James Baquet

    Mark is chatting with his classmate Ming in the common room of their dorm.

    Ming: Hi, Mark. I have to write about the discoverers of the planets in the solar system, and I’m stuck!

    Mark: What’s the problem?

    Ming: I can’t figure out who discovered the very first planets.

    Mark: There’s a simple reason for that: Everyone discovered the first planets — and no one did!

    Ming: That doesn’t help.

    Mark: The first five planets known — as well as the sun and the moon, which once were also considered planets — were visible to the naked eye, that is, the eye unaided by any special equipment, like a telescope.

    Ming: So our earliest ancestors probably noticed Mercury, Venus, Mars, Jupiter, and Saturn as they gazed up at the night sky. But how did they know they were planets, and not just stars?

    Mark: Do you know what the word “planet” means?

    Ming: No, sorry.

    Mark: It means “wanderer.” So the ancient people noticed that the planets don’t stay fixed relative to the other stars.

    Ming: I see.

    Mark: As far as we know, the first to identify the five visible planets — plus the sun and moon — were the Babylonians. Our modern days of the week are based on those seven heavenly bodies.

    Ming: You mentioned that the sun and the moon aren’t planets. Why is that?

    Mark: The modern definition says a planet needs to orbit around a star. As the moon orbits the earth — a planet — and the sun doesn’t orbit any particular body, they don’t meet the definition.

    Ming: Got it. Okay, now that we’ve settled the five visible planets —

    Mark: Plus earth as the sixth —

    Ming: We only have two or three more. Let’s see... William Herschel, a German immigrant to England, discovered Uranus in 1781.

    Mark: That’s right. At first he thought it was a comet.

    Ming: Yeah. He made his own telescopes, and they were so good — and he was so systematic — that he also found several moons that Galileo had missed.

    Mark: Of course! Galileo’s telescopes were quite simple.

    Ming: Two men — a Frenchman named Le Verrier and a German named Galle — are credited with discovering Neptune.

    Mark: Although, in hindsight, it seems that Neptune appears in Galileo’s notes, but he mistook it for a fixed star.

    Ming: So those are the eight official planets. In addition, Pluto was discovered by an American named Clyde W. Tombaugh in 1930, but it’s no longer considered a planet.

    Mark: That’s right. So there you have it — five planets plus the earth, moon, and sun. Two more were discovered by Europeans. And one, found by an American, is no longer a planet.

    Ming: Great! Thanks, Mark.