IAU Planetary Resolutions (Prague, August 2006) RESOLUTION 5 – Definition of a Planet in the Solar System Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation “planets”. The word “planet” originally described “wanderers” that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information. The IAU therefore resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way: (1) A planet1 is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. (2) A “dwarf planet” is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2 (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. (3) All other objects3 except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”. 1. 2. 3.
The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. An IAU process will be established to assign borderline objects into either dwarf planet or other categories. These currently include most of the Solar System asteroids, most trans-Neptunian Objects, comets, and other small bodies.
RESOLUTION 6 – Pluto The IAU further resolves: Pluto is a “dwarf planet” by the above definition and is recognized as the prototype of a new category of Trans-Neptunian Objects1. 1.
An IAU process will be established to select a name for this category.
remains the planets eventually formed, as matter collided and accreted. This means that planets which formed with the Sun would orbit it in the plane that we call ‘the ecliptic’. Should objects that might not have emerged from the primordial dust also be called planets6? Proposing a new definition of a planet is difficult. Once proposed, it has to be approved by astronomy’s official governing body (which, by international agreement, is the IAU). Every three years it has a General Assembly, at which various proposals are presented and put to the vote. The last one, in August 2006 in Prague, formally accepted several proposals on what constitutes a planet in our Solar System. One proposal suggested that Ceres, Pluto’s moon Charon7, and Eris (2003 UB313) all be accepted as planets and that the status of Pluto remain unchanged, which would increase the number of planets to 12. An argument against this proposal was that the number of planets would be constantly increasing as many objects have already been discovered that would need to be assessed to see if they qualified as ‘planets’. Two further proposals, which excluded Pluto as a planet, were voted on and accepted. The definition of a planet now became, in simple terms: ■ an object in orbit around the Sun ■ round in shape ■ the only object in that orbit. This definition eliminates Pluto, which crosses into Neptune’s orbit, so our Solar System is now
officially deemed to have only eight planets8. The future The IAU’s new definition of a planet in our Solar System has not been unanimously accepted. More important, we still lack a general definition to cover all planets – those in the Solar System as well as the exoplanets outside it. Further new discoveries will bring new considerations. If we find that our Solar System is not typical of others, for instance, we might need to change our new definition radically. If small, rocky, Earth-like planets are unusual and large Jupitermass planets far more common, additional terms might be needed. If life is discovered on some distant planet, then an entire subsidiary group of life-sustaining planets is possible. It will be a while before a definition is found that satisfies everyone. I hope, however, that these explanations will give readers greater understanding of the problem and enable them, too, to contribute to the debate. ■ Case Rijsdijk is a consultant on astronomy education at the South African Astronomical Observatory and a researcher in particle physics. For years he has brought astronomy to the public through his articles for the popular press and his SABC radio broadcasts. For more details, consult Bruce Dorminey, Distant Wanderers (Copernicus Books, 2002); John Davies, Beyond Pluto (Cambridge University Press, 2001); and Nigel Hey, Solar System (Weidenfeld and Nicholson, 2002). Visit the following websites: http://saao.ac.za; http://saao.ac.za/assa; www.iau.org; http://skytonight. com; www.eso.org; http://cosmiccontroversy.com
6. The greater the eccentricity, the more ‘egg-shaped’ is the orbit. Most planets have orbits whose eccentricity is hardly noticeable: their orbits are nearly circular. 7. The motivation for classifying Pluto’s moon Charon is the position of the barycentre of the two outside Pluto. The barycentre is the common point around which two gravitationally-bound objects orbit. If they have equal mass, the barycentre lies halfway between them. As the mass of one increases, so the barycentre moves towards the more massive object. If one mass is far greater than the other, the barycentre may actually lie within the body of the larger mass. In the Earth–Moon system, for instance, the barycentre is inside the Earth, which makes this a planet–moon system. In the case of Pluto and Charon, where the barycentre does not lie within Pluto, it is argued that the two make up a double-planet system. 8. A substantial number of the world’s planetary scientists disagree with the IAU’s withdrawal of planetary status from Pluto, and they plan to continue the debate at the next IAU General Assembly in 2009 in Rio de Janeiro.
Q News Photograph of Comet McNaught, taken from Paarden Island, looking over Table Bay towards Kloof Nek, to the right of Table Mountain. Image: Stephen Potter
The Great Comet of 2007 (McNaught) Comets occasionally visit the inner parts of the Solar System, but very bright great comets are rare. Since 1960, ten comets have been visible from Earth with the naked eye, though few qualified as ‘great’. Comet McNaught is the brightest since 1965. Comets have been likened to dirty snowballs, consisting largely of frozen, fluffy water (hence ‘snow’) embedded with dust particles. They reside in the outer Solar System, beyond Pluto’s orbit. Small disturbances to their travel paths, induced by planets, occasionally cause one to deviate towards the Sun, as probably happened to McNaught. When a comet approaches the Sun, the radiant heat begins to evaporate the snow. The water vapour (which also includes other chemicals) drifts away from the comet and is pushed out by the solar wind in a stream of particles, forming a tail. It always points away from the Sun, and can reach lengths as great as many millions of kilometres. But the amount of gas in it is in fact small enough to fit inside a soccer ball. Comets normally revert to the outer Solar System, albeit in an orbit that eventually returns to the Sun. McNaught’s precise trajectory is not known, but current estimates indicate that its orbit may allow it to escape the Sun altogether. This can occur if a comet picked up extra speed, while falling towards the Sun, from coming too close to a planet. If this happened, McNaught will never return, but will leave the Solar System and may eventually (after millions of years) gravitate towards another star. We can expect to see other comets, but it may be decades before one becomes as bright as this. – Stephen Potter, SAAO For more, visit www.saao.ac.za/ public-info/comet-mcnaught/
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