Free The Pluto Files: The Rise and Fall of America's Favorite Planet by Neil deGrasse Tyson Page A

day.
    One of the “families” is simply our star, the Sun, because it’s so much more massive than everything else combined. We then have the terrestrial planets: Mercury, Venus, Earth, and Mars. All of them have more in common with one another than any one of them has with anything else in the solar system. They’re small, they’re rocky, they’re dense, they’re near the Sun. Beyond the terrestrial planets, we have the asteroid belt, made up of hundreds of thousands of craggy chunks of rock and metal—debris that never became part of a planet, as well as the fragmented remains of planetesimals that formed but were subsequently shattered. Then come the so-called Jovian planets, the gas giants: Jupiter, Saturn, Uranus, and Neptune. As with the terrestrials, the Jovians have more in common with each other than any one of them has in common with anything else in the solar system. They’re big, they’re bulbous, they’re low density, they’re ringed, they’re moon-rich, and they’re in sequence. Beyond them we have the Kuiper belt of comets, whose orbits all lie more or less in a plane, and then far beyond the Kuiper belt we have a swarm of comets whose orbits go every which way, called the Oort cloud.
    Where does Pluto fit? The Kuiper belt. End of story.
    We saw no value in counting planets—or counting anything. That exercise to us seemed pedagogically and scientifically vacuous. In an equally unenlightening exercise, consider the answer to “How many countries are there in the world?” There are 192, but that depends on how you define country. 22 The number is as high as 245 if you include places that see themselves as countries, such as Palestine or the Turkish Republic of Northern Cyprus, but are not internationally recognized as such. Want instead to use the official United Nations list? Sounds like a good idea, but that would mean Switzerland was not a country until 2002, when it was finally admitted as a member state. Odd, because the Swiss city of Geneva is where you could always find one of the four world offices of the United Nations itself, as well as the original location for the League of Nations.
    One could also list all the countries alphabetically from somebody’s compilation and check off their individual characteristics. But why not simply group countries by “family” resemblances, using data and demographics that tell you something useful, such as region, population size, per capita income, temperature range, life expectancy, or proportion of arable land. Divisions such as these, taken in turn or together, enable you to compare and contrast countries in meaningful ways.
    The Rose Center’s Cullman Hall of the Universe, named for New York philanthropists Dorothy and Lewis Cullman, is split in four principal sectors: the Planets Zone, the Stars Zone, the Galaxies Zone, and the Universe Zone. In a day gone by, each wall panel in the Planets Zone might have been devoted to a single planet: Mercury and its properties, then Venus and its properties, continuing out to Pluto and its properties. Nine panels. And that would be that.
    We did something different.
    We looked across the solar system and asked ourselves what physical features about planets and other objects could be taken together and discussed as common properties or phenomena, allowing us to compare and contrast families of objects in whatever way those families would naturally delineate. One such feature is storms; wherever you have a thick, rich atmosphere on a rotating object, you have storms. Another feature is rings. Yet another is magnetic fields. So we took nontraditional cuts through the data of our solar system and presented them among these panels. Pluto was displayed among other Kuiper belt objects, but we neither counted these objects nor made a list of who is or is not a planet.
    We knew that no matter how the Pluto debate would ultimately resolve, our familial treatment of the solar system was pedagogically and