Earth. The sun was eventually identified as a star, one of the billions of stars in our galaxy. The word "planet" now no longer referred to the wanderers of the heavens, but to heavenly bodies (of significant size) that orbit the sun. In addition, with better optics than the ancients had at their disposal, we even discovered that most of the planets in our solar system had moons of their own. Better telescopes and observations have allowed us to find Uranus, Neptune, Pluto, Sedna and other minor ("dwarf") planets. And, of course, within the last fifteen years, we've gained the ability to detect planets orbiting other stars and have, to date, discovered 442 of them.
So, what is the true believer to do? If one believes that the Torah says that there are seven planets, and one wants to remain "Torah true," how does one reconcile the astronomy of the last few hundred years with the words of Chazal? Does one say that astronomy is a sham and that the data is faked? Does one say that the pictures NASA released from the Pioneer and Voyager missions are fakes? Does one say that the astronomers know the truth, but are simply faking it to suppress God's truth?
Well, you could take that approach -- although I think it's really very silly. The simple answer is also the best answer -- that when Chazal (or the Rishonim) talk about planets, they are referring to points of light that "wander" against the background sky. Using that definition, they are correct -- there are only seven* planets. But one has to recognize that the definition of the word "planet" has changed over the years and that when we discuss planets today, we are using a different definition of the term. As a result, it is also correct to say that there are well more than seven planets.
I often wonder how many Torah/science arguments could be avoided if we simply recognize that the meanings of words, including planet, species, bird, animal, star and even science among many others have had their definitions change over the years. Of course, not all Torah/science arguments would end -- but a fair number of them could be put to rest.
* Yes, I know Uranus is sometimes visible with the naked eye which would make eight planets -- but it's so infrequent and it's orbital period so long that it was probably just not noticed by the ancients. And, of course, this also excludes comets.