杏吧原创

IN LATE 2006, received an angry letter: 鈥淒ear Mr Tyson鈥� Why do you think Pluto is no longer a planet? I do not like your anser[sic]!!! 鈥� Pluto is a planet !!!!!!!鈥� signed, 鈥測our friend Emerson York鈥�. It was one of a stack of cute hate mail from third-grade students in Pennsylvania.

Read some of the letters complaining about Pluto鈥檚 demotion

The letters were prompted by the controversial new definition adopted by the that year, which required a planet to have 鈥渃leared the neighbourhood around its orbit鈥� in addition to orbiting the sun and being round. This led to an embarrassing downgrade to 鈥渄warf planet鈥� for Pluto, which resides in the Kuiper belt, a zone populated by countless small, icy objects.

Though Tyson was not directly responsible for the definition, he had been considered 鈥渁 public enemy of Pluto lovers the world over鈥� since 2001, he says. That鈥檚 when a drew attention to a provocative solar system display Tyson had installed at New York鈥檚 , where he is director. The display showed only eight planets, Mercury through Neptune. Pluto was placed with other diminutive, frozen denizens of the solar system鈥檚 fringes, with the apparent implication that it was nothing but a glorified chunk of distant debris.

Now, in The Pluto Files, Tyson offers a highly entertaining history of Pluto with an emphasis on its place in culture, especially in the US, where affection is strong for an object jokingly described by one NASA scientist as a 鈥渢rue-blue American planet鈥�.

We learn that the name Pluto, however, was suggested by an 11-year-old English girl, and would have been an unlikely pick from an American. At the time of Pluto鈥檚 discovery in 1930 by US astronomer Clyde Tombaugh, its name was associated in the US with Pluto Water, a laxative promoted with the slogan: 鈥淲hen nature won鈥檛, Pluto will.鈥�

Equally enjoyable are letters from school kids taking Tyson to task. 鈥淒ear Scientest [sic]鈥� some people like Pluto鈥� If it doesn鈥檛 exist then they don鈥檛 have a favorite planet,鈥� adding, 鈥淧lease write back, but not in cursive because I can鈥檛 read in cursive.鈥�

Most fascinating about the book is what it reveals about the public鈥檚 relationship with astronomy. While scientists seeking funding tend to emphasise the potential practical benefits of their work for fear that people will not otherwise support it, the planet controversy shows that there is intense public interest in what鈥檚 out there in the . No matter how distant or disconnected from daily life, planets are personal.

鈥淭here is intense public interest in what鈥檚 out there in the solar system, no matter how distant鈥�

Last year I attended a debate over the planet question between Tyson and , director of the Planetary Science Institute in Tucson, Arizona. Sykes argued that when objects are massive enough for gravity to shape them into spheres, their gravity also radically transforms their interiors. This triggers geological activity 鈥� such as volcanic eruptions soon after the object鈥檚 formation 鈥� that shapes its surface in ways that less massive bodies, like small asteroids, never experience.

For the purpose of understanding the detailed properties of individual objects, then, there is a case to be made for distinguishing between those that are massive enough to become round, which we might call planets, and those that are not 鈥� a point I wish Tyson had made in The Pluto Files. There is little space in the breezy book to delve deeply into planetary science, but this distinction deserved a sentence or two.

While Tyson emphasises the things that set Pluto apart from the eight official planets, like its cluttered and elongated orbit, he does not argue against calling it a planet. We should spend less time classifying objects as planet or non-planet, he says, and more time thinking about the myriad ways to group them, from size and composition to formation history and weather. He makes a good case for moving beyond the definition debate, but it is unlikely to sway the hordes of devoted Plutophiles 鈥� especially the angry correspondent who told Tyson: 鈥淧luto is a planet because I say so.鈥�

Read some of the letters complaining about Pluto鈥檚 demotion