来源：沪江听写酷 2014-11-23 19:30
Well, today I thought we’d talk about some of the reasons why Pluto’s status as a planet has been debated. You see, until recently what makes a planet a planet was one of the simpler concepts in astronomy. It’s always been deemed so, uh…so obvious, so… basic that it was never officially defined…So anyway,…uh improvements in telescopes and related technology have led to a whole host of discoveries in our solar system…with one result being that now even the generally accepted idea of what a planet is is being challenged…or at least qualified. And this directly affects the status of Pluto. Student A So what makes Pluto so different that it could be, um…reclassified? Professor Well, actually, there are several important differences between Pluto and the other planets. First, when you look at the other planets, especially the planets in the outer solar system, where Pluto orbits, you see that Pluto stands out, it’s the oddball…and I’ll give you one guess why. Student B It’s gotta be the size…Jupiter, Saturn and uh, Uranus and Neptune,…they’re the gas giants, and, well, Pluto isn’t. Professor Exactly,…uh compared to the gas giants, Pluto’s very different,…it’s neither gaseous nor a giant. See, uh Pluto is less than half the size of the next smallest planet, Mercury. It’s even smaller than our moon…and smaller than other moons in our solar system. So Pluto is very small for a planet,…maybe it’s not large enough to be considered a planet. Student A But Pluto orbits the Sun and…I mean…well, that’s one of the things planets do. Professor You’re right…Most people agree that a planet orbits a sun, and Pluto certainly does that…every 248 years, but with a highly eccentric orbit. Take a look at this: What I mean when I say ‘eccentric’ is…it’s not like the other planets’ orbits, instead it’s different in uh, two major ways. One, it’s elliptical, but the others are nearly circular. So for part of its orbit, Pluto is closer to the Sun than Neptune and for the rest it’s farther away. And two, Pluto orbits on a different plane. That is, all the planets orbit the Sun on the same plane, except Pluto…which orbits at a seventeen degree angle to the other orbits. Do you see where it looks like it crosses the other orbits? Student A [interrupting] But I don’t see why being small and having an unusual orbit would change Pluto’s status. I mean it still has most of the features that the other planets have, doesn’t it? It’s got an atmosphere, granted it’s thin, but it’s there. It even has a moon! Professor That’s true. In fact, if it wasn’t for the discovery of the Kuiper belt rhymes with “piper”, there probably wouldn’t be a question about Pluto’s status…. [questioning sounds by the students] Student B It’s…I’m sorry, the what belt? Professor Uh, it’s the Kuiper belt. It’s like a swarm of icy-rocky objects out beyond Neptune. It turns out that Kuiper belt objects, which are also called KBOs, have a lot in common with Pluto. For one, KBOs and Pluto are made of the same stuff, namely rock and ice. And for most of its orbit, Pluto is in the Kuiper belt. Remember when I said that Pluto has an eccentric orbit? Well, many KBOs do, too,…for the same reason,…their orbits are influenced by Neptune’s gravity. Now, without going into too much detail,…let me just say that Neptune’s gravity sort of pulls Pluto and the KBOs around…this results in orbits that are elliptical and almost exactly one and one half times longer than Neptune’s. In light of these similarities, some suggest that Pluto’s merely the largest KBO found to date. Now, I’m saying this because several other large Kuiper belt objects have been found, some half as large as Pluto. Some scientists believe that they might find other KBOs as large as Pluto… Student B So you’re saying that Pluto’s more like a KBO than a planet? Student A Yeah…I mean, considering everything you just said, um, if Pluto were discovered today, would it even…well,…would it even be called a planet? Professor Well, let’s see.