Pluto is, isn’t, is, a planet
A question at Perkins Observatory by a very intelligent fourth grader reminded me recently that certain scientific controversies never really go away.
I hesitate, frankly, to write about this topic. I have been stargazing for 50 years. I have seen Pluto exactly twice and only after hours of effort to ferret it out from fields of very faint stars in the Perkins Observatory telescope. Pluto is a speck, even in the largest of telescopes.
Back in 2001, Neil deGrasse Tyson, director of the Hayden Planetarium, excluded Pluto from its public display of planets. He’s borne the brunt of vituperation from people like science writer David Levy. According to Levy, “Tyson is so far off base with Pluto, it’s like he’s in a different universe.”
If we strip away the personal attacks, much can be learned from questioning Pluto’s planetary status. The discussion goes to the heart of how the planets were formed in the first place.
The fact is, problems have always existed with Pluto’s classification. Pluto is in orbit around the sun, but so are many other objects called asteroids, or “minor planets.” At 1400 miles wide, Pluto would be a rather large asteroid, but it’s a scrawny planet at only .7 times the diameter of Earth’s moon.
Some say Pluto must be a planet because it has moons. Yet at least one asteroid, called Ida, has a moon.
We should also consider the shape of Pluto’s orbit. The planets travel in almost circular paths around the sun. Pluto’s orbit is highly elliptical. As a result, from 1979 –1999, Pluto was closer to the sun than Neptune.
Like many asteroids, Pluto’s orbit is tilted with respect to the planets, which orbit the sun near the same plane, called the ecliptic. It is as if they are marbles rolling around on a plate. Pluto rolls on a plate tilted a full 17 degrees from the ecliptic.
Like the planets, Pluto is a spherically shaped. Shape is mostly a function of size. When they coalesced out of the flat swirling cloud of dust and gas that formed our solar system, smaller bodies tended to be irregularly shaped, and larger ones, because of their mass, tended to contract evenly into balls. Thus, the larger asteroids are spherical.
Ceres, the largest, was considered to be a planet for about a year in the early 19th century. It was finally reclassified as an asteroid, basically defining a new class of solar-system objects.
Recent studies of Pluto have helped astronomers to refine one critical issue with regard to its planetary status — what it is made out of. Pluto is composed of a mixture of rock and ice. It looks a lot like a the dozens of other rocky iceballs that orbit the sun out past Neptune in a region called the Kuiper Belt. The other Kuiper Belt objects are somewhat smaller than Pluto, but it fits many of the other criteria.
The presence of rocky asteroids like Ceres outside the orbit of the rocky planets suggests that smaller, asteroid-like objects accumulated by their mutual gravity to form the rocky planets close to the sun — Mercury, Venus, Earth, and Mars. Farther from the sun, rocky ice balls like Pluto collected together in the same way to form the cores of the gas-giant planets — Jupiter, Saturn, Uranus, and Neptune.
Perhaps 4.5 billion years ago, many objects like Pluto existed in the outer solar system. Most of those “protoplanets” disappeared into what we now call the gas giants. A few remain. Pluto is simply the largest of a class of objects that predate the larger planets by hundreds of millions of years.
By classifying Pluto as a Kuiper Belt object, Tyson recognized the possibility that the planets did not spring whole out of the early solar system like Athena from the head of Zeus. They were formed in stages, and objects like Pluto represent a preliminary stage in the planets’ formation.
Pluto is not a fully fledged planet. It is far more important than that. Long before Earth was a ball of rock, objects like Pluto and Ceres graced our solar system waiting for the inevitable gravitational action that would produce our solar system.
Now really, did the Hayden Planetarium deserve public abuse for suggesting that possibility? And did I deserve the scorn of that fourth grader: “What are you doin’ taking away one of my planets?”
Of course, we didn’t. In 2006, astronomers created a category of “dwarf planets,” to cover the leftovers like Pluto and Ceres, of which there are perhaps millions.
Tyson went on to fame and fortune as the world’s greatest front man for astronomy. I still have my fourth graders. Everybody, except perhaps for David Levy and that fourth grader, is happy.
Tom Burns is the director of Perkins Observatory. He can be reached at email@example.com.