Redefining the solar system

I did mention back in July that the International Astronomical Union would be meeting to attempt to come up with a new [in fact the first? – Ed] definition of a planet. Well it seems they have.. at least they have proposed one, it’s not been adopted yet. One result of the new definition by the IAU General Assembly will be to immediately increase the number of planets in the solar system to 12 [and counting? – Ed], now including the admittedly cool view that Pluto and it’s largest moon, Charon, are in fact twin planets [plutons – Ed] orbiting each other, while orbiting the Sun. In the NY Times[subs], several astronomers, including Prof Michael Brown whose team discovered Xena, are not happy.Via this BBC report, there’s a wonderful graphic of our new solar system from the IAU – including the new twin planets, Pluto and Charon, Xena [aka 2003 UB319] and the former minor planet, [aka asteroid] Ceres.

The Times sees Pluto being downgraded, and the Independent report seems to agree... The Guardian’s Alok Jha seems less convinced of that

And it doesn’t seem to be the intention of the IAU to downgrade Pluto..

The IAU’s press release explains the fudge compromise

The part of “IAU Resolution 5 for GA-XXVI” that describes the planet definition, states “A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.” Member of the Planet Definition Committee, Richard Binzel says: “Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor. Nature decides whether or not an object is a planet.”

According to the new draft definition, two conditions must be satisfied for an object to be called a “planet.” First, the object must be in orbit around a star, while not being itself a star. Second, the object must be large enough (or more technically correct, massive enough) for its own gravity to pull it into a nearly spherical shape. The shape of objects with mass above 5 x 1020 kg and diameter greater than 800 km would normally be determined by self-gravity, but all borderline cases would have to be established by observation.

And the other part of the compromise..

If the proposed Resolution is passed, the 12 planets in our Solar System will be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003 UB313. The name 2003 UB313 is provisional, as a “real” name has not yet been assigned to this object. A decision and announcement of a new name are likely not to be made during the IAU General Assembly in Prague, but at a later time. The naming procedures depend on the outcome of the Resolution vote. There will most likely be more planets announced by the IAU in the future. Currently a dozen “candidate planets” are listed on IAU’s “watchlist” which keeps changing as new objects are found and the physics of the existing candidates becomes better known.

The IAU draft Resolution also defines a new category of planet for official use: “pluton”. Plutons are distinguished from classical planets in that they reside in orbits around the Sun that take longer than 200 years to complete (i.e. they orbit beyond Neptune). Plutons typically have orbits that are highly tilted with respect to the classical planets (technically referred to as a large orbital inclination). Plutons also typically have orbits that are far from being perfectly circular (technically referred to as having a large orbital eccentricity). All of these distinguishing characteristics for plutons are scientifically interesting in that they suggest a different origin from the classical planets.

So, Pluto is a pluton? And what of Ceres, with an orbital period of 4.599 years?

And from the NY Times report..

“This will be the talk of the town in Prague,” said Alan P. Boss, a planetary theorist at the Carnegie Institution of Washington, who said the new definition, with four paragraphs and four footnotes, read as if it had been written by lawyers, not scientists. “I don’t think this is the one were looking for.”

Neil deGrasse Tyson, director of the Hayden Planetarium, which was raked over the coals five years ago for demoting Pluto in an exhibit in its new Rose Center at the American Museum of Natural History in New York, was clearly disappointed in the committee’s work. “I’m happy there’s finally a definition that’s unambiguous,” Dr. Tyson said. “There hasn’t been one in 2,500 years.”

But roundness, he said, is not a very interesting attribute to use in classifying astronomical bodies. “A Plutophile is well served by this definition,” he said. “It is one of the few that allow you to utter Pluto and Jupiter in the same breath.”

But Alan Stern of the Southwest Research Institute in Boulder, Colo., called the definition “a nice solution that works both inside and outside the solar system.”

While Dr Brown has indicated a list of more plutons to be added to the mix.. they are like buses you know

But Dr. Brown pointed out that at least 43 other publicly known objects in the Kuiper Belt were big enough to fit the planet definition, and that his group was sitting on a list of dozens more.

Dr. Boss said, “We’re going to have more planets inside the solar system than we have outside.”

He added, “Being a planet used to be an old boys’ club, with eight or nine members.”

And there’s even a further dissenting voice

Geoffrey W. Marcy of the University of California, Berkeley, a widely known hunter of planets around other stars, said in an e-mail message, “I am not attending the I.A.U. meeting, nor do I care about the outcome of any vote about whether Pluto and Xena are ‘planets.’ ”

“The universe,” Dr. Marcy added, “contains so much beauty and so many mysteries that we astronomers already have our hands full figuring out how it all came about.”


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  • Intelligence Insider

    Some astronomers and scientists are also proposing that Pluto be downgraded from a planet.

  • Pete Baker


    Well the IAU is debating a proposal by a body it set up to define what a planet is.. the Planet Definition Committee.. that definition has been 2 years in the making.

    There are references in the original post to the debate on Pluto in particular, including Michael Brown’s views, who’s mentioned in the article you linked – not a bad article… but it missed the inclusion of Ceres as one of the new planets.

    Under the new definitions, Ceres is now a planet… while Pluto is to be both a planet… and a pluton.

  • Crataegus


    Thanks for this one Astronomy generally is fascinating but sometimes Astronomers have strange concerns. Its a debate that has been running a while.

    “The universe,” Dr. Marcy added, “contains so much beauty and so many mysteries that we astronomers already have our hands full figuring out how it all came about.”

    How true and I think it neatly illustrates the relative importance of humanity. Here on earth we fight and argue and yet a few minor events in near space and humanity could exit stage. Pity there wasn’t more finance for research and particularly for travelling and living in space. It is such a hostile environment and it would be a truly challenging field of work. It would lead to major technological advances and also perhaps help us appreciate what we have and its interrelated complexity?

  • Occasional Commentator

    Pete: “Pluto is to be both a planet… and a pluton. ”

    Is there a specific word for a non-pluton planet? Aren’t they divided into terrestrials (up to Mars), gas giants, and plutons?

  • Pete Baker


    Terrestrial – up to Ceres now btw 😉 – and gas giant would be more descriptive terms rather than classes of planet-like bodies, if you see what I mean.

    According to the IAU press release, the non-plutons seem intended to be regarded as “classical planets”

    Plutons are distinguished from classical planets in that they reside in orbits around the Sun that take longer than 200 years to complete (i.e. they orbit beyond Neptune).

    Although with Pluto fitting the definition of a pluton… that would mean it is no longer a classical planet?

  • Rory

    I was wondering if Pluto is no longer to be considered a planet what possible hope can there be for “wee” Ulster to be regarded as other than a damned nuisance?

    p.s. I wouldn’t mind having been the first one to discover Xena. Man! She was well fit!

  • Fanny

    “Good Planets are Hard to Find” – Steve Forbert

  • Crataegus


    Nice quote; It is probably an awful long way to the next one.

  • Pete Baker

    Well, crat, we’ve just seen a new one appear in the asteroid belt – Ceres – perhaps the distance to the next one isn’t so long away after all..

  • Lewis

    Shouldn’t a Plutocracy decide whether Pluto is a palnet rather than a congress of astronomers?