Contents

"Honey,
I Shrunk the Solar System"

(NASA: Published 08.24.2006.)

"The International Astronomical Union, wrapping up its [2006] meeting in Prague, Czech Republic, has resolved one of the most hotly-debated topics in the cosmos by approving a specific definition that gives our solar system eight planets, instead of the nine most of us grew up memorizing.

NASA has already visited all eight planets that retain their official title: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. In addition, the agency has its New Horizons spacecraft en route to Pluto, which the astronomical union has re-assigned to a new category of celestial objects, to be called "dwarf planets."

"NASA will, of course, use the new guidelines established by the International Astronomical Union," said Dr. Paul Hertz, Chief Scientist for the Science Mission Directorate at NASA Headquarters. "We will continue pursuing exploration of the most scientifically interesting objects in the solar system, regardless of how they are categorized."

Ceres, which orbits in a belt between Mars and Jupiter and is the largest known asteroid, is one of those interesting objects. In 2007, NASA will launch the Dawn spacecraft on a mission to study Ceres, which the astronomers have placed in the dwarf planet category, alongside Pluto. The dwarf planet family also includes 2003 UB313, nicknamed "Xena." When Dr. Mike Brown of Caltech and his colleagues announced last summer that they'd discovered the object, which is bigger and farther away than Pluto, many astronomers decided it was time to figure out once and for all, "What exactly is a planet, anyway?"

Here's how it all shakes out. The International Astronomical Union has decided that, to be called a planet, an object must have three traits. It must orbit the sun, be massive enough that its own gravity pulls it into a nearly round shape, and be dominant enough to clear away objects in its neighborhood.

To be admitted to the dwarf planet category, an object must have only two of those traits -- it must orbit the sun and have a nearly round shape. And no, moons don't count as dwarf planets. In addition to Pluto, Ceres and 2003 UB313, the astronomical union has a dozen potential dwarf planets on its watchlist.

What's to become of the other objects in our solar system neighborhood, the ones that are not planets, not dwarf planets and not moons? The organization has decided that most asteroids, comets and other small objects will be called "small solar-system bodies."

Despite the establishment of these three distinct categories, there are bound to be gray areas. As technologies improve and more objects are found, the International Astronomical Union will set up a process to decide which categories are most appropriate for specific objects.

Even before the discovery of Xena, not all was calm in the planetary world. There was debate after Clyde Tombaugh discovered Pluto in 1930. With its small size, distant location and odd orbit, some questioned whether Pluto was really a planet or just an icy remnant of the planet-forming process.

That issue has been resolved by the International Astronomical Union. Among those most keenly following the debate -- Mike Brown, who has been awaiting word on Pluto and the object he found, Xena.

"I'm of course disappointed that Xena will not be the tenth planet, but I definitely support the IAU in this difficult and courageous decision," said Brown. "It is scientifically the right thing to do, and is a great step forward in astronomy."

Although the revamping of our solar system might seem unsettling, it's really nothing new. In fact, when Ceres was first discovered in 1801, it was called a planet, as were several similar objects found later. But when the count kept on growing, astronomers decided "enough is enough," and they demoted Ceres and its siblings, placing them in a new category, called asteroids.

The International Astronomical Union, founded in 1919, assigns names to celestial bodies."

(From NASA's "Honey, I Shrunk the Solar System" Published 08.24.2006. - Montage of planets. Image credit: NASA/JPL)

For more information, please visit:
The International Astronomical Union or IAU General Assembly 2006.  
Press Releases During IAU General Assembly 2006.

 

Final IAU Resolution on the definition of "planet"

RESOLUTIONS:

Resolution 5A is the principal definition for the IAU usage of "planet" and related terms. Resolution 5B adds the word "classical" to the collective name of the eight planets Mercury through Neptune.

Resolution 6A creates for IAU usage a new class of objects, for which Pluto is the prototype. Resolution 6B introduces the name "plutonian objects" for this class. The Merriam-Webster dictionary defines "plutonian" as:
Main Entry: plu • to • ni • an
Pronunciation: plü-'tO-nE-&n
Function: adjective
Usage: often capitalized
: of, relating to, or characteristic of Pluto or the lower world

After having received inputs from many sides -- especially the geological community -- the term "Pluton" is no longer being considered.

IAU Resolution: 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.

RESOLUTION 5A
The IAU therefore resolves that planets and other bodies in our Solar System, except satellites,be defined into three distinct categories in the following way:

1. A "planet"¹ 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".

¹The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
²An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
³These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

RESOLUTION 5B
Insert the word "classical" before the word "planet" in Resolution 5A, Section (1), and footnote 1. Thus reading:
(1) A classical "planet"¹ is a celestial body . . .
and
¹The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

IAU Resolution: Pluto

RESOLUTION 6A
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 objects.

RESOLUTION 6B
The following sentence is added to Resolution 6A: This category is to be called "plutonian objects."

Disclaimer: This article is taken wholly from, or contains information that was originally published by, NASA and International Astronomical Union.  Topic editors and authors for the Encyclopedia of the Cosmos may have edited its content or added new information. The use of information from NASA and International Astronomical Union should not be construed as support for, or endorsement by, that organization for any new information added by EoC personnel, or for any editing of the original content.