Monday, August 14, 2006

What is a Planet?

(An artist representation of Xena ("2003 UB313")

Our solar system is suffering an identity crisis.

Monday, August 14, 2006

...For decades, it has consisted of nine planets, even as scientists debated whether Pluto really belonged. Then the recent discovery of an object larger and farther away than Pluto threatened to throw this slice of the cosmos into chaos.
Should this newly found icy rock known as "2003 UB313" become the 10th planet? Should Pluto be demoted? And what exactly is a planet, anyway?

At a 12-day conference beginning today, scientists will conduct a galactic census of sorts. Among the possibilities at the meeting of the International Astronomical Union in the Czech Republic capital of Prague: Subtract Pluto or christen one more planet, and possibly dozens more...

The debate intensified last summer when astronomer Michael Brown, of the California Institute of Technology, announced the discovery of a celestial object larger than Pluto. Like Pluto, it is a member of the Kuiper Belt, a mysterious disc-shaped zone beyond Neptune containing thousands of comets and planetary objects. (Brown nicknamed his find "Xena" after a warrior heroine in a TV series; pending a formal name, it remains 2003 UB313.)

The Hubble Space Telescope measured the bright, rocky object at 1,490 miles in diameter, roughly 70 miles longer than Pluto. At 9 billion miles from the sun, it is the farthest known object in the solar system...

"Life would be simpler if we went back to eight planets," said Brian Marsden, director of the astronomical union's Minor Planet Center in Cambridge, Mass...

For years, Pluto's inclusion in the solar system has been controversial. Astronomers thought it was the same size as Earth but later found it was smaller than Earth's moon. Pluto is also odd in other ways: With its elongated orbit and funky orbital plane, it acts more like other Kuiper Belt objects than traditional planets....

The trick for astronomers meeting in Prague is to set a criterion that makes sense scientifically. Should planets be grouped by location, size or another marker? If planets are defined by their size, should they be bigger than Pluto or another arbitrary size? The latter could expand the solar system to 23, 39 or even 53 planets.


Ley said...

I don't think that any Kuiper Belt objects should be included as planets, Pluto included. In my opinion, the defininition should hinge on how the object was formed. Planets, in my opinion, form out of a planetary disk that surounds a star in the early stages of its evolution. Hence the planets all occupy a similar plane, while the belt objects do not. To my knowledge we do not fully understand how Kuiper belt objects form, but I think the general concensus is that it is a difference process. The Kuiper Belt objects are also not in the same plane as the planets.

meret said...

The draft resolution from the IAU (International Astronomical Union):

Draft Resolution 5 for GA-XXVI: Definition of a Planet

Contemporary observations are changing our understanding of the Solar System, 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 force us to create a new definition, which we can make using currently available scientific information. (Here we are not concerned with the upper boundary between "planet" and "star".)

The IAU therefore resolves that planets and other Solar System bodies be defined in the following way:

(1) A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape1, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.2

(2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a "dwarf planet."3

(3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call "plutons".

(4) All non-planet objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".4

1 This generally applies to objects with mass above 5 x 1020 kg and diameter greater than 800 km. An IAU process will be established to evaluate planet candidates near this boundary.

2 For two or more objects comprising a multiple object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto-Charon a double planet.

3 If Pallas, Vesta, and/or Hygeia are found to be in hydrostatic equilibrium, they are also planets, and may be referred to as "dwarf planets".

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. In the new nomenclature the concept "minor planet" is not used.


It will be nice to resolve the ambiguity.

meret said...

Also at the IAU website - some interesting answers to questions about planets.

For example:

Q: What is the origin of the word “planet”?

A: The word “planet” comes from the Greek word for “wanderer”, meaning that planets were originally defined as objects that moved in the sky with respect to the background of fixed stars.

Q: What new terms are proposed as official IAU definitions?

A: There are two new terms being proposed for use as official definitions of the IAU. The terms are: “planet” and “pluton”.

Q: Based on this new definition, how many planets are there in our Solar System?

A: There are currently 12. Eight are the classical planets Mercury through Neptune. Three are in a newly defined (and growing in number) category called “plutons”, for which Pluto is the prototype. One is Ceres, which may be described as a dwarf planet.

Q: What are the 12 planets?

A: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003 UB313 (provisional name).


more at link.