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Issue 3.15

The Astrophysics Spectator

September 6, 2006

Last month the number of planets in our Solar System dropped to eight, as Pluto was demoted to a dwarf planet. This week, several pages are added to The Astrophysics Spectator that discuss this change in terminology.

The redefinition was long overdue. New discoveries of bodies in the Kuiper Belt had blurred the boundary between the planets and the asteroids. With the prospect of the Solar System ending up with several dozen planets, the astronomy community through the International Astronomical Union (IAU) chose to clarify the definition of planet. They chose to divide the bodies in our the Solar System into three categories: classical planets, dwarf planets, and small solar system bodies.

The new definition of planet is discussed in new three pages: a news page that describes what the IAU did, a commentary that gives some thoughts on names in astronomy and their effect on the broader world, and a page on the “Planets” topical path that discusses the three types of object orbiting the Sun. Two pages on the web site have been rewritten to correspond to our new eight-planet Solar System.

Next Issue: The next issue of The Astrophysics Spectator is planned for release in two weeks on September 20.

Jim Brainerd


Pluto Falls from the List of Planets (August 24, 2006). The International Astronomical Union approved a definition for “planet.” Whether an object is a planet is set by its physical characteristics. First, the object must be in orbit around the Sun, but not around another body. Second, it must be large enough to be round through its self-gravity. Finally, and this criterion is key, it must have cleared the neighborhood of its orbit of other objects. This last criterion eliminates Pluto as a planet. Pluto is now a dwarf planet, and the prototype of the class of plutonian objects. (continue)


What's in a Name? The recent IAU decision to precisely define the word planet brings the word more in line with how scientists think about objects orbiting the Sun. This evolution of a term in astronomy as our understanding improves is not unusual; terminology is born in phenomenology, but it is refined in the confrontation of theory with observation. The demotion of Pluto to dwarf planet status has more impact outside of the scientific community than within the community. (continue)


What is a Planet? Ancient astronomers knew of only five planets in the sky. Over the past 225 years, however, astronomers have found so many objects of all sizes orbiting the Sun that the concept of the planet became ill-defined, begging for clarification. This came in the summer of 2006 from the International Astronomical Union. Now objects orbiting the Sun fall into three groups: classical planets, dwarf planets, and small solar system bodies. The categories are set both by the physical properties of a body and by the proximity of other bodies. (continue)


Planet Update. Planet Update. When the IAU defined planet in a way that dropped Pluto to dwarf planet status, it made two pages on this web site obsolete. These pages, the Planets page of “The Structure of Our Universe,” and An Introduction to Planets page of “Planets,” have been rewritten to reflect the new classification scheme for bodies in our Solar System.

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