Friday, July 24, 2009

The Pluto Files

The Rise and Fall of America's Favorite Planet
Neil DeGrasse Tyson

The Pluto Files is a rather tongue in cheek look at the demotion of Pluto from it's status as a planet by the International Astronomical Union, written by the director of the Hayden Planetarium at New York's Museum of Natural History.

In 2000, the freshly renovated Hayden Planetarium, under Tyson's direction, opened a new exhibit showing the relative sizes of the planets - excluding Pluto. The exhibit classified the planets as either Earth like - the inner group or gas giants, the outer group. Pluto, being very small and made mostly of ice, did not fit in either group, and was left out of the exhibit. This led to a large controversy, many angry letters from elementary school students and, eventually, a vote, in 2006 at a gathering of the IAU, to define the term planet in a way that excludes Pluto.

Tyson's reason for leaving Pluto out of the exhibit is that it is not a member of either class of planets, neither a rocky Earth like planet nor a gas giant. How can you have a class of one? he asks. Pluto is a very small object, smaller than our own moon. It's orbit is eccentric, dipping in closer to the sun than Neptune's for one part of each turn around the Sun and veering off from the plane of the ecliptic. Pluto is like a small child running around and between the legs of a group of adults.

Later, as other Pluto like objects have been found in what is now known as the Kuiper belt, named after Dutch astronomer Gerard Kuiper. At least one, Eris, is larger than Pluto. We now have a class of objects with several thousands of examples, of which Pluto and Eris are the largest known members. Are they both planets, then? No because the AIU had voted on a definition of a planet with three criteria: 1. that it orbits a star and not another object 2. That it has enough gravity to give it a spherical shape and 3. That it has cleaned up it's orbital zone of debris. Both Pluto and Eris, and also one Asteroid, Ceres, meet the first two of these criteria, but not the third.

This last criteria is problematical. The Earth is always being struck by meteorites, which are debris in it's orbital zone. Is the Earth not a planet? Just last weekend Jupiter was struck by a large object which has created an Earth sized impact disturbance in it's atmosphere, visible by amateur astronomers, arguably an object not previously cleared from Jupiter's orbital zone. It appears to me that this is an arbitrary criteria, with exceptions being made in order to include, really, any planets at all. Besides, since when is science subject to a vote?

I suggest that the orbit clearing criteria be abandoned as a bad job and be replaced to the requirement that a planet must have an atmosphere. this would restore Pluto to the status of a planet, possible Eris, too, and exclude Ceres. We would then have nine or ten planets and counting. Pluto has been redefined as a dwarf planet. OK, but a dwarf planet is still a planet.


  1. Even Tyson has distanced himself from the IAU decision, which he describes as flawed.

    Requiring a body orbiting the sun to have an atmosphere might lead to the exclusion of Mercury as a planet, as it has only a tenuous atmosphere.

    A better definition is simply that a planet is a non-self-luminous spheroidal body orbiting a star. The spherical part is important because it means the object is large enough to be pulled into a round shape by its own gravity--a characteristic of planets and not of shapeless asteroids. This would add Ceres in addition to Haumea, Makemake, Eris and, of course, Pluto to the planet list. Some good books to read from this viewpoint are "Is Pluto A Planet?" by Dr. David Weintraub and "The Case for Pluto" by Alan Boyle, due out in October.

    If we amend the IAU resolution and establish dwarf planets as planets, Pluto will not be in a category of one, as there will now be a third class of planets, the dwarf planets, in addition to the terrestrials and the gas giants.

  2. Tenuous is still an atmosphere. But I'm fine with including Ceres. There is no law saying we can't have 11 planets or a hundred.


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