Reality Czech?
Artist’s concept of 2003 UB313. Image Credit: NASA, ESA, and A. Schaller (for STScI)
It appears that the festivities are getting rather interesting in Prague as astronomers attending the IAU’s XXVIth General Assembly continue trying to hammer out what exactly constitutes a planet. (See my previous entry for more details.)
Richard Fienberg, editor-in-chief of Sky & Telescope reports heated debate taking place over the draft proposal currently under consideration, which is scheduled for a final vote this coming Thursday. If approved, the new guidelines would bump the number of planets in our solar system up to twelve. But the proposed measures have encountered what look like pronounced objections from some scientists, and at least one counter-proposal has been authored thus far. [See Space.com and NewScientist Space for more.]
The devil’s certainly in the details.
I can’t help but wonder though: why even bother trying to bend and stretch the term planet to function in ways never intended? Cultural and emotional attachments aside, that word, which we’ve long since adopted to describe what the ancient Greeks called wanderers in the night sky, offers little room in which to convey meaningful scientific description.
With the caveat that I’m just another interested nobody with his own website, I’ve put some thought into the problem, and would propose an entirely different solution…
Informally, allow for usage of planet to describe all the familiar members of our solar system, any body massive enough to be molded into a sphere by its own gravity — whether orbiting a star, or not. A more elegant term, in my opinion, is world; I adore how eloquently Carl Sagan employed it to describe wondrous, distant destinations in the cosmos not all that different from the one we call home. Smaller bodies which in turn orbit them (rather than a star) would still be known as moons or satellites. For casual reference, I personally see no wrong in this.
Now, to (hopefully) satisfy scientific rigor and offer a well-structured framework which can be amended as necessary without the need for any linguistic gymnastics, my approach is actually quite pragmatic (and I can’t imagine that I’m the only one that’s considered this…).
Design and implement a suitable planetary classification scheme which draws on principles used to identify stars and galaxies. Devise a system of coding any and all relevant planetary attributes, which would describe any given body’s size and mass (within ranges), composition (e.g. terrestrial, Jovian, and so on), the presence of moons or satellites, and so forth, into different classes of worlds using appropriate alphanumeric combinations in predetermined order. It could function much like the Hubble scheme for galaxies, in which our own Milky Way (if I’m reading the situation correctly) is considered to be of type SBbc - denoting a loosely wound, barred spiral. Simple enough?
Does my model have any merit? I have no idea. But it seems more reasonable than trying to mash a square peg into a round hole. I’m just thinking this would allow the layperson room to continue using familiar language, keeping those cultural and emotional investments, while providing a necessary new one for scientists. This would describe, explicitly, what these worlds are rather than attempting to denote what they are not. And — it’s applicable toward the ever-growing tally of extrasolar planets, whose known numbers are certain to climb exponentially in coming decades. There’s no need to expect them to conform to whatever psychological satisfactions we derive from our immediate cosmic neighborhood.
But, what do I know? I can’t help but wonder if any of the astronomers gathered in Prague - those of a caliber I’ll never be - are asking themselves that same question.
3 Responses to “Reality Czech?”
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I think the clearest definition of a planet would be an object whose face points starward at all times — the face determined solely by its rotation period — and which is large enough to be near-spherical in shape (probably based on a limiting value of the dipole of the shape, though this currently seems to be clear).
One possible disadvantage of this system is that Earth-Luna becomes a binary planet system, rather than approaching it in a couple of billions of years, although this might help clarify to the uninitiated why the ‘lunar problem’ of Newtonian physics was so nasty to solve. There would be a handful of asteroids which would become planets, and several Kuiper belt objects would also (at least Quouar [sp?] and Sedna in addition to Pluto-Charon and ‘Xena’-'Gabrielle’). Whichever choice is made, since elimination of Pluto as a planet is apparently a non-starter, there will be a category change — moons may be associated with both planets of a binary system (although Pluto is considerably heavier than Charon, the orbits on Nix and Hydra must be considerably impacted by Charon at least at certain points in the moons’ orbits).
Pluto may receive a demotion yet. Here’s the text of the alternate proposal offered:
New proposal for Resolution 5: Definition of a Planet
(1) A planet is a celestial body that (a) is by far the largest object in its local population[1], (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) does not produce energy by any nuclear fusion mechanism [3].
