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insideBritannica
get inside: A Retrospective on Pluto
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 | | Pictured: Pluto and its large moon Charon. (NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team) |
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The American astronomer Clyde Tombaugh discovered icy Pluto in 1930 during a deliberate search for a ninth planet. By the turn of the 21st century, however, new revelations about this remote object and kindred bodies in the outer solar system were raising doubts about Pluto’s planetary status. A year ago, in August 2006, astronomers at their major triennial meeting in Prague stirred controversy by approving a definition of planet that excluded Pluto while creating a category called dwarf planet, for which Pluto and two other bodies qualified.
Pluto and its Moons
Because of Pluto's remoteness and small size, the best telescopes on Earth and in Earth orbit have been able to resolve little detail on its surface. Indeed, such basic information as its radius and mass have been difficult to determine; most of what is known about Pluto has been learned since the late 1970s as an outcome of the discovery of Charon:
What is a Planet?
The idea of what exactly constitutes a planet of the solar system has been debated for centuries. In August 2006, the general assembly of the IAU approved a definition for a solar system planet that excluded Pluto, instead defining it under a new distinct class of objects called dwarf planets:
Pluto as a Member of the Solar System
Although it is no longer considered a planet, Pluto is still a member of the Milky Way Galaxy that also includes of the following:
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 | | Pictured: One of the discovery photographs of Pluto's moon Charon. (Official U.S. Navy Photograph)
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Before Pluto’s large moon, Charon, was discovered in 1977, astronomers thought the two bodies to be one, and thus Pluto to be significantly larger and more massive than it actually is. Even in the discovery photos, Charon appears only as a bump on the side of Pluto. The nearness of the two objects, their great distance from Earth, and the distorting effects of Earth’s atmosphere made it extremely difficult to distinguish Charon from Pluto, even in the best telescopes of the time.
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Free Web Training for Britannica Online Subscribers!
Britannica Online subscribers are invited to participate in online Encyclopædia Britannica training sessions! This is provided to see what's new with Britannica Online and learn different ways to use it in your school or library!
K12 Teachers and Librarians will enjoy these 1-hour online presentations that they can attend without having to leave the school or library. K12 Teachers and Librarians can choose to attend the Elementary session or the Middle/High School session. Pick a date, complete the online registration form at www.eb.com/webcast, and invite your colleagues to join in this live presentation.
Note that there is a registration deadline 1 week before the session.
Space is limited, so please sign up soon! 
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A brief overview of comets, highlighting their origin and their distinction from meteors, or shooting stars.
(Copyright © 2004 AIMS Multimedia)
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Learn about the major planetary, solar, and intergalactic phenomena, including galaxies, quasars, pulsars, novas, black holes, and the Big Bang.
(Copyright © 2005 New Dimension Media)
View Video
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Asteroids: Is Planetary Armageddon Looming?
by Gregory McNamee
About 4.6 billion years ago, an asteroid bigger than Ceres, perhaps as large as Mars, collided with Earth and sent a vast cloud of sandy fragments and great chunks of rock into the atmosphere. These fragments eventually coalesced into the Moon. Could the heavens have another mega-collision in store for Earth? Could be... (read more)
The Britannica Blog is a place to share smart, lively conversations on just about any topic.
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Find Standards–Based Materials in Britannica
Teachers - let Britannica help you prepare your lesson plans! Britannica Online School Edition includes encyclopedia articles and student Learning Material activities that have been correlated to state curriculum standards and benchmarks. Click the orange “Standards” button to see the links to the standards.
To look up specific standards you are trying to address, click “Teacher Resources” in the Britannica header, and click the “Curriculum Standards” link. Then, simply select your state, subject area, and grade level. The standards and grade level expectations will appear. Click any of the standards to see supporting encyclopedia articles and student activities.
Try it now!
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2007 Compton's by Britannica
Written and visually designed for students ages 10 to 17, the 2007 Compton's by Britannica has been thoroughly reviewed, updated, and revised by educators, expert contributors, and Britannica editors. With over 50% new and revised content since 2004, Compton's is ideal for middle school and high school aged students.
When you order the 2007 Compton's by Britannica, Britannica will also include New Views of the Solar System as a free gift! New Views of the Solar System looks at scientists’ changing perspectives on the solar system. Brilliant photos and drawings provide a stunning collection of vivid and detailed images. This 112 page hardcover volume is a perfect companion to the Compton’s by Britannica set.
Order 2007 Compton's by Britannica today for only $559, and get free S&H and the free gift. Please mention promotion code AUGUST when ordering.
Call Britannica at 1-800-621-3900 or go online to www.eb.com to place your order now.
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