(2) According to point (1) the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane are the only planets of our Solar System. All the other objects in orbit around the Sun are smaller than Mercury. We recognize that there are objects that fulfill the criteria (b) and (c) but not criterion (a). Those objects are defined as “dwarf” planets. Ceres as well as Pluto and several other large Trans-Neptunian objects belong to this category. In contrast to the planets, these objects typically have highly inclined orbits and/or large eccentricities.
(3) All the other natural objects orbiting the Sun that do not fulfill any of the previous criteria shall be referred to collectively as ?Small Solar System Bodies?.[4]
[1] The local population is the collection of objects that cross or close approach the orbit of the body in consideration.
[2] This generally applies to objects with sizes above several hundreds km, depending on the material strength.
[3] This criterion allows the distinction between gas giant planets and brown dwarfs or stars.
[4] This class currently includes most of the Solar System asteroids, near-Earth objects (NEOs), Mars-, Jupiter- and Neptune-Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs), and comets.
Further Considerations
There has been a long discussion about what a planet is. This problem appears at both ends: for the very massive bodies and for the smaller ones. At the large end, the limit seems to be clearer; it is now widely accepted that planets must not generate any energy from nuclear fusion, while brown dwarfs generate some nuclear energy from the fusion of deuterium. More problematic is the small end. We think that the definition should be kept as simple as possible and based on physical and cosmogonic reasons.
There is a wide consensus that planets formed by the accretion of small bodies [missing term] the planetesimals. The accretion process led to the formation of embryo planets that, as they grew in size and acquired more powerful gravitational fields, went to a process of runaway accretion in which the size of a few of them detached from the rest of the bodies of their neighboring zones. Given the powerful gravitational fields of these massive bodies - that we can call at this stage protoplanets - they were able to clean the population that had close encounters with them. The bodies interacting with the protoplanets were finally incorporated to the planets or scattered to other regions.
From a cosmogonic point of view, it therefore makes more sense to consider a planet as an object that acquired a mass large enough to clean a zone around its orbit. According to this definition, only eight planets, Mercury (perhaps marginally), Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune fulfill this condition. It is obvious that, at least for our solar system, this cosmogonic definition implicitly carries the condition of objects with a roundish shape determined by self-gravity.
From our definition, Pluto, Ceres and other large Trans-Neptunian objects in quasi-hydrostatic equilibrium [1] should be not considered as planets, since they never were the dominant bodies in their accretion zones. It is suggested that Pluto be kept unnumbered by historical reasons.
Is may be possible that in the near future cases of objects not foreseen at present could appear beyond our solar system, as for instance free-floating planets, stray planets, or double planets. We think that we should not advance definitions at this point for these exotic cases and leave their discussion when if they became a part of the observed world.
[1] From our present knowledge of the Solar System, we know that objects as small as Mimas (D~400km) are roundish. If this were the lower limit for an icy body to be in hydrostatic equilibrium, then we would already have several tens of bodies fulfilling this requirement.
List of adherents to the above proposal:
Name/Country
Julio A. Fernandez - Uruguay
Marcello Fulchignoni - France
Daniela Lazzaro - Brazil
Gonzalo Tancredi - Uruguay
Alessandro Morbidelli - France
Mario Di Martino - Italy
Paolo Paolicchi - Italy
Antonella Barucci - France
Giovanni Gronchi - Italy
David Vokrovhlicki - Czech Republic
David Nesvorny - USA
Fernando Roig - Brazil
Hugo Levato - Argentina
Steven Chesley - USA
Alsonso Sena - Mexico
J. E. Arlot - France
I. Shevchenko - Russia
Patrick Michel - France
I have read thoroughly the above paragraph of the “definition of planet”, whatever unanimously made the definitions become the part of the book previously and it was mostly practiced for 9 planets, for the current boost in that erena another 3 items came which are rigid body and sufficient in mass, so herein after all rigid bodies in the universe will be call as planets? or yet the exact measurement has to do for further planets exists within the solar system which are unknown. Infact this resolution is good one to include the 3 new plantes, nothing is wrong in adding the new one.
The current technology is substantially adding the knowledge among the people of earth, common people don’t see beyond our solar system even within what is in our solar system they mostly unknown, but today’s scientists and astronomers are making good efforts to increase in the astronomical awareness and curiocity of universe, its a nice decision, the union is taking.
sunil.